This file documents GNU Libtool, a script that allows package developers to provide generic shared library support. This edition documents version 2.5.3.
See Reporting bugs, for information on how to report problems with GNU Libtool.
libtool
libtool
script contentsIn the past, if you were a source code package developer and wanted to take advantage of the power of shared libraries, you needed to write custom support code for each platform on which your package ran. You also had to design a configuration interface so that the package installer could choose what sort of libraries were built.
GNU Libtool simplifies your job by encapsulating both the platform-specific dependencies, and the user interface, in a single script. GNU Libtool is designed so that the complete functionality of each host type is available via a generic interface, but nasty quirks are hidden from the programmer.
GNU Libtool’s consistent interface is reassuring… users don’t need
to read obscure documentation to have their favorite source
package build shared libraries. They just run your package
configure
script (or equivalent), and libtool does all the dirty
work.
There are several examples throughout this document. All assume the same environment: we want to build a library, libhello, in a generic way.
libhello could be a shared library, a static library, or both… whatever is available on the host system, as long as libtool has been ported to it.
This chapter explains the original design philosophy of libtool. Feel free to skip to the next chapter, unless you are interested in history, or want to write code to extend libtool in a consistent way.
Since early 1995, several different GNU developers have recognized the importance of having shared library support for their packages. The primary motivation for such a change is to encourage modularity and reuse of code (both conceptually and physically) in GNU programs.
Such a demand means that the way libraries are built in GNU packages needs to be general, to allow for any library type the package installer might want. The problem is compounded by the absence of a standard procedure for creating shared libraries on different platforms.
The following sections outline the major issues facing shared library support in GNU, and how shared library support could be standardized with libtool.
The following specifications were used in developing and evaluating this system:
The following issues need to be addressed in any reusable shared library system, specifically libtool:
LD_LIBRARY_PATH
must be set properly (if
it is supported), or programs fail to run.
LD_LIBRARY_PATH
or equivalent),
or run ldconfig
.
Even before libtool was developed, many free software packages built and installed their own shared libraries. At first, these packages were examined to avoid reinventing existing features.
Now it is clear that none of these packages have documented the details of shared library systems that libtool requires. So, other packages have been more or less abandoned as influences.
In all fairness, each of the implementations that were examined do the job that they were intended to do, for a number of different host systems. However, none of these solutions seem to function well as a generalized, reusable component.
Most were too complex to use (much less modify) without understanding exactly what the implementation does, and they were generally not documented.
The main difficulty is that different vendors have different views of what libraries are, and none of the packages that were examined seemed to be confident enough to settle on a single paradigm that just works.
Ideally, libtool would be a standard that would be implemented as series of extensions and modifications to existing library systems to make them work consistently. However, it is not an easy task to convince operating system developers to mend their evil ways, and people want to build shared libraries right now, even on buggy, broken, confused operating systems.
For this reason, libtool was designed as an independent shell script. It isolates the problems and inconsistencies in library building that plague Makefile writers by wrapping the compiler suite on different platforms with a consistent, powerful interface.
With luck, libtool will be useful to and used by the GNU community, and that the lessons that were learned in writing it will be taken up by designers of future library systems.
At first, libtool was designed to support an arbitrary number of library object types. After libtool was ported to more platforms, a new paradigm gradually developed for describing the relationship between libraries and programs.
In summary, “libraries are programs with multiple entry points, and more formally defined interfaces.”
The best way to introduce the libtool paradigm is to contrast it with the paradigm of existing library systems, with examples from each. It is a new way of thinking, so it may take a little time to absorb, but when you understand it, the world becomes simpler.
It makes little sense to talk about using libtool in your own packages until you have seen how it makes your life simpler. The examples in this chapter introduce the main features of libtool by comparing the standard library building procedure to libtool’s operation on two different platforms:
An Ultrix 4.2 platform with only static libraries.
A NetBSD/i386 1.2 platform with shared libraries.
Source files for the following examples are taken from the Autotest file tests/demo.at. The files can be extracted by running a demo test and preserving the artifacts:
burger$ make check TESTSUITEFLAGS="-d -k 'preloaded static library'" burger$ cp -r tests/testsuite.dir/027 demo/
You can follow these examples on your own platform, using the preconfigured libtool script that was installed with libtool (see Configuring libtool). Assume that we are building a library, libhello, out of the files foo.c and hello.c.
Note that the foo.c source file uses the cos
math library
function, which is usually found in the standalone math library, and not
the C library (see Trigonometric Functions in The GNU C Library Reference Manual). So, we need to add -lm to
the end of the link line whenever we link foo.lo into an
executable or a library (see Inter-library dependencies).
The same rule applies whenever you use functions that don’t appear in the standard C library… you need to add the appropriate -lname flag to the end of the link line when you link against those objects.
After we have built that library, we want to create a program by linking main.o against libhello.
To create an object file from a source file, the compiler is invoked with the -c flag (and any other desired flags):
burger$ gcc -I. -g -O -c main.c burger$
The above compiler command produces an object file, usually named main.o, from the source file main.c.
For most library systems, creating object files that become part of a static library is as simple as creating object files that are linked to form an executable:
burger$ gcc -I. -g -O -c foo.c burger$ gcc -I. -g -O -c hello.c burger$
Shared libraries, however, may only be built from position-independent code (PIC). So, special flags must be passed to the compiler to tell it to generate PIC rather than the standard position-dependent code.
Since this is a library implementation detail, libtool hides the complexity of PIC compiler flags and uses separate library object files (the PIC one lives in the .libs subdirectory and the static one lives in the current directory). On systems without shared libraries, the PIC library object files are not created, whereas on systems where all code is PIC, such as AIX, the static ones are not created.
To create library object files for foo.c and hello.c, simply invoke libtool with the standard compilation command as arguments (see Compile mode):
a23$ libtool --mode=compile gcc -I. -g -O -c foo.c gcc -I. -g -O -c foo.c -o foo.o a23$ libtool --mode=compile gcc -I. -g -O -c hello.c gcc -I. -g -O -c hello.c -o hello.o a23$
Note that libtool silently creates an additional control file on each ‘compile’ invocation. The .lo file is the libtool object, which Libtool uses to determine what object file may be built into a shared library. On ‘a23’, only static libraries are supported so the library objects look like this:
# foo.lo - a libtool object file # Generated by ltmain.sh (GNU libtool) 2.5.3 # # Please DO NOT delete this file! # It is necessary for linking the library. # Name of the PIC object. pic_object=none # Name of the non-PIC object. non_pic_object='foo.o'
On shared library systems, libtool automatically generates an additional PIC object by inserting the appropriate PIC generation flags into the compilation command:
burger$ libtool --mode=compile gcc -I. -g -O -c foo.c mkdir .libs gcc -I. -g -O -c foo.c -fPIC -DPIC -o .libs/foo.o gcc -I. -g -O -c foo.c -o foo.o >/dev/null 2>&1 burger$
Note that Libtool automatically created the .libs directory upon its first execution, where PIC library object files will be stored.
Since ‘burger’ supports shared libraries, and requires PIC objects to build them, Libtool has compiled a PIC object and made a note of it in the libtool object:
# foo.lo - a libtool object file # Generated by ltmain.sh (GNU libtool) 2.5.3 # # Please DO NOT delete this file! # It is necessary for linking the library. # Name of the PIC object. pic_object='.libs/foo.o' # Name of the non-PIC object. non_pic_object='foo.o'
Notice the second run of GCC has its output discarded. This is done so the compiler warnings aren’t annoyingly duplicated. If you need to see both sets of warnings (you might have conditional code inside ‘#ifdef PIC’ for example), you can turn off suppression by passing the -no-suppress option to libtool’s compile mode:
burger$ libtool --mode=compile gcc -no-suppress -I. -g -O -c hello.c gcc -I. -g -O -c hello.c -fPIC -DPIC -o .libs/hello.o gcc -I. -g -O -c hello.c -o hello.o burger$
Without libtool, the programmer would invoke the ar
command to
create a static library:
burger$ ar cr libhello.a hello.o foo.o burger$
But of course, that would be too simple, so many systems require that
you run the ranlib
command on the resulting library in order to
generate a symbol table:
burger$ ranlib libhello.a burger$
It seems more natural to use the C compiler for this task, given
libtool’s “libraries are programs” approach. So, on platforms without
shared libraries, libtool simply acts as a wrapper for the system
ar
(and possibly ranlib
) commands.
Again, the libtool control file name (.la suffix) differs from the standard library name (.a suffix). The arguments to libtool are the same ones you would use to produce an executable named libhello.la with your compiler (see Link mode):
a23$ libtool --mode=link gcc -g -O -o libhello.la foo.o hello.o *** Warning: Linking the shared library libhello.la against the *** non-libtool objects foo.o hello.o is not portable! ar cr .libs/libhello.a ranlib .libs/libhello.a creating libhello.la (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la) a23$
Aha! Libtool caught a common error… trying to build a library from standard objects instead of special .lo object files. This doesn’t matter so much for static libraries, but on shared library systems, it is of great importance. (Note that you may replace libhello.la with libhello.a in which case libtool won’t issue the warning any more. But although this method works, this is not intended to be used because it makes you lose the benefits of using Libtool.)
So, let’s try again, this time with the library object files. Remember
also that we need to add -lm to the link command line because
foo.c uses the cos
math library function (see Using libtool).
Another complication in building shared libraries is that we need to specify the path to the directory where they will (eventually) be installed (in this case, /usr/local/lib)1:
a23$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \ -rpath /usr/local/lib -lm ar cr .libs/libhello.a foo.o hello.o ranlib .libs/libhello.a creating libhello.la (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la) a23$
Now, let’s try the same trick on the shared library platform:
burger$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \ -rpath /usr/local/lib -lm rm -fr .libs/libhello.a .libs/libhello.la ld -Bshareable -o .libs/libhello.so.0.0 .libs/foo.o .libs/hello.o -lm ar cr .libs/libhello.a foo.o hello.o ranlib .libs/libhello.a creating libhello.la (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la) burger$
Now that’s significantly cooler… Libtool just ran an obscure
ld
command to create a shared library, as well as the static
library.
Note how libtool creates extra files in the .libs subdirectory, rather than the current directory. This feature makes it easier to clean up the build directory, and helps ensure other programs fail horribly if you accidentally forget to use libtool when you should.
Again, you should look at the .la file to see what Libtool stores in it. You will see Libtool uses this file to remember the destination directory for the library (the argument to -rpath) as well as the dependency on the math library (‘-lm’).
If you choose at this point to install the library (put it in a permanent location) before linking executables against it, then you don’t need to use libtool to do the linking. Simply use the appropriate -L and -l flags to specify the library’s location.
Some system linkers insist on encoding the full directory name of each shared library in the resulting executable. Libtool has to work around this misfeature by special magic to ensure that only permanent directory names are put into installed executables.
The importance of this bug must not be overlooked: it won’t cause programs to crash in obvious ways. It creates a security hole, and possibly even worse, if you are modifying the library source code after you have installed the package, you will change the behaviour of the installed programs!
So, if you want to link programs against the library before you install it, you must use libtool to do the linking.
Here’s the old way of linking against an uninstalled library:
burger$ gcc -g -O -o hell.old main.o libhello.a -lm burger$
Libtool’s way is almost the same2 (see Link mode):
a23$ libtool --mode=link gcc -g -O -o hell main.o libhello.la gcc -g -O -o hell main.o ./.libs/libhello.a -lm a23$
That looks too simple to be true. All libtool did was transform
libhello.la to ./.libs/libhello.a, but remember
that ‘a23’ has no shared libraries. Notice Libtool also
remembered libhello.la depends on -lm, so even
though we didn’t specify -lm on the libtool command
line3 Libtool has added it to the gcc
link line for us.
On ‘burger’ Libtool links against the uninstalled shared library:
burger$ libtool --mode=link gcc -g -O -o hell main.o libhello.la gcc -g -O -o .libs/hell main.o -L./.libs -R/usr/local/lib -lhello -lm creating hell burger$
Now assume libhello.la had already been installed, and you want to link a new program with it. You could figure out where it lives by yourself, then run:
burger$ gcc -g -O -o test test.o -L/usr/local/lib -lhello -lm
However, unless /usr/local/lib is in the standard library search
path, you won’t be able to run test
. If instead you use libtool
to link the already-installed libtool library, it will do The Right
Thing (TM) for you:
burger$ libtool --mode=link gcc -g -O -o test test.o \ /usr/local/lib/libhello.la gcc -g -O -o .libs/test test.o -Wl,--rpath \ -Wl,/usr/local/lib /usr/local/lib/libhello.a -lm creating test burger$
Note that libtool added the necessary run-time path flag, as well as -lm, the library libhello.la depended upon. Nice, huh?
Notice the executable, hell
, was actually created in the
.libs subdirectory. Then, a wrapper script (or, on
certain platforms, a wrapper executable see Wrapper executables for uninstalled programs) was
created in the current directory.
Since libtool created a wrapper script, you should use libtool to install it and debug it too. However, since the program does not depend on any uninstalled libtool library, it is probably usable even without the wrapper script.
On NetBSD 1.2, libtool encodes the installation directory of libhello, by using the ‘-R/usr/local/lib’ compiler flag. Then, the wrapper script guarantees the executable finds the correct shared library (the one in ./.libs) so it can be properly installed.
Let’s compare the two different programs:
burger$ time ./hell.old Welcome to GNU Hell! ** This is not GNU Hello. There is no built-in mail reader. ** 0.21 real 0.02 user 0.08 sys burger$ time ./hell Welcome to GNU Hell! ** This is not GNU Hello. There is no built-in mail reader. ** 0.63 real 0.09 user 0.59 sys burger$
The wrapper script takes significantly longer to execute, but at least the results are correct, even though the shared library hasn’t been installed yet.
So, what about all the space savings that shared libraries are supposed to yield?
burger$ ls -l hell.old libhello.a -rwxr-xr-x 1 gord gord 15481 Nov 14 12:11 hell.old -rw-r--r-- 1 gord gord 4274 Nov 13 18:02 libhello.a burger$ ls -l .libs/hell .libs/libhello.* -rwxr-xr-x 1 gord gord 11647 Nov 14 12:10 .libs/hell -rw-r--r-- 1 gord gord 4274 Nov 13 18:44 .libs/libhello.a -rwxr-xr-x 1 gord gord 12205 Nov 13 18:44 .libs/libhello.so.0.0 burger$
Well, that sucks. Maybe I should just scrap this project and take up basket weaving.
Actually, it just proves an important point: shared libraries incur overhead because of their (relative) complexity. In this situation, the price of being dynamic is eight kilobytes, and the payoff is about four kilobytes. So, having a shared libhello won’t be an advantage until we link it against at least a few more programs.
Some platforms, notably those hosted on Windows such as Cygwin
and MinGW, use a wrapper executable rather than a wrapper script
to ensure proper operation of uninstalled programs linked by libtool
against uninstalled shared libraries. The wrapper executable thus
performs the same function as the wrapper script used on other
platforms, but allows to satisfy the make
rules for the
program, whose name ends in $(EXEEXT)
. The actual program
executable is created below .libs, and its name will end
in $(EXEEXT)
and may or may not contain an lt-
prefix.
This wrapper executable sets various environment values so that the
program executable may locate its (uninstalled) shared libraries,
and then launches the program executable.
The wrapper executable provides a debug mode, enabled by passing the
command-line option --lt-debug
(see below). When executing in
debug mode, diagnostic information will be printed to stderr
before the program executable is launched.
Finally, the wrapper executable supports a number of command line
options that may be useful when debugging the operation of the wrapper
system. All of these options begin with --lt-
, and if present
they and their arguments will be removed from the argument list passed
on to the program executable. Therefore, the program executable may not
employ command line options that begin with --lt-
. (In fact, the
wrapper executable will detect any command line options that begin with
--lt-
and abort with an error message if the option is not
recognized.) If this presents a problem, please contact the Libtool
team at the Libtool bug reporting address bug-libtool@gnu.org.
These command line options include:
Causes the wrapper to print a copy of the wrapper script
to stdout
, and exit.
Causes the wrapper to print diagnostic information to stdout
,
before launching the program executable.
For consistency, both the wrapper script and the wrapper executable support these options.
If hell was a complicated program, you would certainly want to test and debug it before installing it on your system. In the above section, you saw how the libtool wrapper script makes it possible to run the program directly, but unfortunately, this mechanism interferes with the debugger:
burger$ gdb hell GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is no warranty for GDB; type "show warranty" for details. GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc. "hell": not in executable format: File format not recognized (gdb) quit burger$
Sad. It doesn’t work because GDB doesn’t know where the executable lives. So, let’s try again, by invoking GDB directly on the executable:
burger$ gdb .libs/hell GNU gdb 5.3 (i386-unknown-netbsd) Copyright 2002 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is no warranty for GDB. Type "show warranty" for details. (gdb) break main Breakpoint 1 at 0x8048547: file main.c, line 29. (gdb) run Starting program: /home/src/libtool/demo/.libs/hell /home/src/libtool/demo/.libs/hell: can't load library 'libhello.so.0' Program exited with code 020. (gdb) quit burger$
Argh. Now GDB complains because it cannot find the shared library that hell is linked against. So, we must use libtool to properly set the library path and run the debugger. Fortunately, we can forget all about the .libs directory, and just run it on the executable wrapper (see Execute mode):
burger$ libtool --mode=execute gdb hell GNU gdb 5.3 (i386-unknown-netbsd) Copyright 2002 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is no warranty for GDB. Type "show warranty" for details. (gdb) break main Breakpoint 1 at 0x8048547: file main.c, line 29. (gdb) run Starting program: /home/src/libtool/demo/.libs/hell Breakpoint 1, main (argc=1, argv=0xbffffc40) at main.c:29 29 printf ("Welcome to GNU Hell!\n"); (gdb) quit The program is running. Quit anyway (and kill it)? (y or n) y burger$
Installing libraries on a non-libtool system is quite straightforward… just copy them into place:4
burger$ su Password: ******** burger# cp libhello.a /usr/local/lib/libhello.a burger#
Oops, don’t forget the ranlib
command:
burger# ranlib /usr/local/lib/libhello.a burger#
Libtool installation is quite simple, as well. Just use the
install
or cp
command that you normally would
(see Install mode):
a23# libtool --mode=install cp libhello.la /usr/local/lib/libhello.la cp libhello.la /usr/local/lib/libhello.la cp .libs/libhello.a /usr/local/lib/libhello.a ranlib /usr/local/lib/libhello.a a23#
Note that the libtool library libhello.la is also installed, to help libtool with uninstallation (see Uninstall mode) and linking (see Linking executables) and to help programs with dlopening (see Dlopened modules).
Here is the shared library example:
burger# libtool --mode=install install -c libhello.la \ /usr/local/lib/libhello.la install -c .libs/libhello.so.0.0 /usr/local/lib/libhello.so.0.0 install -c libhello.la /usr/local/lib/libhello.la install -c .libs/libhello.a /usr/local/lib/libhello.a ranlib /usr/local/lib/libhello.a burger#
It is safe to specify the -s (strip symbols) flag if you use a BSD-compatible install program when installing libraries. Libtool will either ignore the -s flag, or will run a program that will strip only debugging and compiler symbols from the library.
Once the libraries have been put in place, there may be some additional configuration that you need to do before using them. First, you must make sure that where the library is installed actually agrees with the -rpath flag you used to build it.
Then, running ‘libtool -n finish libdir’ can give you further hints on what to do (see Finish mode):
burger# libtool -n finish /usr/local/lib PATH="$PATH:/sbin" ldconfig -m /usr/local/lib ----------------------------------------------------------------- Libraries have been installed in: /usr/local/lib To link against installed libraries in a given directory, LIBDIR, you must use the '-LLIBDIR' flag during linking. You will also need to do one of the following: - add LIBDIR to the 'LD_LIBRARY_PATH' environment variable during execution - add LIBDIR to the 'LD_RUN_PATH' environment variable during linking - use the '-RLIBDIR' linker flag See any operating system documentation about shared libraries for more information, such as the ld and ld.so manual pages. ----------------------------------------------------------------- burger#
After you have completed these steps, you can go on to begin using the installed libraries. You may also install any executables that depend on libraries you created.
If you used libtool to link any executables against uninstalled libtool libraries (see Linking executables), you need to use libtool to install the executables after the libraries have been installed (see Installing libraries).
So, for our Ultrix example, we would run:
a23# libtool --mode=install install -c hell /usr/local/bin/hell install -c hell /usr/local/bin/hell a23#
On shared library systems that require wrapper scripts, libtool just ignores the wrapper script and installs the correct binary:
burger# libtool --mode=install install -c hell /usr/local/bin/hell install -c .libs/hell /usr/local/bin/hell burger#
Why return to ar
and ranlib
silliness when you’ve had a
taste of libtool? Well, sometimes it is desirable to create a static
archive that can never be shared. The most frequent case is when you
have a set of object files that you use to build several different
libraries. You can create a “convenience library” out of those
objects, and link against that with the other libraries, instead of
listing all the object files every time.
If you just want to link this convenience library into programs, then
you could just ignore libtool entirely, and use the old ar
and
ranlib
commands (or the corresponding GNU Automake
‘_LIBRARIES’ rules). You can even install a convenience library
using GNU Libtool, though you probably don’t want to and hence GNU
Automake doesn’t allow you to do so.
burger$ libtool --mode=install ./install-sh -c libhello.a \ /local/lib/libhello.a ./install-sh -c libhello.a /local/lib/libhello.a ranlib /local/lib/libhello.a burger$
Using libtool for static library installation protects your library from
being accidentally stripped (if the installer used the -s flag),
as well as automatically running the correct ranlib
command.
But libtool libraries are more than just collections of object files: they can also carry library dependency information, which old archives do not. If you want to create a libtool static convenience library, you can omit the -rpath flag and use -static to indicate that you’re only interested in a static library. When you link a program with such a library, libtool will actually link all object files and dependency libraries into the program.
If you omit both -rpath and -static, libtool will create a convenience library that can be used to create other libtool libraries, even shared ones. Just like in the static case, the library behaves as an alias to a set of object files and dependency libraries, but in this case the object files are suitable for inclusion in shared libraries. But be careful not to link a single convenience library, directly or indirectly, into a single program or library, otherwise you may get errors about symbol redefinitions.
The key is remembering that a convenience library contains PIC objects, and can be linked where a list of PIC objects makes sense; i.e. into a shared library. A static convenience library contains non-PIC objects, so can be linked into an old static library, or a program.
When GNU Automake is used, you should use noinst_LTLIBRARIES
instead of lib_LTLIBRARIES
for convenience libraries, so that
the -rpath option is not passed when they are linked.
As a rule of thumb, link a libtool convenience library into at most one libtool library, and never into a program, and link libtool static convenience libraries only into programs, and only if you need to carry library dependency information to the user of the static convenience library.
Another common situation where static linking is desirable is in creating a standalone binary. Use libtool to do the linking and add the -all-static flag.
libtool
¶The libtool
program has the following synopsis:
libtool [option]... [mode-arg]...
and accepts the following options:
Display libtool configuration variables and exit.
Dump a trace of shell script execution to standard output. This
produces a lot of output, so you may wish to pipe it to less
(or
more
) or redirect to a file.
Don’t create, modify, or delete any files, just show what commands would be executed by libtool.
Display basic configuration options. This provides a way for packages to determine whether shared or static libraries will be built.
Same as --mode=finish.
Display short help message.
Display a help message and exit. If --mode=mode is specified, then detailed help for mode is displayed.
Display help for the general options as well as detailed help for each operation mode, and exit.
Use mode as the operation mode. When using libtool from the command line, you can give just mode (or a unique abbreviation of it) as the first argument as a shorthand for the full --mode=mode. For example, the following are equivalent:
$ libtool --mode=execute --dry-run gdb prog.exe $ libtool execute --dry-run gdb prog.exe $ libtool exe --dry-run gdb prog.exe $ libtool e --dry-run gdb prog.exe
mode must be set to one of the following:
Compile a source file into a libtool object.
Automatically set the library path so that another program can use uninstalled libtool-generated programs or libraries.
Create a library or an executable.
Install libraries or executables.
Complete the installation of libtool libraries on the system.
Delete installed libraries or executables.
Delete uninstalled libraries or executables.
Use configuration variables from tag tag (see Tags).
Do not remove duplicate dependencies in libraries. When building packages with static libraries, the libraries may depend circularly on each other (shared libs can too, but for those it doesn’t matter), so there are situations, where -la -lb -la is required, and the second -la may not be stripped or the link will fail. In cases where these duplications are required, this option will preserve them, only stripping the libraries that libtool knows it can safely.
Do not print out any progress or informational messages.
Print out progress and informational messages (enabled by default), as well as additional messages not ordinary seen by default.
Print out the progress and informational messages that are seen by default. This option has no effect on whether the additional messages seen in --verbose mode are shown.
Do not print out any additional informational messages beyond those ordinarily seen by default. This option has no effect on whether the ordinary progress and informational messages enabled by --no-quiet are shown.
Thus, there are now three different message levels (not counting --debug), depending on whether the normal messages and/or the additional verbose messages are displayed. Note that there is no mechanism to display verbose messages, without also displaying normal messages.
Normal messages are displayed, verbose messages are not displayed. In addition to being the default mode, it can be forcibly achieved by using both option --no-verbose and either option --no-silent or option --no-quiet.
Neither normal messages nor verbose messages are displayed. This mode can be achieved using either option --silent or option --quiet.
Both normal messages and verbose messages are displayed. This mode can be achieved using either option -v or option --verbose.
Print libtool version information and exit.
Report the warnings falling in category CATEGORY. The default
category is all
. To disable warnings, use the category
none
.
The current libtool
implementation is done with a shell script
that needs to be invoked by the shell that configure
chose for
configuring libtool
(see config.status Invocation in The Autoconf Manual). This shell is set in the she-bang
(‘#!’) line of the libtool
script. Using a different
shell may cause undefined behavior.
The mode-args are a variable number of arguments, depending on the selected operation mode. In general, each mode-arg is interpreted by programs libtool invokes, rather than libtool itself.
For compile mode, mode-args is a compiler command to be used in creating a “standard” object file. These arguments should begin with the name of the C compiler, and contain the -c compiler flag so that only an object file is created.
Libtool determines the name of the output file by removing the directory component from the source file name, then substituting the source code suffix (e.g. ‘.c’ for C source code) with the library object suffix, ‘.lo’.
If shared libraries are being built, any necessary PIC generation flags are substituted into the compilation command.
The following components of mode-args are treated specially:
Note that the -o option is now fully supported. It is emulated on the platforms that don’t support it (by locking and moving the objects), so it is really easy to use libtool, just with minor modifications to your Makefiles. Typing for example
libtool --mode=compile gcc -c foo/x.c -o foo/x.lo
will do what you expect.
Note, however, that, if the compiler does not support -c and -o, it is impossible to compile foo/x.c without overwriting an existing ./x.o. Therefore, if you do have a source file ./x.c, make sure you introduce dependencies in your Makefile to make sure ./x.o (or ./x.lo) is re-created after any sub-directory’s x.lo:
x.o x.lo: foo/x.lo bar/x.lo
This will also ensure that make won’t try to use a temporarily corrupted x.o to create a program or library. It may cause needless recompilation on platforms that support -c and -o together, but it’s the only way to make it safe for those that don’t.
If both PIC and non-PIC objects are being built, libtool will normally suppress the compiler output for the PIC object compilation to save showing very similar, if not identical duplicate output for each object. If the -no-suppress option is given in compile mode, libtool will show the compiler output for both objects.
Libtool will try to build only PIC objects.
Libtool will try to build only non-PIC objects.
Even if Libtool was configured with --enable-static, the object file Libtool builds will not be suitable for static linking. Libtool will signal an error if it was configured with --disable-shared, or if the host does not support shared libraries.
Even if libtool was configured with --disable-static, the object file Libtool builds will be suitable for static linking.
Pass a flag directly to the compiler. With -Wc,
, multiple flags
may be separated by commas, whereas -Xcompiler
passes through
commas unchanged.
Link mode links together object files (including library objects) to form another library or to create an executable program.
mode-args consist of a command using the C compiler to create an output file (with the -o flag) from several object files.
The following components of mode-args are treated specially:
If output-file is a program, then do not link it against any
shared libraries at all. If output-file is a library, then only
create a static library. In general, this flag cannot be used together
with ‘disable-static’ (see The LT_INIT
macro).
Tries to avoid versioning (see Library interface versions) for libraries and modules, i.e. no version information is stored and no symbolic links are created. If the platform requires versioning, this option has no effect.
Pass the absolute name of the directory for installing executable
programs (see Directory Variables in The GNU Coding Standards). libtool
may use this value to
install shared libraries there on systems that do not provide for any
library hardcoding and use the directory of a program and the PATH
variable as library search path. This is typically used for DLLs on
Windows or other systems using the PE (Portable Executable) format.
On other systems, -bindir is ignored. The default value used
is libdir/../bin for libraries installed to
libdir. You should not use -bindir for modules.
Same as -dlpreopen file, if native dlopening is not
supported on the host platform (see Dlopened modules) or if
the program is linked with -static,
-static-libtool-libs, or -all-static. Otherwise, no
effect. If file is self
Libtool will make sure that the
program can dlopen
itself, either by enabling
-export-dynamic or by falling back to -dlpreopen self.
Link file into the output program, and add its symbols to the
list of preloaded symbols (see Dlpreopening). If file is
self
, the symbols of the program itself will be added to
preloaded symbol lists. If file is force
Libtool will
make sure that a preloaded symbol list is always defined,
regardless of whether it’s empty or not.
Allow symbols from output-file to be resolved with dlsym
(see Dlopened modules).
Tells the linker to export only the symbols listed in symfile. The symbol file should end in .sym and must contain the name of one symbol per line. This option has no effect:
By default all symbols are exported.
Same as -export-symbols, except that only symbols matching the regular expression regex are exported. By default all symbols are exported.
Search libdir for required libraries that have already been installed.
output-file requires the installed library libname. This option is required even when output-file is not an executable.
Creates a library that can be dlopened (see Dlopened modules). This option doesn’t work for programs. Module names don’t need to be prefixed with ‘lib’. In order to prevent name clashes, however, libname and name must not be used at the same time in your package.
Disable fast-install mode for the executable output-file. Useful if the program won’t be necessarily installed.
Link an executable output-file that can’t be installed and therefore doesn’t need a wrapper script on systems that allow hardcoding of library paths. Useful if the program is only used in the build tree, e.g., for testing or generating other files.
Declare that output-file does not depend on any libraries other
than the ones listed on the command line, i.e., after linking, it will
not have unresolved symbols. Some platforms require all symbols in
shared libraries to be resolved at library creation (see Inter-library dependencies), and using this parameter allows libtool
to
assume that this will not happen.
Create output-file from the specified objects and libraries.
Use a list of object files found in file to specify objects.
Use this to change the DLL base name on OS/2 to name, to keep within the 8 character base name limit on this system.
Prevents removal of files from the temporary output directory whose
names match this regular expression. You might specify ‘\.bbg?$’
to keep those files created with gcc -ftest-coverage
for example.
Specify that the library was generated by release release of your package, so that users can easily tell what versions are newer than others. Be warned that no two releases of your package will be binary compatible if you use this flag. If you want binary compatibility, use the -version-info flag instead (see Library interface versions).
If output-file is a library, it will eventually be installed in libdir. If output-file is a program, add libdir to the run-time path of the program. On platforms that don’t support hardcoding library paths into executables and only search PATH for shared libraries, such as when output-file is a Windows (or other PE platform) DLL, the .la control file will be installed in libdir, but see -bindir above for the eventual destination of the .dll or other library file itself.
If output-file is a program, add libdir to its run-time path. If output-file is a library, add -Rlibdir to its dependency_libs, so that, whenever the library is linked into a program, libdir will be added to its run-time path.
If output-file is a program, then link it against any uninstalled shared libtool libraries (this is the default behavior). If output-file is a library, then only create a shared library. In the later case, libtool will signal an error if it was configured with --disable-shared, or if the host does not support shared libraries.
If output-file is a libtool library, replace the system’s standard file name extension for shared libraries with suffix (most systems use .so here). This option is helpful in certain cases where an application requires that shared libraries (typically modules) have an extension other than the default one. Please note you must supply the full file name extension including any leading dot.
If output-file is a program, then do not link it against any uninstalled shared libtool libraries. If output-file is a library, then only create a static library.
If output-file is a program, then do not link it against any shared libtool libraries. If output-file is a library, then only create a static library.
If output-file is a libtool library, use interface version information current, revision, and age to build it (see Library interface versions). Do not use this flag to specify package release information, rather see the -release flag.
If output-file is a libtool library, compute interface version information so that the resulting library uses the specified major, minor and revision numbers. This is designed to permit libtool to be used with existing projects where identical version numbers are already used across operating systems. New projects should use the -version-info flag instead.
if output-file is a libtool library, declare that it provides a weak libname interface. This is a hint to libtool that there is no need to append libname to the list of dependency libraries of output-file, because linking against output-file already supplies the same interface (see Linking with dlopened modules).
Pass a linker-specific flag directly to the compiler. With -Wc,
,
multiple flags may be separated by commas, whereas -Xcompiler
passes through commas unchanged.
Pass a linker-specific flag directly to the assembler. With -Wa,
,
multiple flags may be separated by commas, whereas -Xassembler
passes through commas unchanged.
Pass a linker-specific flag directly to the linker.
Pass a link-specific flag to the compiler driver (CC
) during linking.
If the output-file ends in .la, then a libtool library is created, which must be built only from library objects (.lo files). The -rpath option is required. In the current implementation, libtool libraries may not depend on other uninstalled libtool libraries (see Inter-library dependencies).
If the output-file ends in .a, then a standard library is
created using ar
and possibly ranlib
.
If output-file ends in .o or .lo, then a reloadable object file is created from the input files (generally using ‘ld -r’). This method is often called partial linking.
Otherwise, an executable program is created.
For execute mode, the library path is automatically set, then a program is executed.
The first of the mode-args is treated as a program name, with the rest as arguments to that program.
The following components of mode-args are treated specially:
Add the directory containing file to the library path.
This mode sets the library path environment variable according to any -dlopen flags.
If any of the args are libtool executable wrappers, then they are translated into the name of their corresponding uninstalled binary, and any of their required library directories are added to the library path.
In install mode, libtool interprets most of the elements of
mode-args as an installation command beginning with
cp
, or a BSD-compatible install
program.
The following components of mode-args are treated specially:
When installing into a temporary staging area, rather than the
final prefix
, this argument is used to reflect the
temporary path, in much the same way automake
uses
DESTDIR
. For instance, if prefix
is /usr/local,
but inst-prefix-dir is /tmp, then the object will be
installed under /tmp/usr/local/. If the installed object
is a libtool library, then the internal fields of that library
will reflect only prefix
, not inst-prefix-dir:
# Directory that this library needs to be installed in: libdir='/usr/local/lib'
not
# Directory that this library needs to be installed in: libdir='/tmp/usr/local/lib'
inst-prefix
is also used to ensure that if the installed
object must be relinked upon installation, that it is relinked
against the libraries in inst-prefix-dir/prefix
,
not prefix
.
In truth, this option is not really intended for use when calling
libtool directly; it is automatically used when libtool --mode=install
calls libtool --mode=relink
. Libtool does this by
analyzing the destination path given in the original
libtool --mode=install
command and comparing it to the
expected installation path established during libtool --mode=link
.
Thus, end-users need change nothing, and automake
-style
make install DESTDIR=/tmp
will Just Work(tm) most of the time.
For systems where fast installation cannot be turned on, relinking
may be needed. In this case, a ‘DESTDIR’ install will fail.
Currently it is not generally possible to install into a temporary staging area that contains needed third-party libraries that are not yet visible at their final location.
The rest of the mode-args are interpreted as arguments to the
cp
or install
command.
The command is run, and any necessary unprivileged post-installation commands are also completed.
Finish mode has two functions. One is to help system administrators install libtool libraries so that they can be located and linked into user programs. To invoke this functionality, pass the name of a library directory as mode-arg. Running this command may require superuser privileges, and the --dry-run option may be useful.
The second is to facilitate transferring libtool libraries to a native compilation environment after they were built in a cross-compilation environment. Cross-compilation environments may rely on recent libtool features, and running libtool in finish mode will make it easier to work with older versions of libtool. This task is performed whenever the mode-arg is a .la file.
Uninstall mode deletes installed libraries, executables and objects.
The first mode-arg is the name of the program to use to delete
files (typically /bin/rm
).
The remaining mode-args are either flags for the deletion program (beginning with a ‘-’), or the names of files to delete.
Clean mode deletes uninstalled libraries, executables, objects and libtool’s temporary files associated with them.
The first mode-arg is the name of the program to use to delete
files (typically /bin/rm
).
The remaining mode-args are either flags for the deletion program (beginning with a ‘-’), or the names of files to delete.
This chapter describes how to integrate libtool with your packages so that your users can install hassle-free shared libraries.
There are several ways that Libtool may be integrated in your
package, described in the following sections. Typically, the Libtool
macro files as well as ltmain.sh are copied into your package
using libtoolize
and aclocal
after setting up the
configure.ac and toplevel Makefile.am, then
autoconf
adds the needed tests to the configure script.
These individual steps are often automated with autoreconf
.
Here is a diagram showing how such a typical Libtool configuration works when preparing a package for distribution, assuming that m4 has been chosen as the location for additional Autoconf macros, and build-aux as the location for auxiliary build tools (see The Autoconf Manual in The Autoconf Manual):
libtool.m4 -----. .--> aclocal.m4 -----. ltoptions.m4 ---+ .-> aclocal* -+ +--> autoconf* ltversion.m4 ---+--+ `--> [copy in m4/] --+ | ltsugar.m4 -----+ | ^ | \/ lt~obsolete.m4 -+ +-> libtoolize* -----' | configure [ltdl.m4] ------+ | | `----------------------------------' ltmain.sh -----------> libtoolize* -> [copy in build-aux/]
During configuration, the libtool script is generated either
through config.status
or config.lt
:
.--> config.status* --. configure* --+ +--> libtool `--> [config.lt*] ----' ^ | ltmain.sh --------------------------------'
At make
run time, libtool
is then invoked as needed
as a wrapper around compilers, linkers, install and cleanup programs.
There are alternatives choices to several parts of the setup; for
example, the Libtool macro files can either be copied or symlinked into
the package, or copied into aclocal.m4. As another example, an
external, pre-configured libtool
script may be used,
by-passing most of the tests and package-specific setup for Libtool.
Libtool uses a number of macros to interrogate the host system when it is being built, and you can use some of them yourself too. Although there are a great many other macros in the libtool installed m4 files, these do not form part of the published interface, and are subject to change between releases.
Macros in the ‘LT_CMD_’ namespace check for various shell commands:
Finds the longest command line that can be safely passed to ‘$SHELL’ without being truncated, and store in the shell variable ‘$max_cmd_len’. It is only an approximate value, but command lines of this length or shorter are guaranteed not to be truncated.
Macros in the ‘LT_FUNC_’ namespace check characteristics of library functions:
‘AC_DEFINE’ the preprocessor symbol ‘DLSYM_USCORE’ if we have to add an underscore to symbol-names passed in to ‘dlsym’.
Macros in the ‘LT_LIB_’ namespace check characteristics of system libraries:
Set ‘LIBM’ to the math library or libraries required on this machine, if any.
This is the macro used by ‘libltdl’ to determine what dlloaders to use on this machine, if any. Several shell variables are set (and ‘AC_SUBST’ed) depending on the dlload interfaces are available on this machine. ‘LT_DLLOADERS’ contains a list of libtool libraries that can be used, and if necessary also sets ‘LIBADD_DLOPEN’ if additional system libraries are required by the ‘dlopen’ loader, and ‘LIBADD_SHL_LOAD’ if additional system libraries are required by the ‘shl_load’ loader, respectively. Finally some symbols are set in config.h depending on the loaders that are found to work: ‘HAVE_LIBDL’, ‘HAVE_SHL_LOAD’, ‘HAVE_DYLD’, ‘HAVE_DLD’.
Macros in the ‘LT_PATH_’ namespace search the system for the full path to particular system commands:
Add a --with-gnu-ld option to configure. Try to find
the path to the linker used by ‘$CC’, and whether it is the
GNU linker. The result is stored in the shell variable
‘$LD’, which is AC_SUBST
ed.
Try to find a BSD-compatible nm
or a MS-compatible
dumpbin
command on this machine. The result is stored in the
shell variable ‘$NM’, which is AC_SUBST
ed.
Macros in the ‘LT_SYS_’ namespace probe for system characteristics:
Tests whether a program can dlopen itself, and then also whether the same program can still dlopen itself when statically linked. Results are stored in the shell variables ‘$enable_dlopen_self’ and ‘enable_dlopen_self_static’ respectively.
Define the preprocessor symbol ‘LTDL_DLOPEN_DEPLIBS’ if the OS needs help to load dependent libraries for ‘dlopen’ (or equivalent).
Define the preprocessor symbol ‘LT_DLSEARCH_PATH’ to the system default library search path.
Define the preprocessor symbol ‘LT_MODULE_EXT’ to the extension used for runtime loadable modules. If you use libltdl to open modules, then you can simply use the libtool library extension, .la.
Define the preprocessor symbol ‘LT_MODULE_PATH_VAR’ to the name of the shell environment variable that determines the run-time module search path.
Set the shell variable ‘sys_symbol_underscore’ to ‘no’ unless the compiler prefixes global symbols with an underscore.
Libtool is fully integrated with Automake (see Introduction in The Automake Manual), starting with Automake version 1.2.
If you want to use libtool in a regular Makefile (or Makefile.in), you are on your own. If you’re not using Automake, and you don’t know how to incorporate libtool into your package you need to do one of the following:
Libtool library support is implemented under the ‘LTLIBRARIES’ primary.
Here are some samples from the Automake Makefile.am in the libtool distribution’s demo subdirectory.
First, to link a program against a libtool library, just use the ‘program_LDADD’5 variable:
bin_PROGRAMS = hell hell_static # Build hell from main.c and libhello.la hell_SOURCES = main.c hell_LDADD = libhello.la # Create a statically linked version of hell. hell_static_SOURCES = main.c hell_static_LDADD = libhello.la hell_static_LDFLAGS = -static
You may use the ‘program_LDFLAGS’ variable to stuff in any flags you want to pass to libtool while linking program (such as -static to avoid linking uninstalled shared libtool libraries).
Building a libtool library is almost as trivial… note the use of ‘libhello_la_LDFLAGS’ to pass the -version-info (see Library interface versions) option to libtool:
# Build a libtool library, libhello.la for installation in libdir. lib_LTLIBRARIES = libhello.la libhello_la_SOURCES = hello.c foo.c libhello_la_LDFLAGS = -version-info 3:12:1
The -rpath option is passed automatically by Automake (except for
libraries listed as noinst_LTLIBRARIES
), so you
should not specify it.
See The Automake Manual in The Automake Manual, for more information.
When building libtool archives which depend on built sources (for example a generated header file), you may find it necessary to manually record these dependencies. Because libtool archives generate object file names manually recording these dependencies is not as straightforward as the examples in Automake’s manual describe. This affects header files in particular, because simply listing them as ‘nodist_libfoo_la_SOURCES’ will not cause Automake to establish a dependent relationship for the object files of libfoo.la. A useful trick (although somewhat imprecise) is to manually record built sources used by a libtool archive as dependencies of all the objects for that library as shown below (as opposed to a particular object file):
# Build a libtool library, libhello.la which depends on a generated header. hello.h: echo '#define HELLO_MESSAGE "Hello, World!"' > $@ BUILT_SOURCES = hello.h CLEANFILES = hello.h nodist_libhello_la_SOURCES = hello.h libhello_la_SOURCES = hello.c foo.h foo.c bar.h bar.c # Manually record hello.h as a prerequisite for all objects in libhello.la $(libhello_la_OBJECTS): hello.h
See The Automake Manual in The Automake Manual, for more information.
Libtool requires intimate knowledge of your compiler suite and operating system to be able to create shared libraries and link against them properly. When you install the libtool distribution, a system-specific libtool script is installed into your binary directory.
However, when you distribute libtool with your own packages (see Including libtool in your package), you do not always know the compiler suite and operating system that are used to compile your package.
For this reason, libtool must be configured before it can be
used. This idea should be familiar to anybody who has used a GNU
configure
script. configure
runs a number of tests for
system features, then generates the Makefiles (and possibly a
config.h header file), after which you can run make
and
build the package.
Libtool adds its own tests to your configure
script to
generate a libtool script for the installer’s host machine.
LT_INIT
macro ¶If you are using GNU Autoconf (or Automake), you should add a call to
LT_INIT
to your configure.ac file. This macro
adds many new tests to the configure
script so that the generated
libtool script will understand the characteristics of the host. It’s the
most important of a number of macros defined by Libtool:
Ensure that a recent enough version of Libtool is being used. If the
version of Libtool used for LT_INIT
is earlier than
version, print an error message to the standard
error output and exit with failure (exit status is 63). For example:
LT_PREREQ([2.5.3])
Add support for the --enable-shared, --disable-shared,
--enable-static, --disable-static, --enable-pic, and
--disable-pic configure
flags.6 AC_PROG_LIBTOOL
and
AM_PROG_LIBTOOL
are deprecated names for older versions of this macro;
autoupdate
will upgrade your configure.ac files.
By default, this macro turns on shared libraries if they are available,
and also enables static libraries if they don’t conflict with the shared
libraries. You can modify these defaults by passing either
disable-shared
or disable-static
in the option list to
LT_INIT
, or using AC_DISABLE_SHARED
or AC_DISABLE_STATIC
.
# Turn off shared libraries during beta-testing, since they # make the build process take too long. LT_INIT([disable-shared])
The user may specify modified forms of the configure flags
--enable-shared and --enable-static to choose whether
shared or static libraries are built based on the name of the package.
For example, to have shared ‘bfd’ and ‘gdb’ libraries built,
but not shared ‘libg++’, you can run all three configure
scripts as follows:
trick$ ./configure --enable-shared=bfd,gdb
In general, specifying --enable-shared=pkgs is the same as configuring with --enable-shared every package named in the comma-separated pkgs list, and every other package with --disable-shared. The --enable-static=pkgs flag behaves similarly, but it uses --enable-static and --disable-static. The same applies to the --enable-fast-install=pkgs flag, which uses --enable-fast-install and --disable-fast-install.
The package name ‘default’ matches any packages that have not set
their name in the PACKAGE
environment variable.
The --enable-pic and --disable-pic configure flags can be
used to specify whether or not libtool
uses PIC objects. By default,
libtool
uses PIC objects for shared libraries and non-PIC objects for
static libraries.
The --enable-pic option also accepts a comma-separated
list of package names. Specifying --enable-pic=pkgs is the same
as configuring every package in pkgs with --enable-pic and every
other package with the default configuration. The package name ‘default’
is treated the same as for --enable-shared and
--enable-static.
This macro also sets the shell variable LIBTOOL_DEPS
, that you
can use to automatically update the libtool script if it becomes
out-of-date. In order to do that, add to your configure.ac:
LT_INIT AC_SUBST([LIBTOOL_DEPS])
and, to Makefile.in or Makefile.am:
LIBTOOL_DEPS = @LIBTOOL_DEPS@ libtool: $(LIBTOOL_DEPS) $(SHELL) ./config.status libtool
If you are using GNU Automake, you can omit the assignment, as Automake will take care of it. You’ll obviously have to create some dependency on libtool.
Aside from disable-static
and disable-shared
, there are
other options that you can pass to LT_INIT
to modify its
behaviour. Here is a full list:
Enable checking for dlopen support. This option should be used if the package makes use of the -dlopen and -dlpreopen libtool flags, otherwise libtool will assume that the system does not support dlopening.
This option should be used if the package has been ported to build clean
dlls on win32 platforms. Usually this means that any library data items
are exported with __declspec(dllexport)
and imported with
__declspec(dllimport)
. If this option is not used, libtool will
assume that the package libraries are not dll clean and will build only
static libraries on win32 hosts.
Provision must be made to pass -no-undefined to libtool
in link mode from the package Makefile
. Naturally, if you pass
-no-undefined, you must ensure that all the library symbols
really are defined at link time!
Enable the --enable-aix-soname to configure
, which the
user can pass to override the given default.
By default (and always in releases prior to 2.4.4), Libtool always
behaves as if aix-soname=aix
is given, with no configure
option for the user to override. Specifically, when the -brtl linker
flag is seen in LDFLAGS
at build-time, static archives are built from
static objects only, otherwise, traditional AIX shared library archives of
shared objects using in-archive versioning are built (with the .a
file
extension!). Similarly, with -brtl in LDFLAGS
, libtool
shared archives are built from shared objects, without any filename-based
versioning; and without -brtl no shared archives are built at all.
When aix-soname=svr4
option is given, or the
--enable-aix-soname=svr4 configure
option is passed, static
archives are always created from static objects, even without -brtl
in LDFLAGS
. Shared archives are made from shared objects, and filename
based versioning is enabled.
When aix-soname=both
option is given, or the
--enable-aix-soname=svr4 configure
option is passed, static
archives are built traditionally (as aix-soname=aix), and both
kinds of shared archives are built. The .la
pseudo-archive specifies
one or the other depending on whether -brtl is specified in
LDFLAGS
when the library is built.
Change the default behaviour for LT_INIT
to disable
optimization for fast installation. The user may still override this
default, depending on platform support, by specifying
--enable-fast-install to configure
.
Change the default behaviour for LT_INIT
to enable
shared libraries. This is the default on all systems where
Libtool knows how to create shared libraries.
The user may still override this default by specifying
--disable-shared to configure
.
Change the default behaviour for LT_INIT
to disable
shared libraries. The user may still override this default by
specifying --enable-shared to configure
.
Change the default behaviour for LT_INIT
to enable
static libraries. This is the default on all systems where
shared libraries have been disabled for some reason, and on
most systems where shared libraries have been enabled.
If shared libraries are enabled, the user may still override
this default by specifying --disable-static to
configure
.
Change the default behaviour for LT_INIT
to disable
static libraries. The user may still override this default by
specifying --enable-static to configure
.
Change the default behaviour for libtool
to try to use only
PIC objects. The user may still override this default by specifying
--disable-pic to configure
.
Change the default behaviour of libtool
to try to use only
non-PIC objects. The user may still override this default by
specifying --enable-pic to configure
.
Enable libtool
support for the language given if it
has not yet already been enabled. Languages accepted are “C++”,
“Fortran 77”, “Java”, “Go”, and “Windows Resource”.
If Autoconf language support macros such as AC_PROG_CXX
are
used in your configure.ac, Libtool language support will automatically
be enabled.
Conversely using LT_LANG
to enable language support for Libtool
will automatically enable Autoconf language support as well.
Both of the following examples are therefore valid ways of adding C++ language support to Libtool.
LT_INIT LT_LANG([C++])
LT_INIT AC_PROG_CXX
This macro is deprecated, the ‘dlopen’ option to LT_INIT
should be
used instead.
This macro is deprecated, the ‘win32-dll’ option to LT_INIT
should
be used instead.
This macro is deprecated, the ‘disable-fast-install’ option to LT_INIT
should be used instead.
Change the default behaviour for LT_INIT
to disable shared libraries.
The user may still override this default by specifying ‘--enable-shared’.
The option ‘disable-shared’ to LT_INIT
is a shorthand for this.
AM_DISABLE_SHARED
is a deprecated alias for AC_DISABLE_SHARED
.
Change the default behaviour for LT_INIT
to enable shared libraries.
This is the default on all systems where Libtool knows how to create
shared libraries. The user may still override this default by specifying
‘--disable-shared’. The option ‘shared’ to LT_INIT
is a
shorthand for this.
AM_ENABLE_SHARED
is a deprecated alias for AC_ENABLE_SHARED
.
Change the default behaviour for LT_INIT
to disable static libraries.
The user may still override this default by specifying ‘--enable-static’.
The option ‘disable-static’ to LT_INIT
is a shorthand for this.
AM_DISABLE_STATIC
is a deprecated alias for AC_DISABLE_STATIC
.
Change the default behaviour for LT_INIT
to enable static libraries.
This is the default on all systems where shared libraries have been disabled
for some reason, and on most systems where shared libraries have been enabled.
If shared libraries are enabled, the user may still override this default by
specifying ‘--disable-static’. The option ‘static’ to LT_INIT
is a shorthand for this.
AM_ENABLE_STATIC
is a deprecated alias for AC_ENABLE_STATIC
.
The tests in LT_INIT
also recognize the following
environment variables:
The C compiler that will be used by the generated libtool
. If
this is not set, LT_INIT
will look for gcc
or
cc
.
Compiler flags used to generate standard object files. If this is not
set, LT_INIT
will not use any such flags. It affects
only the way LT_INIT
runs tests, not the produced
libtool
.
C preprocessor flags. If this is not set, LT_INIT
will
not use any such flags. It affects only the way LT_INIT
runs tests, not the produced libtool
.
The system linker to use (if the generated libtool
requires one).
If this is not set, LT_INIT
will try to find out what is
the linker used by CC
.
The flags to be used by libtool
when it links a program. If
this is not set, LT_INIT
will not use any such flags. It
affects only the way LT_INIT
runs tests, not the produced
libtool
.
The libraries to be used by LT_INIT
when it links a
program. If this is not set, LT_INIT
will not use any
such flags. It affects only the way LT_INIT
runs tests,
not the produced libtool
.
Program to use rather than checking for nm
.
Program to use rather than checking for ranlib
.
A command that creates a link of a program, a soft-link if possible, a
hard-link otherwise. LT_INIT
will check for a suitable
program if this variable is not set.
Program to use rather than checking for dlltool
. Only meaningful
for Cygwin/MS-Windows.
Program to use rather than checking for objdump
. Only meaningful
for Cygwin/MS-Windows.
Program to use rather than checking for as
. Only used on
Cygwin/MS-Windows at the moment.
Program to use rather than checking for mt
, the Manifest Tool.
Only used on Cygwin/MS-Windows at the moment.
Libtool has heuristics for the system search path for runtime-loaded
libraries. If the guessed default does not match the setup of the host
system, this variable can be used to modify that path list, as follows
(LT_SYS_LIBRARY_PATH
is a colon-delimited list like PATH
):
path:
The heuristically determined paths will be appended after the trailing
colon;
:path
The heuristically determined paths will be prepended before the leading
colon;
path::path
The heuristically determined paths will be inserted between the double
colons;
path
With no dangling colons, the heuristically determined paths will be
ignored entirely.
With 1.3 era libtool, if you wanted to know any details of what
libtool had discovered about your architecture and environment, you
had to run the script with --config and grep through the
results. This idiom was supported up to and including 1.5.x era
libtool, where it was possible to call the generated libtool script
from configure.ac as soon as LT_INIT
had
completed. However, one of the features of libtool 1.4 was that the
libtool configuration was migrated out of a separate ltconfig
file, and added to the LT_INIT
macro (nee AC_PROG_LIBTOOL
),
so the results of the configuration tests were available directly to code in
configure.ac, rendering the call out to the generated libtool
script obsolete.
Starting with libtool 2.0, the multipass generation of the libtool script has been consolidated into a single config.status pass, which happens after all the code in configure.ac has completed. The implication of this is that the libtool script does not exist during execution of code from configure.ac, and so obviously it cannot be called for --config details anymore. If you are upgrading projects that used this idiom to libtool 2.0 or newer, you should replace those calls with direct references to the equivalent Autoconf shell variables that are set by the configure time tests before being passed to config.status for inclusion in the generated libtool script.
By default, the configured libtool script is generated by the
call to AC_OUTPUT
command, and there is rarely any need to use
libtool from configure. However, sometimes it is
necessary to run configure time compile and link tests using
libtool. You can add LT_OUTPUT
to your
configure.ac any time after LT_INIT
and any
LT_LANG
calls; that done, libtool will be created by a
specially generated config.lt file, and available for use in
later tests.
Also, when LT_OUTPUT
is used, for backwards compatibility with
Automake regeneration rules, config.status will call
config.lt to regenerate libtool, rather than generating
the file itself.
When you invoke the libtoolize
program (see Invoking libtoolize
), it will tell you where to find a definition of
LT_INIT
. If you use Automake, the aclocal
program
will automatically add LT_INIT
support to your
configure script when it sees the invocation of LT_INIT
in configure.ac.
Because of these changes, and the runtime version compatibility checks
Libtool now executes, we now advise against including a copy of
libtool.m4 (and brethren) in acinclude.m4. Instead,
you should set your project macro directory with
AC_CONFIG_MACRO_DIRS
. When you libtoolize
your
project, a copy of the relevant macro definitions will be placed in
your AC_CONFIG_MACRO_DIRS
, where aclocal
can reference
them directly from aclocal.m4.
While Libtool tries to hide as many platform-specific features as possible, some have to be taken into account when configuring either the Libtool package or a libtoolized package.
LDFLAGS=-Wl,-brtl
for the latter style.
AR=/usr/bin/ar LD=/usr/bin/ld NM='/usr/bin/nm -B'
.
/bin/sh
is very slow due to its inefficient handling
of here-documents. A modern shell is preferable:
CONFIG_SHELL=/bin/bash; export $CONFIG_SHELL $CONFIG_SHELL ./configure [...]
CXX='pgCC --one_instantiation_per_object'
and avoid parallel make
.
-fsanitize=address
you need to specify the -lasan
library,
like so: g++ -o libx.la -fsanitize=address -lasan -rpath [...]
.
MACOSX_DEPLOYMENT_TARGET
is set to
10.3 or later at configure
time. See rdar://problem/4135857
for more information on this issue.
sed
programs are horribly broken, and cannot handle
libtool’s requirements, so users may report unusual problems. There
is no workaround except to install a working sed
(such as GNU sed)
on these systems.
cc
programs emits copyright
on standard error that confuse tests on size of conftest.err. The
workaround is to specify CC
when run configure with
CC='cc -Hnocopyr'
.
gcc
provided by Marco Walther.
libtool
sometimes guesses the wrong paths that the linker and dynamic linker search by
default. If this occurs for the dynamic library path, you may use the
LT_SYS_LIBRARY_PATH
environment variable to adjust. Otherwise, at
configure
time you may override libtool’s guesses by setting the
autoconf
cache variables lt_cv_sys_lib_search_path_spec
and
lt_cv_sys_lib_dlsearch_path_spec
respectively.
In order to use libtool, you need to include the following files with your package:
Attempt to guess a canonical system name.
Canonical system name validation subroutine script.
BSD-compatible install
replacement script.
A generic script implementing basic libtool functionality.
Note that the libtool script itself should not be included with your package. See Configuring libtool.
You should use the libtoolize
program, rather than manually
copying these files into your package.
libtoolize
¶The libtoolize
program provides a standard way to add libtool
support to your package. In the future, it may implement better usage
checking, or other features to make libtool even easier to use.
The libtoolize
program has the following synopsis:
libtoolize [option]...
and accepts the following options:
Copy files from the libtool data directory rather than creating symlinks.
Dump a trace of shell script execution to standard output. This
produces a lot of output, so you may wish to pipe it to less
(or
more
) or redirect to a file.
Don’t run any commands that modify the file system, just print them out.
Replace existing libtool files. By default, libtoolize
won’t
overwrite existing files.
Display a help message and exit.
Install libltdl in the target-directory-name subdirectory of
your package. Normally, the directory is extracted from the argument
to LT_CONFIG_LTDL_DIR
in configure.ac, though you can
also specify a subdirectory name here if you are not using Autoconf
for example. If libtoolize
can’t determine the target
directory, ‘libltdl’ is used as the default.
Normally, Libtoolize tries to diagnose use of deprecated libtool macros and other stylistic issues. If you are deliberately using outdated calling conventions, this option prevents Libtoolize from explaining how to update your project’s Libtool conventions.
If passed in conjunction with --ltdl, this option will cause
the libltdl
installed by ‘libtoolize’ to be set up for
use with a non-recursive automake
build. To make use of it,
you will need to add the following to the Makefile.am of the
parent project:
## libltdl/ltdl.mk appends to the following variables ## so we set them here before including it: BUILT_SOURCES = AM_CPPFLAGS = AM_LDFLAGS = include_HEADERS = noinst_LTLIBRARIES = lib_LTLIBRARIES = EXTRA_LTLIBRARIES = EXTRA_DIST = CLEANFILES = MOSTLYCLEANFILES = include libltdl/ltdl.mk
Work silently. ‘libtoolize --quiet’ is used by GNU Automake to add libtool files to your package if necessary.
If passed in conjunction with --ltdl, this option will cause
the libtoolize
installed ‘libltdl’ to be set up for use
with a recursive automake
build. To make use of it, you
will need to adjust the parent project’s configure.ac:
AC_CONFIG_FILES([libltdl/Makefile])
and Makefile.am:
SUBDIRS += libltdl
If passed in conjunction with --ltdl, this option will cause
the libtoolize
installed ‘libltdl’ to be set up for
independent configuration and compilation as a self-contained
subproject. To make use of it, you should arrange for your build to
call libltdl/configure
, and then run make
in the
libltdl directory (or the subdirectory you put libltdl into).
If your project uses Autoconf, you can use the supplied
‘LT_WITH_LTDL’ macro, or else call ‘AC_CONFIG_SUBDIRS’
directly.
Previous releases of ‘libltdl’ built exclusively in this mode, but now it is the default mode both for backwards compatibility and because, for example, it is suitable for use in projects that wish to use ‘libltdl’, but not use the Autotools for their own build process.
Work noisily! Give a blow by blow account of what
libtoolize
is doing.
Print libtoolize
version information and exit.
Sometimes it can be useful to pass options to libtoolize
even
though it is called by another program, such as autoreconf
. A
limited number of options are parsed from the environment variable
LIBTOOLIZE_OPTIONS
: currently --debug, --no-warn,
--quiet and --verbose. Multiple options passed in
LIBTOOLIZE_OPTIONS
must be separated with a space, comma or a
colon.
By default, a warning is issued for unknown options found in
LIBTOOLIZE_OPTIONS
unless the first such option is
--no-warn. Where libtoolize
has always quit
on receipt of an unknown option at the command line, this and all
previous releases of libtoolize
will continue unabated whatever
the content of LIBTOOLIZE_OPTIONS
(modulo some possible warning
messages).
trick$ LIBTOOLIZE_OPTIONS=--no-warn,--quiet autoreconf --install
If libtoolize
detects an explicit call to
AC_CONFIG_MACRO_DIRS
(see The Autoconf Manual in The Autoconf Manual) in your configure.ac, it will
put the Libtool macros in the specified directory.
In the future other Autotools will automatically check the contents of
AC_CONFIG_MACRO_DIRS
, but at the moment it is more portable to
add the macro directory to ACLOCAL_AMFLAGS
in
Makefile.am, which is where the tools currently look. If
libtoolize
doesn’t see AC_CONFIG_MACRO_DIRS
, it too
will honour the first ‘-I’ argument in ACLOCAL_AMFLAGS
when choosing a directory to store libtool configuration macros in.
It is perfectly sensible to use both AC_CONFIG_MACRO_DIRS
and
ACLOCAL_AMFLAGS
, as long as they are kept in synchronisation.
ACLOCAL_AMFLAGS = -I m4
When you bootstrap your project with aclocal
, then you will
need to explicitly pass the same macro directory with
aclocal
’s ‘-I’ flag:
trick$ aclocal -I m4
If libtoolize
detects an explicit call to
AC_CONFIG_AUX_DIR
(see The Autoconf Manual in The Autoconf Manual) in your configure.ac, it
will put the other support files in the specified directory.
Otherwise they too end up in the project root directory.
Unless --no-warn is passed, libtoolize
displays
hints for adding libtool support to your package, as well.
LTLIBOBJS
¶People used to add code like the following to their configure.ac:
LTLIBOBJS=`echo "$LIBOBJS" | sed 's/\.[^.]* /.lo /g;s/\.[^.]*$/.lo/'` AC_SUBST([LTLIBOBJS])
This is no longer required (since Autoconf 2.54), and doesn’t take Automake’s deansification support into account either, so doesn’t work correctly even with ancient Autoconfs!
Provided you are using a recent (2.54 or better) incarnation of
Autoconf, the call to AC_OUTPUT
takes care of setting
LTLIBOBJS
up correctly, so you can simply delete such snippets
from your configure.ac if you had them.
When you are developing a package, it is often worthwhile to configure
your package with the --disable-shared flag, or to override the
defaults for LT_INIT
by using the disable-shared
option
(see The LT_INIT
macro). This prevents libtool
from building shared libraries, which has several advantages:
You may want to put a small note in your package README to let other developers know that --disable-shared can save them time. The following example note is taken from the GIMP7 distribution README:
The GIMP uses GNU Libtool to build shared libraries on a variety of systems. While this is very nice for making usable binaries, it can be a pain when trying to debug a program. For that reason, compilation of shared libraries can be turned off by specifying the --disable-shared option to configure.
Libtool was first implemented to add support for writing shared libraries in the C language. However, over time, libtool is being integrated with other languages, so that programmers are free to reap the benefits of shared libraries in their favorite programming language.
This chapter describes how libtool interacts with other languages, and what special considerations you need to make if you do not use C.
++
¶Creating libraries of C++ code should be a fairly straightforward process, because its object files differ from C ones in only three ways:
ld
directly to link such libraries, and
we should use the C++ compiler instead.
ld
to link a C++ program or library is deemed
to fail.
Because of these three issues, Libtool has been designed to always use
the C++ compiler to compile and link C++ programs and libraries. In
some instances the main()
function of a program must also be
compiled with the C++ compiler for static C++ objects to be properly
initialized.
Libtool supports multiple languages through the use of tags. Technically
a tag corresponds to a set of configuration variables associated with a
language. These variables tell libtool
how it should create
objects and libraries for each language.
Tags are defined at configure
-time for each language activated
in the package (see LT_LANG
in The LT_INIT
macro). Here is the
correspondence between language names and tags names.
Language name | Tag name |
C | CC |
C++ | CXX |
Java | GCJ |
Fortran 77 | F77 |
Fortran | FC |
Go | GO |
Windows Resource | RC |
libtool
tries to automatically infer what tag to use from
the compiler command being used to compile or link. If it can’t infer
a tag, then it defaults to the configuration for the C
language.
The tag can also be specified using libtool
’s
--tag=tag option (see Invoking libtool
). It is a good
idea to do so in Makefile rules, because that will allow users to
substitute the compiler without relying on libtool
inference
heuristics. When no tag is specified, libtool
will default
to CC
; this tag always exists.
Finally, the set of tags available in a particular project can be
retrieved by tracing for the LT_SUPPORTED_TAG
macro (see Libtool’s trace interface).
The most difficult issue introduced by shared libraries is that of
creating and resolving runtime dependencies. Dependencies on programs
and libraries are often described in terms of a single name, such as
sed
. So, one may say “libtool depends on sed,” and that is
good enough for most purposes.
However, when an interface changes regularly, we need to be more specific: “Gnus 5.1 requires Emacs 19.28 or above.” Here, the description of an interface consists of a name, and a “version number.”
Even that sort of description is not accurate enough for some purposes. What if Emacs 20 changes enough to break Gnus 5.1?
The same problem exists in shared libraries: we require a formal version system to describe the sorts of dependencies that programs have on shared libraries, so that the dynamic linker can guarantee that programs are linked only against libraries that provide the interface they require.
Interfaces for libraries may be any of the following (and more):
Note that static functions do not count as interfaces, because they are not directly available to the user of the library.
Libtool has its own formal versioning system. It is not as flexible as some, but it is definitely the simplest of the more powerful versioning systems.
Think of a library as exporting several sets of interfaces, arbitrarily represented by integers. When a program is linked against a library, it may use any subset of those interfaces.
Libtool’s description of the interfaces that a program uses is simple: it encodes the least and the greatest interface numbers in the resulting binary (first-interface, last-interface).
The dynamic linker is guaranteed that if a library supports every interface number between first-interface and last-interface, then the program can be relinked against that library.
Note that this can cause problems because libtool’s compatibility requirements are actually stricter than is necessary.
Say libhello supports interfaces 5, 16, 17, 18, and 19, and that libtool is used to link test against libhello.
Libtool encodes the numbers 5 and 19 in test, and the dynamic linker will only link test against libraries that support every interface between 5 and 19. So, the dynamic linker refuses to link test against libhello!
In order to eliminate this problem, libtool only allows libraries to declare consecutive interface numbers. So, libhello can declare at most that it supports interfaces 16 through 19. Then, the dynamic linker will link test against libhello.
So, libtool library versions are described by three integers:
The most recent interface number that this library implements.
The implementation number of the current interface.
The difference between the newest and oldest interfaces that this
library implements. In other words, the library implements all the
interface numbers in the range from number current -
age
to current
.
If two libraries have identical current and age numbers, then the dynamic linker chooses the library with the greater revision number.
If you want to use libtool’s versioning system, then you must specify the version information to libtool using the -version-info flag during link mode (see Link mode).
This flag accepts an argument of the form ‘current[:revision[:age]]’. So, passing -version-info 3:12:1 sets current to 3, revision to 12, and age to 1.
If either revision or age are omitted, they default to 0. Also note that age must be less than or equal to the current interface number.
Here are a set of rules to help you update your library version information:
Never try to set the interface numbers so that they correspond to the release number of your package. This is an abuse that only fosters misunderstanding of the purpose of library versions. Instead, use the -release flag (see Managing release information), but be warned that every release of your package will not be binary compatible with any other release.
The following explanation may help to understand the above rules a bit better: consider that there are three possible kinds of reactions from users of your library to changes in a shared library:
In the above description, programs using the library in question may also be replaced by other libraries using it.
Often, people want to encode the name of the package release into the shared library so that it is obvious to the user what package their programs are linked against. This convention is used especially on GNU/Linux:
trick$ ls /usr/lib/libbfd* /usr/lib/libbfd.a /usr/lib/libbfd.so.2.7.0.2 /usr/lib/libbfd.so trick$
On ‘trick’, /usr/lib/libbfd.so is a symbolic link to libbfd.so.2.7.0.2, which was distributed as a part of ‘binutils-2.7.0.2’.
Unfortunately, this convention conflicts directly with libtool’s idea of library interface versions, because the library interface rarely changes at the same time that the release number does, and the library suffix is never the same across all platforms.
So, to accommodate both views, you can use the -release flag to set release information for libraries for which you do not want to use -version-info. For the libbfd example, the next release that uses libtool should be built with ‘-release 2.9.0’, which will produce the following files on GNU/Linux:
trick$ ls /usr/lib/libbfd* /usr/lib/libbfd-2.9.0.so /usr/lib/libbfd.a /usr/lib/libbfd.so trick$
In this case, /usr/lib/libbfd.so is a symbolic link to libbfd-2.9.0.so. This makes it obvious that the user is dealing with ‘binutils-2.9.0’, without compromising libtool’s idea of interface versions.
Note that this option causes a modification of the library name, so do not use it unless you want to break binary compatibility with any past library releases. In general, you should only use -release for package-internal libraries or for ones whose interfaces change very frequently.
Writing a good library interface takes a lot of practice and thorough understanding of the problem that the library is intended to solve.
If you design a good interface, it won’t have to change often, you won’t have to keep updating documentation, and users won’t have to keep relearning how to use the library.
Here is a brief list of tips for library interface design that may help you in your exploits:
Try to make every interface truly minimal, so that you won’t need to delete entry points very often.
Some people love redesigning and changing entry points just for the heck of it (note: renaming a function is considered changing an entry point). Don’t be one of those people. If you must redesign an interface, then try to leave compatibility functions behind so that users don’t need to rewrite their existing code.
The fewer data type definitions a library user has access to, the better. If possible, design your functions to accept a generic pointer (that you can cast to an internal data type), and provide access functions rather than allowing the library user to directly manipulate the data. That way, you have the freedom to change the data structures without changing the interface.
This is essentially the same thing as using abstract data types and inheritance in an object-oriented system.
If you are careful to document each of your library’s global functions and variables in header files, and include them in your library source files, then the compiler will let you know if you make any interface changes by accident (see Writing C header files).
static
keyword (or equivalent) whenever possible ¶The fewer global functions your library has, the more flexibility you’ll have in changing them. Static functions and variables may change forms as often as you like… your users cannot access them, so they aren’t interface changes.
The number of elements in a global array is part of an interface, even
if the header just declares extern int foo[];
. This is because
on i386 and some other SVR4/ELF systems, when an application
references data in a shared library the size of that data (whatever
its type) is included in the application executable. If you might
want to change the size of an array or string then provide a pointer
not the actual array.
Writing portable C header files can be difficult, since they may be read by different types of compilers:
C++ compilers require that functions be declared with full prototypes,
since C++ is more strongly typed than C. C functions and variables also
need to be declared with the extern "C"
directive, so that the
names aren’t mangled. See Writing libraries for C++
, for other issues relevant
to using C++ with libtool.
ANSI C compilers are not as strict as C++ compilers, but functions
should be prototyped to avoid unnecessary warnings when the header file
is #include
d.
Non-ANSI compilers will report errors if functions are prototyped.
These complications mean that your library interface headers must use some C preprocessor magic to be usable by each of the above compilers.
foo.h in the tests/demo subdirectory of the libtool distribution serves as an example for how to write a header file that can be safely installed in a system directory.
Here are the relevant portions of that file:
/* BEGIN_C_DECLS should be used at the beginning of your declarations, so that C++ compilers don't mangle their names. Use END_C_DECLS at the end of C declarations. */ #undef BEGIN_C_DECLS #undef END_C_DECLS #ifdef __cplusplus # define BEGIN_C_DECLS extern "C" { # define END_C_DECLS } #else # define BEGIN_C_DECLS /* empty */ # define END_C_DECLS /* empty */ #endif /* PARAMS is a macro used to wrap function prototypes, so that compilers that don't understand ANSI C prototypes still work, and ANSI C compilers can issue warnings about type mismatches. */ #undef PARAMS #if defined __STDC__ || defined _AIX \ || (defined __mips && defined _SYSTYPE_SVR4) \ || defined WIN32 || defined __cplusplus # define PARAMS(protos) protos #else # define PARAMS(protos) () #endif
These macros are used in foo.h as follows:
#ifndef FOO_H #define FOO_H 1 /* The above macro definitions. */ #include "..." BEGIN_C_DECLS int foo PARAMS((void)); int hello PARAMS((void)); END_C_DECLS #endif /* !FOO_H */
Note that the #ifndef FOO_H prevents the body of foo.h from being read more than once in a given compilation.
Also the only thing that must go outside the
BEGIN_C_DECLS
/END_C_DECLS
pair are #include
lines.
Strictly speaking it is only C symbol names that need to be protected,
but your header files will be more maintainable if you have a single
pair of these macros around the majority of the header contents.
You should use these definitions of PARAMS
, BEGIN_C_DECLS
,
and END_C_DECLS
into your own headers. Then, you may use them to
create header files that are valid for C++, ANSI, and non-ANSI
compilers8.
Do not be naive about writing portable code. Following the tips given above will help you miss the most obvious problems, but there are definitely other subtle portability issues. You may need to cope with some of the following issues:
void *
generic
pointer type, and so need to use char *
in its place.
const
, inline
and signed
keywords are not
supported by some compilers, especially pre-ANSI compilers.
long double
type is not supported by many compilers.
By definition, every shared library system provides a way for executables to depend on libraries, so that symbol resolution is deferred until runtime.
An inter-library dependency is where a library depends on
other libraries. For example, if the libtool library libhello
uses the cos
function, then it has an inter-library dependency
on libm, the math library that implements cos
.
Some shared library systems provide this feature in an internally-consistent way: these systems allow chains of dependencies of potentially infinite length.
However, most shared library systems are restricted in that they only allow a single level of dependencies. In these systems, programs may depend on shared libraries, but shared libraries may not depend on other shared libraries.
In any event, libtool provides a simple mechanism for you to declare
inter-library dependencies: for every library libname that
your own library depends on, simply add a corresponding
-lname
option to the link line when you create your
library. To make an example of our libhello that depends on
libm:
burger$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \ -rpath /usr/local/lib -lm burger$
When you link a program against libhello, you don’t need to specify the same ‘-l’ options again: libtool will do that for you, to guarantee that all the required libraries are found. This restriction is only necessary to preserve compatibility with static library systems and simple dynamic library systems.
Some platforms, such as Windows, do not even allow you this flexibility. In order to build a shared library, it must be entirely self-contained or it must have dependencies known at link time (that is, have references only to symbols that are found in the .lo files or the specified ‘-l’ libraries), and you need to specify the -no-undefined flag. By default, libtool builds only static libraries on these kinds of platforms.
The simple-minded inter-library dependency tracking code of libtool releases prior to 1.2 was disabled because it was not clear when it was possible to link one library with another, and complex failures would occur. A more complex implementation of this concept was re-introduced before release 1.3, but it has not been ported to all platforms that libtool supports. The default, conservative behavior is to avoid linking one library with another, introducing their inter-dependencies only when a program is linked with them.
It can sometimes be confusing to discuss dynamic linking, because the term is used to refer to two different concepts:
dlopen
that load
arbitrary, user-specified modules at runtime. This type of dynamic
linking is explicitly controlled by the application.
To mitigate confusion, this manual refers to the second type of dynamic linking as dlopening a module.
The main benefit to dlopening object modules is the ability to access compiled object code to extend your program, rather than using an interpreted language. In fact, dlopen calls are frequently used in language interpreters to provide an efficient way to extend the language.
Libtool provides support for dlopened modules. However, you should
indicate that your package is willing to use such support, by using the
LT_INIT
option ‘dlopen’ in configure.ac. If this
option is not given, libtool will assume no dlopening mechanism is
available, and will try to simulate it.
This chapter discusses how you as a dlopen application developer might use libtool to generate dlopen-accessible modules.
On some operating systems, a program symbol must be specially declared
in order to be dynamically resolved with the dlsym
(or
equivalent) function. Libtool provides the -export-dynamic and
-module link flags (see Link mode), for you to make that
declaration. You need to use these flags if you are linking an
application program that dlopens other modules or a libtool library
that will also be dlopened.
For example, if we wanted to build a shared library, hello, that would later be dlopened by an application, we would add -module to the other link flags:
burger$ libtool --mode=link gcc -module -o hello.la foo.lo \ hello.lo -rpath /usr/local/lib -lm burger$
If symbols from your executable are needed to satisfy unresolved references in a library you want to dlopen you will have to use the flag -export-dynamic. You should use -export-dynamic while linking the executable that calls dlopen:
burger$ libtool --mode=link gcc -export-dynamic -o helldl main.o burger$
Libtool provides special support for dlopening libtool object and
libtool library files, so that their symbols can be resolved
even on platforms without any dlopen
and dlsym
functions.
Consider the following alternative ways of loading code into your program, in order of increasing “laziness”:
Libtool emulates -dlopen on static platforms by linking objects into the program at compile time, and creating data structures that represent the program’s symbol table. In order to use this feature, you must declare the objects you want your application to dlopen by using the -dlopen or -dlpreopen flags when you link your program (see Link mode).
The name attribute is a null-terminated character string of the
symbol name, such as "fprintf"
. The address attribute is a
generic pointer to the appropriate object, such as &fprintf
.
const lt_dlsymlist
lt_preloaded_symbols[] ¶An array of lt_dlsymlist
structures, representing all the preloaded
symbols linked into the program proper. For each module
-dlpreopened by the Libtool linked program
there is an element with the name of the module and an address
of 0
, followed by all symbols exported from this file.
For the executable itself the special name ‘@PROGRAM@’ is used.
The last element of all has a name and address of
0
.
To facilitate inclusion of symbol lists into libraries,
lt_preloaded_symbols
is ‘#define’d to a suitably unique name
in ltdl.h.
This variable may not be declared const
on some systems due to
relocation issues.
Some compilers may allow identifiers that are not valid in ANSI C, such
as dollar signs. Libtool only recognizes valid ANSI C symbols (an
initial ASCII letter or underscore, followed by zero or more ASCII
letters, digits, and underscores), so non-ANSI symbols will not appear
in lt_preloaded_symbols
.
int
lt_dlpreload (const lt_dlsymlist *preloaded)
¶Register the list of preloaded modules preloaded.
If preloaded is NULL
, then all previously registered
symbol lists, except the list set by lt_dlpreload_default
,
are deleted. Return 0 on success.
int
lt_dlpreload_default (const lt_dlsymlist *preloaded)
¶Set the default list of preloaded modules to preloaded, which
won’t be deleted by lt_dlpreload
. Note that this function does
not require libltdl to be initialized using lt_dlinit
and
can be used in the program to register the default preloaded modules.
Instead of calling this function directly, most programs will use the
macro LTDL_SET_PRELOADED_SYMBOLS
.
Return 0 on success.
Set the default list of preloaded symbols. Should be used in your program to initialize libltdl’s list of preloaded modules.
#include <ltdl.h> int main() { /* ... */ LTDL_SET_PRELOADED_SYMBOLS(); /* ... */ }
int
lt_dlpreload_callback_func (lt_dlhandle handle)
¶Functions of this type can be passed to lt_dlpreload_open
,
which in turn will call back into a function thus passed for each
preloaded module that it opens.
int
lt_dlpreload_open (const char *originator, lt_dlpreload_callback_func *func)
¶Load all of the preloaded modules for originator. For every module opened in this way, call func.
To open all of the modules preloaded into libhell.la (presumably from within the libhell.a initialisation code):
#define preloaded_symbols lt_libhell_LTX_preloaded_symbols static int hell_preload_callback (lt_dlhandle handle); int hell_init (void) { ... if (lt_dlpreload (&preloaded_symbols) == 0) { lt_dlpreload_open ("libhell", preload_callback); } ... }
Note that to prevent clashes between multiple preloaded modules, the preloaded symbols are accessed via a mangled symbol name: to get the symbols preloaded into ‘libhell’, you must prefix ‘preloaded_symbols’ with ‘lt_’; the originator name, ‘libhell’ in this case; and ‘_LTX_’. That is, ‘lt_libhell_LTX_preloaded_symbols’ here.
When, say, an interpreter application uses dlopened modules to extend the list of methods it provides, an obvious abstraction for the maintainers of the interpreter is to have all methods (including the built in ones supplied with the interpreter) accessed through dlopen. For one thing, the dlopening functionality will be tested even during routine invocations. For another, only one subsystem has to be written for getting methods into the interpreter.
The downside of this abstraction is, of course, that environments that provide only static linkage can’t even load the intrinsic interpreter methods. Not so! We can statically link those methods by dlpreopening them.
Unfortunately, since platforms such as AIX and cygwin require that all library symbols must be resolved at compile time, the interpreter maintainers will need to provide a library to both its own dlpreopened modules, and third-party modules loaded by dlopen. In itself, that is not so bad, except that the interpreter too must provide those same symbols otherwise it will be impossible to resolve all the symbols required by the modules as they are loaded. Things are even worse if the code that loads the modules for the interpreter is itself in a library – and that is usually the case for any non-trivial application. Modern platforms take care of this by automatically loading all of a module’s dependency libraries as the module is loaded (libltdl can do this even on platforms that can’t do it by themselves). In the end, this leads to problems with duplicated symbols and prevents modules from loading, and prevents the application from compiling when modules are preloaded.
,-------------. ,------------------. ,-----------------. | Interpreter |----> Module------------> Third-party | `-------------' | Loader | |Dlopened Modules | | | | `-----------------' |,-------v--------.| | || Dlpreopened || | || Modules || | |`----------------'| | | | | | |,-------v--------.| ,--------v--------. ||Module Interface|| |Module Interface | || Library || | Library | |`----------------'| `-----------------' `------------------'
Libtool has the concept of weak library interfaces to circumvent
this problem. Recall that the code that dlopens method-provider
modules for the interpreter application resides in a library: All of
the modules and the dlopener library itself should be linked against
the common library that resolves the module symbols at compile time.
To guard against duplicate symbol definitions, and for dlpreopened
modules to work at all in this scenario, the dlopener library must
declare that it provides a weak library interface to the common
symbols in the library it shares with the modules. That way, when
libtool
links the Module Loader library with some
Dlpreopened Modules that were in turn linked against the
Module Interface Library, it knows that the Module
Loader provides an already loaded Module Interface Library
to resolve symbols for the Dlpreopened Modules, and doesn’t
ask the compiler driver to link an identical Module Interface
Library dependency library too.
In conjunction with Automake, the Makefile.am for the Module Loader might look like this:
lib_LTLIBRARIES = libinterface.la libloader.la libinterface_la_SOURCES = interface.c interface.h libinterface_la_LDFLAGS = -version-info 3:2:1 libloader_la_SOURCES = loader.c libloader_la_LDFLAGS = -weak libinterface.la \ -version-info 3:2:1 \ -dlpreopen ../modules/intrinsics.la libloader_la_LIBADD = $(libinterface_la_OBJECTS)
And the Makefile.am for the intrinsics.la module in a sibling modules directory might look like this:
AM_CPPFLAGS = -I$(srcdir)/../libloader AM_LDFLAGS = -no-undefined -module -avoid-version \ -export-dynamic noinst_LTLIBRARIES = intrinsics.la intrinsics_la_LIBADD = ../libloader/libinterface.la ../libloader/libinterface.la: cd ../libloader && $(MAKE) $(AM_MAKEFLAGS) libinterface.la
For a more complex example, see the sources of libltdl in the Libtool distribution, which is built with the help of the -weak option.
After a library has been linked with -module, it can be dlopened. Unfortunately, because of the variation in library names, your package needs to determine the correct file to dlopen.
The most straightforward and flexible implementation is to determine the name at runtime, by finding the installed .la file, and searching it for the following lines:
# The name that we can dlopen
.
dlname='dlname'
If dlname is empty, then the library cannot be dlopened. Otherwise, it gives the dlname of the library. So, if the library was installed as /usr/local/lib/libhello.la, and the dlname was libhello.so.3, then /usr/local/lib/libhello.so.3 should be dlopened.
If your program uses this approach, then it should search the
directories listed in the LD_LIBRARY_PATH
9 environment variable, as well as
the directory where libraries will eventually be installed. Searching
this variable (or equivalent) will guarantee that your program can find
its dlopened modules, even before installation, provided you have linked
them using libtool.
The following problems are not solved by using libtool’s dlopen support:
dlopen
family, which do package-specific tricks when dlopening
is unsupported or not available on a given platform.
dlopen
family of functions. Some platforms do not even use the same function
names (notably HP-UX, with its shl_load
family).
dlopen
.
Libtool provides a small library, called libltdl, that aims at hiding the various difficulties of dlopening libraries from programmers. It consists of a few headers and small C source files that can be distributed with applications that need dlopening functionality. On some platforms, whose dynamic linkers are too limited for a simple implementation of libltdl services, it requires GNU DLD, or it will only emulate dynamic linking with libtool’s dlpreopening mechanism.
libltdl supports currently the following dynamic linking mechanisms:
dlopen
(POSIX compliant systems, GNU/Linux, etc.)
shl_load
(HP-UX)
LoadLibrary
(Win16 and Win32)
load_add_on
(BeOS)
NSAddImage
or NSLinkModule
(Darwin and Mac OS X)
libltdl is licensed under the terms of the GNU Lesser General Public License, with the following exception:
As a special exception to the GNU Lesser General Public License, if you distribute this file as part of a program or library that is built using GNU Libtool, you may include it under the same distribution terms that you use for the rest of that program.
dlopen
edThe libltdl API is similar to the POSIX dlopen interface, which is very simple but powerful.
To use libltdl in your program you have to include the header file ltdl.h:
#include <ltdl.h>
The early releases of libltdl used some symbols that violated the POSIX namespace conventions. These symbols are now deprecated, and have been replaced by those described here. If you have code that relies on the old deprecated symbol names, defining ‘LT_NON_POSIX_NAMESPACE’ before you include ltdl.h provides conversion macros. Whichever set of symbols you use, the new API is not binary compatible with the last, so you will need to recompile your application to use this version of libltdl.
Note that libltdl is not well tested in a multithreaded environment, though the intention is that it should work (see Using libltdl in a multi threaded environment). If there are any issues, working around them is left as an exercise for the reader; contributions are certainly welcome.
The following macros are defined by including ltdl.h:
LT_PATHSEP_CHAR
is the system-dependent path separator,
that is, ‘;’ on Windows and ‘:’ everywhere else.
If LT_DIRSEP_CHAR
is defined, it can be used as directory
separator in addition to ‘/’. On Windows, this contains
‘\’.
The following types are defined in ltdl.h:
lt_dlhandle
is a module “handle”.
Every lt_dlopened module has a handle associated with it.
lt_dladvise
is used to control optional module loading modes.
If it is not used, the default mode of the underlying system module
loader is used.
lt_dlsymlist
is a symbol list for dlpreopened modules
(see Dlpreopening).
libltdl provides the following functions:
int
lt_dlinit (void)
¶Initialize libltdl. This function must be called before using libltdl and may be called several times. Return 0 on success, otherwise the number of errors.
int
lt_dlexit (void)
¶Shut down libltdl and close all modules.
This function will only then shut down libltdl when it was called as
many times as lt_dlinit
has been successfully called.
Return 0 on success, otherwise the number of errors.
lt_dlhandle
lt_dlopen (const char *filename)
¶Open the module with the file name filename and return a
handle for it. lt_dlopen
is able to open libtool dynamic
modules, preloaded static modules, the program itself and
native dynamic modules10.
Unresolved symbols in the module are resolved using its dependency libraries and previously dlopened modules. If the executable using this module was linked with the -export-dynamic flag, then the global symbols in the executable will also be used to resolve references in the module.
If filename is NULL
and the program was linked with
-export-dynamic or -dlopen self, lt_dlopen
will
return a handle for the program itself, which can be used to access its
symbols.
If libltdl cannot find the library and the file name filename does not have a directory component it will additionally look in the following search paths for the module (in the following order):
lt_dlsetsearchpath
, lt_dladdsearchdir
and
lt_dlinsertsearchdir
.
LTDL_LIBRARY_PATH
.
LD_LIBRARY_PATH
).
Each search path must be a list of absolute directories separated by
LT_PATHSEP_CHAR
, for example, "/usr/lib/mypkg:/lib/foo"
.
The directory names may not contain the path separator.
If the same module is loaded several times, the same handle is returned.
If lt_dlopen
fails for any reason, it returns NULL
.
lt_dlhandle
lt_dlopenext (const char *filename)
¶The same as lt_dlopen
, except that it tries to append
different file name extensions to the file name.
If the file with the file name filename cannot be found
libltdl tries to append the following extensions:
This lookup strategy was designed to allow programs that don’t
have knowledge about native dynamic libraries naming conventions
to be able to dlopen
such libraries as well as libtool modules
transparently.
lt_dlhandle
lt_dlopenadvise (const char *filename, lt_dladvise advise)
¶The same as lt_dlopen
, except that it also requires an additional
argument that may contain additional hints to the underlying system
module loader. The advise parameter is opaque and can only be
accessed with the functions documented below.
Note that this function does not change the content of advise, so
unlike the other calls in this API takes a direct lt_dladvise
type, and not a pointer to the same.
int
lt_dladvise_init (lt_dladvise *advise)
¶The advise parameter can be used to pass hints to the module
loader when using lt_dlopenadvise
to perform the loading.
The advise parameter needs to be initialised by this function
before it can be used. Any memory used by advise needs to be
recycled with lt_dladvise_destroy
when it is no longer needed.
On failure, lt_dladvise_init
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_destroy (lt_dladvise *advise)
¶Recycle the memory used by advise. For an example, see the
documentation for lt_dladvise_ext
.
On failure, lt_dladvise_destroy
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_ext (lt_dladvise *advise)
¶Set the ext
hint on advise. Passing an advise
parameter to lt_dlopenadvise
with this hint set causes it to
try to append different file name extensions like lt_dlopenext
.
The following example is equivalent to calling
lt_dlopenext (filename)
:
lt_dlhandle my_dlopenext (const char *filename) { lt_dlhandle handle = 0; lt_dladvise advise; if (!lt_dladvise_init (&advise) && !lt_dladvise_ext (&advise)) handle = lt_dlopenadvise (filename, advise); lt_dladvise_destroy (&advise); return handle; }
On failure, lt_dladvise_ext
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_global (lt_dladvise *advise)
¶Set the symglobal
hint on advise. Passing an advise
parameter to lt_dlopenadvise
with this hint set causes it to try
to make the loaded module’s symbols globally available for resolving
unresolved symbols in subsequently loaded modules.
If neither the symglobal
nor the symlocal
hints are set,
or if a module is loaded without using the lt_dlopenadvise
call
in any case, then the visibility of the module’s symbols will be as per
the default for the underlying module loader and OS. Even if a
suitable hint is passed, not all loaders are able to act upon it in
which case lt_dlgetinfo
will reveal whether the hint was actually
followed.
On failure, lt_dladvise_global
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_local (lt_dladvise *advise)
¶Set the symlocal
hint on advise. Passing an advise
parameter to lt_dlopenadvise
with this hint set causes it to try
to keep the loaded module’s symbols hidden so that they are not
visible to subsequently loaded modules.
If neither the symglobal
nor the symlocal
hints are set,
or if a module is loaded without using the lt_dlopenadvise
call
in any case, then the visibility of the module’s symbols will be as per
the default for the underlying module loader and OS. Even if a
suitable hint is passed, not all loaders are able to act upon it in
which case lt_dlgetinfo
will reveal whether the hint was actually
followed.
On failure, lt_dladvise_local
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_resident (lt_dladvise *advise)
¶Set the resident
hint on advise. Passing an advise
parameter to lt_dlopenadvise
with this hint set causes it to try
to make the loaded module resident in memory, so that it cannot be
unloaded with a later call to lt_dlclose
.
On failure, lt_dladvise_resident
returns non-zero and sets an error
message that can be retrieved with lt_dlerror
.
int
lt_dladvise_preload (lt_dladvise *advise)
¶Set the preload
hint on advise. Passing an advise
parameter to lt_dlopenadvise
with this hint set causes it to
load only preloaded modules, so that if a suitable preloaded module is
not found, lt_dlopenadvise
will return NULL
.
int
lt_dlclose (lt_dlhandle handle)
¶Decrement the reference count on the module handle. If it drops to zero and no other module depends on this module, then the module is unloaded. Return 0 on success.
void *
lt_dlsym (lt_dlhandle handle, const char *name)
¶Return the address in the module handle, where the symbol given
by the null-terminated string name is loaded.
If the symbol cannot be found, NULL
is returned.
const char *
lt_dlerror (void)
¶Return a human readable string describing the most
recent error that occurred from any of libltdl’s functions.
Return NULL
if no errors have occurred since initialization
or since it was last called.
int
lt_dladdsearchdir (const char *search_dir)
¶Append the search directory search_dir to the current user-defined library search path. Return 0 on success.
int
lt_dlinsertsearchdir (const char *before, const char *search_dir)
¶Insert the search directory search_dir into the user-defined library
search path, immediately before the element starting at address
before. If before is ‘NULL’, then search_dir is
appending as if lt_dladdsearchdir
had been called. Return 0 on success.
int
lt_dlsetsearchpath (const char *search_path)
¶Replace the current user-defined library search path with
search_path, which must be a list of absolute directories separated
by LT_PATHSEP_CHAR
. Return 0 on success.
const char *
lt_dlgetsearchpath (void)
¶Return the current user-defined library search path.
int
lt_dlforeachfile (const char *search_path, int (*func) (const char *filename, void * data), void * data)
¶In some applications you may not want to load individual modules with
known names, but rather find all of the modules in a set of
directories and load them all during initialisation. With this function
you can have libltdl scan the LT_PATHSEP_CHAR
-delimited directory list
in search_path for candidates, and pass them, along with
data to your own callback function, func. If search_path is
‘NULL’, then search all of the standard locations that
lt_dlopen
would examine. This function will continue to make
calls to func for each file that it discovers in search_path
until one of these calls returns non-zero, or until the files are
exhausted. ‘lt_dlforeachfile’ returns the value returned by the last
call made to func.
For example you could define func to build an ordered argv-like vector of files using data to hold the address of the start of the vector.
int
lt_dlmakeresident (lt_dlhandle handle)
¶Mark a module so that it cannot be ‘lt_dlclose’d. This can be useful if a module implements some core functionality in your project that would cause your code to crash if removed. Return 0 on success.
If you use ‘lt_dlopen (NULL)’ to get a handle for the running binary, that handle will always be marked as resident, and consequently cannot be successfully ‘lt_dlclose’d.
int
lt_dlisresident (lt_dlhandle handle)
¶Check whether a particular module has been marked as resident, returning 1
if it has or 0 otherwise. If there is an error while executing this
function, return -1 and set an error message for retrieval with
lt_dlerror
.
dlopen
ed ¶Libtool modules are created like normal libtool libraries with a few exceptions:
You have to link the module with libtool’s -module switch, and you should link any program that is intended to dlopen the module with -dlopen modulename.la where possible, so that libtool can dlpreopen the module on platforms that do not support dlopening. If the module depends on any other libraries, make sure you specify them either when you link the module or when you link programs that dlopen it. If you want to disable versioning (see Library interface versions) for a specific module you should link it with the -avoid-version switch. Note that libtool modules don’t need to have a "lib" prefix. However, Automake 1.4 or higher is required to build such modules.
Usually a set of modules provide the same interface, i.e. exports the same symbols, so that a program can dlopen them without having to know more about their internals: In order to avoid symbol conflicts all exported symbols must be prefixed with "modulename_LTX_" (modulename is the name of the module). Internal symbols must be named in such a way that they won’t conflict with other modules, for example, by prefixing them with "_modulename_". Although some platforms support having the same symbols defined more than once it is generally not portable and it makes it impossible to dlpreopen such modules.
libltdl will automatically cut the prefix off to get the real name of the symbol. Additionally, it supports modules that do not use a prefix so that you can also dlopen non-libtool modules.
foo1.c gives an example of a portable libtool module. Exported symbols are prefixed with "foo1_LTX_", internal symbols with "_foo1_". Aliases are defined at the beginning so that the code is more readable.
/* aliases for the exported symbols */ #define foo foo1_LTX_foo #define bar foo1_LTX_bar /* a global variable definition */ int bar = 1; /* a private function */ int _foo1_helper() { return bar; } /* an exported function */ int foo() { return _foo1_helper(); }
The Makefile.am contains the necessary rules to build the module foo1.la:
... lib_LTLIBRARIES = foo1.la foo1_la_SOURCES = foo1.c foo1_la_LDFLAGS = -module ...
Libltdl provides a wrapper around whatever dynamic run-time object loading mechanisms are provided by the host system, many of which are themselves not thread safe. Consequently libltdl cannot itself be consistently thread safe.
If you wish to use libltdl in a multithreaded environment, then you must mutex lock around libltdl calls, since they may in turn be calling non-thread-safe system calls on some target hosts.
Some old releases of libtool provided a mutex locking API that was unusable with POSIX threads, so callers were forced to lock around all libltdl API calls anyway. That mutex locking API was next to useless, and is not present in current releases.
Some future release of libtool may provide a new POSIX thread compliant mutex locking API.
Some of the internal information about each loaded module that is maintained by libltdl is available to the user, in the form of this structure:
struct
lt_dlinfo { char *filename; char *name; int ref_count; int is_resident; int is_symglobal; int is_symlocal;}
¶lt_dlinfo
is used to store information about a module.
The filename attribute is a null-terminated character string of
the real module file name. If the module is a libtool module then
name is its module name (e.g. "libfoo"
for
"dir/libfoo.la"
), otherwise it is set to NULL
. The
ref_count attribute is a reference counter that describes how
often the same module is currently loaded. The remaining fields can
be compared to any hints that were passed to lt_dlopenadvise
to determine whether the underlying loader was able to follow them.
The following function will return a pointer to libltdl’s internal copy of this structure for the given handle:
const lt_dlinfo *
lt_dlgetinfo (lt_dlhandle handle)
¶Return a pointer to a struct that contains some information about
the module handle. The contents of the struct must not be modified.
Return NULL
on failure.
Furthermore, to save you from having to keep a list of the handles of all the modules you have loaded, these functions allow you to iterate over libltdl’s list of loaded modules:
The opaque type used to hold the module interface details for each registered libltdl client.
int
lt_dlhandle_interface (lt_dlhandle handle, const char *id_string)
¶Functions of this type are called to check that a handle conforms to a
library’s expected module interface when iterating over the global
handle list. You should be careful to write a callback function of
this type that can correctly identify modules that belong to this
client, both to prevent other clients from accidentally finding your
loaded modules with the iterator functions below, and vice versa. The
best way to do this is to check that module handle conforms
to the interface specification of your loader using lt_dlsym
.
The callback may be given every module loaded by all the libltdl module clients in the current address space, including any modules loaded by other libraries such as libltdl itself, and should return non-zero if that module does not fulfill the interface requirements of your loader.
int
my_interface_cb (lt_dlhandle handle, const char *id_string)
{
char *(*module_id) (void) = NULL;
/* A valid my_module must provide all of these symbols. */
if (!((module_id = (char*(*)(void)) lt_dlsym ("module_version"))
&& lt_dlsym ("my_module_entrypoint")))
return 1;
if (strcmp (id_string, module_id()) != 0)
return 1;
return 0;
}
lt_dlinterface_id
lt_dlinterface_register (const char *id_string, lt_dlhandle_interface *iface)
¶Use this function to register your interface validator with libltdl,
and in return obtain a unique key to store and retrieve per-module data.
You supply an id_string and iface so that the resulting
lt_dlinterface_id
can be used to filter the module handles
returned by the iteration functions below. If iface is NULL
,
all modules will be matched.
void
lt_dlinterface_free (lt_dlinterface_id iface)
¶Release the data associated with iface.
int
lt_dlhandle_map (lt_dlinterface_id iface, int (*func) (lt_dlhandle handle, void * data), void * data)
¶For each module that matches iface, call the function
func. When writing the func callback function, the
argument handle is the handle of a loaded module, and
data is the last argument passed to lt_dlhandle_map
. As
soon as func returns a non-zero value for one of the handles,
lt_dlhandle_map
will stop calling func and immediately
return that non-zero value. Otherwise 0 is eventually returned when
func has been successfully called for all matching modules.
lt_dlhandle
lt_dlhandle_iterate (lt_dlinterface_id iface, lt_dlhandle place)
¶Iterate over the module handles loaded by iface, returning the
first matching handle in the list if place is NULL
, and
the next one on subsequent calls. If place is the last element
in the list of eligible modules, this function returns NULL
.
lt_dlhandle handle = 0; lt_dlinterface_id iface = my_interface_id; while ((handle = lt_dlhandle_iterate (iface, handle))) { ... }
lt_dlhandle
lt_dlhandle_fetch (lt_dlinterface_id iface, const char *module_name)
¶Search through the module handles loaded by iface for a module named
module_name, returning its handle if found or else NULL
if no such named module has been loaded by iface.
However, you might still need to maintain your own list of loaded module handles (in parallel with the list maintained inside libltdl) if there were any other data that your application wanted to associate with each open module. Instead, you can use the following API calls to do that for you. You must first obtain a unique interface id from libltdl as described above, and subsequently always use it to retrieve the data you stored earlier. This allows different libraries to each store their own data against loaded modules, without interfering with one another.
void *
lt_dlcaller_set_data (lt_dlinterface_id key, lt_dlhandle handle, void * data)
¶Set data as the set of data uniquely associated with key and
handle for later retrieval. This function returns the data
previously associated with key and handle if any. A result of
0, may indicate that a diagnostic for the last error (if any) is available
from lt_dlerror()
.
For example, to correctly remove some associated data:
void *stale = lt_dlcaller_set_data (key, handle, 0); if (stale != NULL) { free (stale); } else { char *error_msg = lt_dlerror (); if (error_msg != NULL) { my_error_handler (error_msg); return STATUS_FAILED; } }
void *
lt_dlcaller_get_data (lt_dlinterface_id key, lt_dlhandle handle)
¶Return the address of the data associated with key and
handle, or else NULL
if there is none.
Old versions of libltdl also provided a simpler, but similar, API
based around lt_dlcaller_id
. Unfortunately, it had no
provision for detecting whether a module belonged to a particular
interface as libltdl didn’t support multiple loaders in the same
address space at that time. Those APIs are no longer supported
as there would be no way to stop clients of the old APIs from
seeing (and accidentally altering) modules loaded by other libraries.
Sometimes libltdl’s many ways of gaining access to modules are not
sufficient for the purposes of a project. You can write your own
loader, and register it with libltdl so that lt_dlopen
will be
able to use it.
Writing a loader involves writing at least three functions that can be
called by lt_dlopen
, lt_dlsym
and lt_dlclose
.
Optionally, you can provide a finalisation function to perform any
cleanup operations when lt_dlexit
executes, and a symbol prefix
string that will be prepended to any symbols passed to lt_dlsym
.
These functions must match the function pointer types below, after
which they can be allocated to an instance of lt_user_dlloader
and registered.
Registering the loader requires that you choose a name for it, so that it
can be recognised by lt_dlloader_find
and removed with
lt_dlloader_remove
. The name you choose must be unique, and not
already in use by libltdl’s builtin loaders:
The system dynamic library loader, if one exists.
The GNU dld loader, if libdld was installed when libltdl was built.
The loader for lt_dlopen
ing of preloaded static modules.
The prefix "dl" is reserved for loaders supplied with future versions of libltdl, so you should not use that for your own loader names.
The following types are defined in ltdl.h:
lt_module
is a dlloader dependent module.
The dynamic module loader extensions communicate using these low
level types.
lt_dlloader
is a handle for module loader types.
lt_user_data
is used for specifying loader instance data.
struct
lt_user_dlloader {const char *sym_prefix; lt_module_open *module_open; lt_module_close *module_close; lt_find_sym *find_sym; lt_dlloader_exit *dlloader_exit; }
¶If you want to define a new way to open dynamic modules, and have the
lt_dlopen
API use it, you need to instantiate one of these
structures and pass it to lt_dlloader_add
. You can pass whatever
you like in the dlloader_data field, and it will be passed back as
the value of the first parameter to each of the functions specified in
the function pointer fields.
lt_module
lt_module_open (const char *filename)
¶The type of the loader function for an lt_dlloader
module
loader. The value set in the dlloader_data field of the struct
lt_user_dlloader
structure will be passed into this function in the
loader_data parameter. Implementation of such a function should
attempt to load the named module, and return an lt_module
suitable for passing in to the associated lt_module_close
and
lt_sym_find
function pointers. If the function fails it should
return NULL
, and set the error message with lt_dlseterror
.
int
lt_module_close (lt_user_data loader_data, lt_module module)
¶The type of the unloader function for a user defined module loader.
Implementation of such a function should attempt to release
any resources tied up by the module module, and then unload it
from memory. If the function fails for some reason, set the error
message with lt_dlseterror
and return non-zero.
void *
lt_find_sym (lt_module module, const char *symbol)
¶The type of the symbol lookup function for a user defined module loader.
Implementation of such a function should return the address of the named
symbol in the module module, or else set the error message
with lt_dlseterror
and return NULL
if lookup fails.
int
lt_dlloader_exit (lt_user_data loader_data)
¶The type of the finalisation function for a user defined module loader.
Implementation of such a function should free any resources associated
with the loader, including any user specified data in the
dlloader_data
field of the lt_user_dlloader
. If non-NULL
,
the function will be called by lt_dlexit
, and
lt_dlloader_remove
.
For example:
int register_myloader (void) { lt_user_dlloader dlloader; /* User modules are responsible for their own initialisation. */ if (myloader_init () != 0) return MYLOADER_INIT_ERROR; dlloader.sym_prefix = NULL; dlloader.module_open = myloader_open; dlloader.module_close = myloader_close; dlloader.find_sym = myloader_find_sym; dlloader.dlloader_exit = myloader_exit; dlloader.dlloader_data = (lt_user_data)myloader_function; /* Add my loader as the default module loader. */ if (lt_dlloader_add (lt_dlloader_next (NULL), &dlloader, "myloader") != 0) return ERROR; return OK; }
Note that if there is any initialisation required for the loader, it must be performed manually before the loader is registered – libltdl doesn’t handle user loader initialisation.
Finalisation is handled by libltdl however, and it is important
to ensure the dlloader_exit
callback releases any resources claimed
during the initialisation phase.
libltdl provides the following functions for writing your own module loaders:
int
lt_dlloader_add (lt_dlloader *place, lt_user_dlloader *dlloader, const char *loader_name)
¶Add a new module loader to the list of all loaders, either as the
last loader (if place is NULL
), else immediately before the
loader passed as place. loader_name will be returned by
lt_dlloader_name
if it is subsequently passed a newly
registered loader. These loader_names must be unique, or
lt_dlloader_remove
and lt_dlloader_find
cannot
work. Returns 0 for success.
/* Make myloader be the last one. */ if (lt_dlloader_add (NULL, myloader) != 0) perror (lt_dlerror ());
int
lt_dlloader_remove (const char *loader_name)
¶Remove the loader identified by the unique name, loader_name.
Before this can succeed, all modules opened by the named loader must
have been closed. Returns 0 for success, otherwise an error message can
be obtained from lt_dlerror
.
/* Remove myloader. */ if (lt_dlloader_remove ("myloader") != 0) perror (lt_dlerror ());
lt_dlloader *
lt_dlloader_next (lt_dlloader *place)
¶Iterate over the module loaders, returning the first loader if place is
NULL
, and the next one on subsequent calls. The handle is for use with
lt_dlloader_add
.
/* Make myloader be the first one. */ if (lt_dlloader_add (lt_dlloader_next (NULL), myloader) != 0) return ERROR;
lt_dlloader *
lt_dlloader_find (const char *loader_name)
¶Return the first loader with a matching loader_name identifier, or else
NULL
, if the identifier is not found.
The identifiers that may be used by libltdl itself, if the host architecture supports them are dlopen11, dld and dlpreload.
/* Add a user loader as the next module loader to be tried if the standard dlopen loader were to fail when lt_dlopening. */ if (lt_dlloader_add (lt_dlloader_find ("dlopen"), myloader) != 0) return ERROR;
const char *
lt_dlloader_name (lt_dlloader *place)
¶Return the identifying name of place, as obtained from
lt_dlloader_next
or lt_dlloader_find
. If this function fails,
it will return NULL
and set an error for retrieval with
lt_dlerror
.
lt_user_data *
lt_dlloader_data (lt_dlloader *place)
¶Return the address of the dlloader_data
of place, as
obtained from lt_dlloader_next
or lt_dlloader_find
. If
this function fails, it will return NULL
and set an error for
retrieval with lt_dlerror
.
int
lt_dladderror (const char *diagnostic)
¶This function allows you to integrate your own error messages into
lt_dlerror
. Pass in a suitable diagnostic message for return by
lt_dlerror
, and an error identifier for use with
lt_dlseterror
is returned.
If the allocation of an identifier fails, this function returns -1.
int myerror = lt_dladderror ("doh!"); if (myerror < 0) perror (lt_dlerror ());
int
lt_dlseterror (int errorcode)
¶When writing your own module loaders, you should use this function to
raise errors so that they are propagated through the lt_dlerror
interface. All of the standard errors used by libltdl are declared in
ltdl.h, or you can add more of your own with
lt_dladderror
. This function returns 0 on success.
if (lt_dlseterror (LTDL_ERROR_NO_MEMORY) != 0) perror (lt_dlerror ());
Even though libltdl is installed together with libtool, you may wish to include libltdl in the distribution of your package, for the convenience of users of your package that don’t have libtool or libltdl installed, or if you are using features of a very new version of libltdl that you don’t expect your users to have yet. In such cases, you must decide what flavor of libltdl you want to use: a convenience library or an installable libtool library.
The most simplistic way to add libltdl
to your package is to
copy all the libltdl source files to a subdirectory within
your package and to build and link them along with the rest of your
sources. To help you do this, the m4 macros for Autoconf are
available in ltdl.m4. You must ensure that they are available
in aclocal.m4 before you run Autoconf12. Having made the macros available, you must add a call to the
‘LTDL_INIT’ macro (after the call to ‘LT_INIT’)
to your package’s configure.ac to
perform the configure time checks required to build the library
correctly. Unfortunately, this method has problems if you then try to
link the package binaries with an installed libltdl, or a library that
depends on libltdl, because of the duplicate symbol definitions. For
example, ultimately linking against two different versions of libltdl,
or against both a local convenience library and an installed libltdl
is bad. Ensuring that only one copy of the libltdl sources are linked
into any program is left as an exercise for the reader.
Declare directory to be the location of the libltdl
source files, for libtoolize --ltdl
to place
them. See Invoking libtoolize
, for more details. Provided that you
add an appropriate LT_CONFIG_LTDL_DIR
call in your
configure.ac before calling libtoolize
, the
appropriate libltdl
files will be installed automatically.
AC_WITH_LTDL
and LT_WITH_LTDL
are deprecated names for
older versions of this macro; autoupdate
will update your
configure.ac file.
This macro adds the following options to the configure
script:
The LTDL_INIT
macro will look in the standard header file
locations to find the installed libltdl
headers. If
LTDL_INIT
can’t find them by itself, the person who builds
your package can use this option to tell configure
where
the installed libltdl
headers are.
Similarly, the person building your package can use this option to
help configure
find the installed libltdl.la.
If there is no installed libltdl
, or in any case if the
person building your package would rather use the libltdl
sources shipped with the package in the subdirectory named by
LT_CONFIG_LTDL_DIR
, they should pass this option to
configure
.
If the --with-included-ltdl is not passed at
configure time, and an installed libltdl
is not
found13, then configure
will exit immediately with an error that
asks the user to either specify the location of an installed
libltdl
using the --with-ltdl-include and
--with-ltdl-lib options, or to build with the
libltdl
sources shipped with the package by passing
--with-included-ltdl.
If an installed libltdl
is found, then LIBLTDL
is set to
the link flags needed to use it, and LTDLINCL
to the preprocessor
flags needed to find the installed headers, and LTDLDEPS
will
be empty. Note, however, that no version checking is performed. You
should manually check for the libltdl
features you need in
configure.ac:
LT_INIT([dlopen]) LTDL_INIT # The lt_dladvise_init symbol was added with libtool-2.2 if test yes != "$with_included_ltdl"; then save_CFLAGS=$CFLAGS save_LDFLAGS=$LDFLAGS CFLAGS="$CFLAGS $LTDLINCL" LDFLAGS="$LDFLAGS $LIBLTDL" AC_CHECK_LIB([ltdl], [lt_dladvise_init], [], [AC_MSG_ERROR([installed libltdl is too old])]) LDFLAGS=$save_LDFLAGS CFLAGS=$save_CFLAGS fi
options may include no more than one of the following build
modes depending on how you want your project to build libltdl
:
‘nonrecursive’, ‘recursive’, or ‘subproject’. In order
for libtoolize
to detect this option correctly, if you
supply one of these arguments, they must be given literally (i.e.,
macros or shell variables that expand to the correct ltdl mode will not
work).
This is how the Libtool project distribution builds the libltdl
we ship and install. If you wish to use Automake to build
libltdl
without invoking a recursive make to descend into the
libltdl
subdirectory, then use this option. You will need to set
your configuration up carefully to make this work properly, and you will
need releases of Autoconf and Automake that support
subdir-objects
and LIBOBJDIR
properly. In your
configure.ac, add:
AM_INIT_AUTOMAKE([subdir-objects]) AC_CONFIG_HEADERS([config.h]) LT_CONFIG_LTDL_DIR([libltdl]) LT_INIT([dlopen]) LTDL_INIT([nonrecursive])
You have to use a config header, but it may have a name different than config.h.
Also, add the following near the top of your Makefile.am:
AM_CPPFLAGS = AM_LDFLAGS = BUILT_SOURCES = EXTRA_DIST = CLEANFILES = MOSTLYCLEANFILES = include_HEADERS = noinst_LTLIBRARIES = lib_LTLIBRARIES = EXTRA_LTLIBRARIES = include libltdl/ltdl.mk
Unless you build no other libraries from this Makefile.am,
you will also need to change lib_LTLIBRARIES
to assign with
‘+=’ so that the libltdl
targets declared in
ltdl.mk are not overwritten.
This build mode still requires that you use Automake, but (in contrast
with ‘nonrecursive’) uses the more usual device of starting another
make
process in the libltdl subdirectory. To use this
mode, you should add to your configure.ac:
AM_INIT_AUTOMAKE AC_CONFIG_HEADERS([config.h]) LT_CONFIG_LTDL_DIR([libltdl]) LT_INIT([dlopen]) LTDL_INIT([recursive]) AC_CONFIG_FILES([libltdl/Makefile])
Again, you have to use a config header, but it may have a name different than config.h if you like.
Also, add this to your Makefile.am:
SUBDIRS = libltdl
This mode is the default unless you explicitly add recursive
or
nonrecursive
to your LTDL_INIT
options; subproject
is the only mode supported by previous releases of libltdl. Even if you
do not use Autoconf in the parent project, then, in ‘subproject’
mode, still libltdl
contains all the necessary files to configure
and build itself – you just need to arrange for your build system to
call libltdl/configure with appropriate options, and then run
make
in the libltdl
subdirectory.
If you are using Autoconf and Automake, then you will need to add the following to your configure.ac:
LT_CONFIG_LTDL_DIR([libltdl]) LTDL_INIT
and to Makefile.am:
SUBDIRS = libltdl
Aside from setting the libltdl build mode, there are other keywords
that you can pass to LTDL_INIT
to modify its behavior when
--with-included-ltdl has been given:
This is the default unless you explicitly add installable
to
your LTDL_INIT
options.
This keyword will cause options to be passed to the configure
script in the subdirectory named by LT_CONFIG_LTDL_DIR
to cause it to be built as a convenience library. If you’re not
using automake, you will need to define top_build_prefix
,
top_builddir
, and top_srcdir
in your makefile so that
LIBLTDL
, LTDLDEPS
, and LTDLINCL
expand correctly.
One advantage of the convenience library is that it is not installed,
so the fact that you use libltdl
will not be apparent to the
user, and it won’t overwrite a pre-installed version of
libltdl
the system might already have in the installation
directory. On the other hand, if you want to upgrade libltdl
for any reason (e.g. a bugfix) you’ll have to recompile your package
instead of just replacing the shared installed version of
libltdl
. However, if your programs or libraries are linked
with other libraries that use such a pre-installed version of
libltdl
, you may get linker errors or run-time crashes.
Another problem is that you cannot link the convenience library into
more than one libtool library, then link a single program with those
libraries, because you may get duplicate symbols. In general you can
safely use the convenience library in programs that don’t depend on
other libraries that might use libltdl
too.
This keyword will pass options to the configure
script in the subdirectory named by LT_CONFIG_LTDL_DIR
to cause it to be built as an installable library. If you’re not
using automake, you will need to define top_build_prefix
,
top_builddir
and top_srcdir
in your makefile so that
LIBLTDL
, LTDLDEPS
, and LTDLINCL
are expanded
properly.
Be aware that you could overwrite another libltdl
already
installed to the same directory if you use this option.
Whatever method you use, ‘LTDL_INIT’ will define the shell variable
LIBLTDL
to the link flag that you should use to link with
libltdl
, the shell variable LTDLDEPS
to the files that
can be used as a dependency in Makefile rules, and the shell
variable LTDLINCL
to the preprocessor flag that you should use to
compile programs that include ltdl.h. So, when you want to link a
program with libltdl, be it a convenience, installed or installable
library, just use ‘$(LTDLINCL)’ for preprocessing and compilation,
and ‘$(LIBLTDL)’ for linking.
libltdl
,
LIBLTDL
will be set to the compiler flags needed to link against
the installed library, LTDLDEPS
will be empty, and LTDLINCL
will be set to the compiler flags needed to find the libltdl
header files.
LIBLTDL
and LTDLDEPS
will be the pathname for the convenience version of
libltdl (starting with ‘${top_builddir}/’ or
‘${top_build_prefix}’) and LTDLINCL
will be -I
followed by the directory that contains ltdl.h (starting with
‘${top_srcdir}/’).
libltdl
is being
built, its pathname starting with ‘${top_builddir}/’ or
‘${top_build_prefix}’, will be stored in LIBLTDL
and
LTDLDEPS
, and LTDLINCL
will be set just like in the case of
convenience library.
You should probably also use the ‘dlopen’ option to LT_INIT
in your configure.ac, otherwise libtool will assume no dlopening
mechanism is supported, and revert to dlpreopening, which is probably not
what you want. Avoid using the -static,
-static-libtool-libs, or -all-static
switches when linking programs with libltdl. This will not work on
all platforms, because the dlopening functions may not be available
for static linking.
The following example shows you how to embed an installable libltdl in
your package. In order to use the convenience variant, just replace the
LTDL_INIT
option ‘installable’ with ‘convenience’. We
assume that libltdl was embedded using ‘libtoolize --ltdl’.
configure.ac:
... # Name the subdirectory that contains libltdl sources LT_CONFIG_LTDL_DIR([libltdl]) # Configure libtool with dlopen support if possible LT_INIT([dlopen]) # Enable building of the installable libltdl library LTDL_INIT([installable]) ...
Makefile.am:
... SUBDIRS = libltdl AM_CPPFLAGS = $(LTDLINCL) myprog_LDFLAGS = -export-dynamic myprog_LDADD = $(LIBLTDL) -dlopen self -dlopen foo1.la myprog_DEPENDENCIES = $(LTDLDEPS) foo1.la ...
These macros are deprecated, the ‘installable’ option to
LTDL_INIT
should be used instead.
These macros are deprecated, the ‘convenience’ option to
LTDL_INIT
should be used instead.
This section describes macros whose sole purpose is to be traced using Autoconf’s --trace option (see The Autoconf Manual in The Autoconf Manual) to query the Libtool configuration of a project. These macros are called by Libtool internals and should never be called by user code; they should only be traced.
This macro is called once for each language enabled in the package. Its only argument, tag, is the tag-name corresponding to the language (see Tags).
You can therefore retrieve the list of all tags enabled in a project using the following command:
autoconf --trace 'LT_SUPPORTED_TAG:$1'
This chapter covers some questions that often come up on the mailing lists.
When creating a shared library, but not when compiling or creating
a program, libtool
drops some flags from the command line
provided by the user. This is done because flags unknown to
libtool
may interfere with library creation or require
additional support from libtool
, and because omitting
flags is usually the conservative choice for a successful build.
If you encounter flags that you think are useful to pass, as a
work-around you can prepend flags with -Wc,
or -Xcompiler
to allow them to be passed through to the compiler driver
(see Link mode). Another possibility is to add flags already
to the compiler command at configure
run time:
./configure CC='gcc -m64'
If you think libtool
should let some flag through by default,
here’s how you can test such an inclusion: grab the Libtool development
tree, edit the ltmain.in file in the libltdl/config
subdirectory to pass through the flag (search for ‘Flags to be
passed through’), re-bootstrap and build with the flags in question
added to LDFLAGS
, CFLAGS
, CXXFLAGS
, etc. on the
configure
command line as appropriate. Run the testsuite
as described in the README file and report results to
the Libtool bug reporting address bug-libtool@gnu.org.
Libtool is under constant development, changing to remain up-to-date with modern operating systems. If libtool doesn’t work the way you think it should on your platform, you should read this chapter to help determine what the problem is, and how to resolve it.
Libtool comes with an integrated set of tests to check that your build is sane, that test its capabilities, and report obvious bugs in the libtool program. The test suite is based on Autotest from Autoconf (see Generating Test Suites with Autotest in The Autoconf Manual). These tests, too, are constantly evolving, based on past problems with libtool, and known deficiencies in other operating systems.
As described in the README file, you may run make -k check after you have built libtool (possibly before you install it) to make sure that it meets basic functional requirements.
The test suite uses keywords to classify certain test groups:
The test group exercises one of these libtool
language tags.
These keywords denote that the respective external program is needed
by the test group. The tests are typically skipped if the program is
not installed. The ‘automake’ keyword may also denote use of the
aclocal
program.
This test group may require user interaction on some systems. Typically, this means closing a popup window about a DLL load error on Windows.
Denote that the libltdl library is exercised by the test group.
Denote that the libtool
or libtoolize
scripts are
exercised by the test group, respectively.
Denote that this test group may recursively re-invoke the test suite
itself, with changed settings and maybe a changed libtool
script. You may use the INNER_TESTSUITEFLAGS
variable to pass
additional settings to this recursive invocation. Typically, recursive
invocations delimit the set of tests with another keyword, for example
by passing -k libtool
right before the expansion of the
INNER_TESTSUITEFLAGS
variable (without an intervening space, so
you get the chance for further delimitation).
Test groups with the keyword ‘recursive’ should not be denoted with keywords, in order to avoid infinite recursion. As a consequence, recursive test groups themselves should never require user interaction, while the test groups they invoke may do so.
There is a convenience target ‘check-noninteractive’ that runs all tests from both test suites that do not cause user interaction on Windows. Conversely, the target ‘check-interactive’ runs the complement of tests and might require closing popup windows about DLL load errors on Windows.
Here is a list of the some of the current files in the test suite, and what they test for:
Tests that a binary can be built and ran from outside of the sub-directory that it is built and ran in.
Tests convenience archive within another convenience archive.
Compiles foo()
in libfoo, then compiles libfoo (and
bar()
function) into libbar.
Tests include a demonstration of various scenarios related to using the
-bindir option with libtool
, verifying that installed files
are correctly linked and executed, and demonstrating flexibility in handling
different installation paths and adapting to changing directory structures.
Tests that there are no conflicting warnings about AC_PROG_RANLIB by verifying no autoscan AC_PROG_RANLIB warning and checking that AC_PROG_RANLIB declaration has a warning.
Tests that the -no-canonical-prefixes flag is not removed from the linking command, but it is instead passed through to the linker.
Tests that the local version of an installed program is used both with and without an external library.
Tests include a demonstration of libtool
convenience libraries, a
mechanism that allows build-time static libraries to be created, in a way that
their components can be later linked into programs or other libraries, even
shared ones.
Tests include a verification that libtool
operates properly when the
maximum command line length is very small.
Tests creation of shell functions shared with configure
and
libtool
.
Tests that exercise the configure interface to libltdl.
Tests demonstrate libtool
’s convenience archive capabilities for C,
C++, Fortran, and Java languages.
Test that libtool
can handle code with C++ constructors.
Test cwrapper compliance with standards for uninstalled executables, string length, and installed shared libraries.
Tests for macOS X including compilation, concurrent library extraction,
GDB
debug information, ld
warnings, and verifying
.dylib and .so files can be used with lt_dlopen
.
Tests include a demonstration of a trivial package that uses libtool
.
The tests include scenarios to build both static and shared libraries, only
static libraries, only shared libraries, disabling fast-install mode, building
PIC code, and building non-PIC code.
Tests include a demonstration of inter-library dependencies with
libtool
. The test programs link some interdependent libraries under
different scenarios.
Tests building and linking various libraries within various directories and
sub-directories while changing directories as well. It should be possible to
use a nontrivial relative path to the output file name when creating libraries
and programs. The deplibs
of these might have relative paths as well.
When executing uninstalled programs, the paths relative to $PWD at build
time needs to be translated to a path valid at execution time. Also test
installing these libraries and programs; however, use consistent relative paths
between libtool
--mode=link and libtool
--mode=install in this test.
Tests the correct detection and handling of identical dependency libraries when
using libtool
.
Tests include various scenarios related to detecting deplibs correctly,
including cases where there is no file
command installed as well
as the host OS being MinGW.
Installs some libs in $DESTDIR, moves them to a different dir, then installs some false libraries in $DESTDIR that should not be linked against. If the program refers to these false libraries, there is a bug.
Tests that lt_dlopen
can open a shared library and call a function from
the library, verifies that lt_dlsym
can find a symbol in the opened
library, and make sure that lt_dlclose
can properly close the library.
Tests whether on Windows a convenience symbol in a section of a .lib file is present even if that section is hidden in the .obj file.
Tests two convenience archives with the same name, and also containing an object with the same name.
Tests circular call of dependencies between two libraries, liba and
libb. Function a1()
from liba calls b1()
from
libb and function b1()
from libb calls a2()
from
liba.
Tests a library with multiple files of the same name (from different directories), such as 1/a.c, 2/a.c, 3/a.c, etc.
Tests building binaries using libtool
using two configure approaches.
Tests C++ exception handling with libtool
.
Tests the --mode=execute feature of libtool
.
Tests ensure that $EXEEXT
handling works by linking and installing an
executable.
Tests symbol exports if shared libraries are enabled.
Test exporting from a DLL with module definition (.def files). This test only runs if shared libraries are enabled and building a DLL is supported.
Tests test Fortran 77 support in libtool
by creating libraries from
Fortran 77 sources, and mixed Fortran and C sources, and a Fortran 77 program
to use the former library, and a C program to use the latter library.
Tests to ensure that libtool
really fails when it should, including
after compile failure, program creation failure, and shared library creation
failure.
Tests are similar to the tests/f77demo.at tests, except that Fortran 90 is used in combination with the ‘FC’ interface provided by Autoconf and Automake.
Tests include checks that compile and linker flags get passed through
libtool
. Tests flags for C, C++, Fortran 77, and Fortran 90.
Tests a variety of mode commands, including mode short-hands, to ensure basic command line functionality. Also verify that the --debug flag is handled correctly in each mode.
Tests indirect dependencies (or nested dependencies). libd depends on libconv, which depends on libb, which depends on liba.
Tests that func_infer_tag
works by compiling various code snippets in
various languages (C, C++, Fortran, Java) without a --tag flag.
Tests the functionality of the inherited_linker_flags
variable in
libtool
library files.
Tests install mode and ensures that install_override_mode
overrides the
mode of the shared library (and only the shared library).
Tests parsing of .la files including both correctly formed .la files and malformed or bogus .la files.
Tests basic libtool
functionality including shell meta-character removal, lack
of supplied mode option, file extension handling, simple linking,
objectlist flag usage, and LT_SUPPORTED_TAG
usage.
Tests that errors and warnings are reported for invalid and unknown
LIBTOOLIZE_OPTIONS
as well as checking the --no-warn flag
suppresses libtoolize
warnings.
Tests to make sure that depdepls
are added right after the libraries
that pull them in which is necessary at least for static linking and on systems
where libraries do not link against other libraries.
Tests for problems with linking libraries in different order.
Tests libltdl (a libdl API for dlopen
) support of
LoadLibrary
for dynamically linking DLLs in Windows environments.
Tests to verify that invoking C compiler language localization options do not
cause problems with libtool
.
Tests libltdl (a libdl API for dlopen
) module loading
advisor functions.
Tests libltdl (a libdl API for dlopen
) for a memory use
after free bug in lt_dlexit
.
Tests lt_dlopen
with an archive file. Verifies that lt_dlopen
can load an archive file and successfully return a handle to it.
Tests libltdl (a libdl API for dlopen
) with a basic C
example.
Tests libltdl (a libdl API for dlopen
) with an extern C++
function.
Tests that libtool
doesn’t mangle gnulib argv
names.
Tests if ltdl can find an installed module using the libdir
variable in the .la file. Tests also include a MinGW test which uses a
Windows-style libdir
name.
Tests include a demonstration of a package that uses libtool
and the
system independent dlopen
wrapper libltdl to load modules. The
library libltdl provides a dlopen
wrapper for various platforms
(POSIX) including support for dlpreopened
modules
(see Dlpreopening).
Tests to check for failures on systems that require libraries to be prefixed with lib.
Tests the want_nocaseglob
configuration option to search for libraries
regardless of case.
Tests AC_NO_EXECUTABLES
macro with gcc
.
Tests non-recursive DLL libraries with nonrecursive Automake libltdl
(a libdl API for dlopen
) build.
Tests various aspects of libtool
’s old m4
interface.
Tests the -fpic flag with gcc
and g++
.
Tests recursive DLL libraries with recursive Automake libltdl (a
libdl API for dlopen
) build.
Tests that resident modules are not unloaded at program exit, as they need to
be able to invoke atexit
handlers. A module being a resident module
means it is prevented from being lt_dlclosed
.
Tests the runpath configuration options in libtool
(-R and
-rpath) and that the resulting .la files are installed in the
directory appended to their respective library’s dependency_libs
by
-R.
Tests sys_lib_search_path_spec
, which is an expression to get the
compile-time system library search path. Tests include determining if it is
possible to link an executable to system libraries and if
sys_lib_search_path_spec
also works on MSVC (Microsoft Visual C++).
Tests include verifying proper behaviour of the shlibpath_var
, which is
the variable responsible for telling the linker where to find shared libraries.
Additionally, this tests the behaviour of the shlibpath_overrides_runpath
variable, which determines if it possible to override an executable’s
hardcoded library search path with an environment variable.
Tests the functionality of the SList datastructure, which is an implementation of singly linked lists. Tests include verifying the usual linked list functions such as finding, removing, deleting, reversing, and sorting the element(s) in SLists.
Tests libltdl functionality as a standalone tool. Tests include compiling a symlinked version of libltdl and a copied version of libltdl. Next libltdl is installed locally and linked in a project without the use of Autoconf or Automake.
Tests various flags related to static and dynamic linking including -Bstatic and -Bdynamic.
Tests various libtool
flag and option combinations, tests linking
various types of objects in different sections, and tests regular expressions
to cover edge cases and unusual configurations over multiple iterations.
Tests the libltdl flag for building subprojects in individual directories. Tests cover soft-linked libltdl trees, copied libltdl trees, and linking libltdl without Autotools.
Tests that libtool
runs properly in sandboxed sysroot
directories.
Tests include a demonstration of a package that uses libtool
’s
multi-language support through configuration tags. It generates a library from
C++ sources, which is then linked to a C++ program.
Tests that libtool
can handle C++ code utilizing templates.
Main testsuite framework file processed by autom4te
processes enable
running the libtool
test cases.
Tests libtool
’s versioning system. Tests begin with verifying the
behaviour of libtool
versioning flags -version-info and
-version-number. Next, this tests installing a library, then updating
the library with a new revision, a compatible update, and an incompatible
update. In each case, the tests verify that the original library will link and
install as expected.
Tests the function of the --enable-pic flag. The --enable-pic
flag is used to specify whether or not libtool
uses PIC objects.
This includes tests for setting --enable-pic to no, yes, or a comma
delimited list of package names.
The Autotest-based test suite produces as output a file tests/testsuite.log that contains information about failed tests.
You can pass options to the test suite through the make
variable
TESTSUITEFLAGS
(see Making testsuite Scripts in The Autoconf Manual).
If you think you have discovered a bug in libtool, you should think twice: the libtool maintainer is notorious for passing the buck (or maybe that should be “passing the bug”). Libtool was invented to fix known deficiencies in shared library implementations, so, in a way, most of the bugs in libtool are actually bugs in other operating systems. However, the libtool maintainer would definitely be happy to add support for somebody else’s buggy operating system. [I wish there was a good way to do winking smiley-faces in Texinfo.]
Genuine bugs in libtool include problems with shell script portability, documentation errors, and failures in the test suite (see The libtool test suite).
First, check the documentation and help screens to make sure that the behaviour you think is a problem is not already mentioned as a feature.
Then, you should read the Emacs guide to reporting bugs (see Reporting Bugs in The Emacs Manual). Some of the details listed there are specific to Emacs, but the principle behind them is a general one.
Finally, send a bug report to the Libtool bug reporting address bug-libtool@gnu.org with any appropriate facts, such as test suite output (see When tests fail), all the details needed to reproduce the bug, and a brief description of why you think the behaviour is a bug. Be sure to include the word “libtool” in the subject line, as well as the version number you are using (which can be found by typing libtool --version).
This chapter contains information that the libtool maintainer finds important. It will be of no use to you unless you are considering porting libtool to new systems, or writing your own libtool.
libtool
script contentsBefore you embark on porting libtool to an unsupported system, it is worthwhile to send e-mail to the Libtool mailing list libtool@gnu.org, to make sure that you are not duplicating existing work.
If you find that any porting documentation is missing, please complain! Complaints with patches and improvements to the documentation, or to libtool itself, are more than welcome.
Once it is clear that a new port is necessary, you’ll generally need the following information:
You need the output of config.guess
for this system, so that you
can make changes to the libtool configuration process without affecting
other systems.
ld
and cc
These generally describe what flags are used to generate PIC, to create shared libraries, and to link against only static libraries. You may need to follow some cross references to find the information that is required.
ld.so
, rtld
, or equivalentThese are a valuable resource for understanding how shared libraries are loaded on the system.
ldconfig
, or equivalentThis page usually describes how to install shared libraries.
This shows the naming convention for shared libraries on the system, including what names should be symbolic links.
Some systems have special documentation on how to build and install shared libraries.
If you know how to program the Bourne shell, then you can complete the port yourself; otherwise, you’ll have to find somebody with the relevant skills who will do the work. People on the libtool mailing list are usually willing to volunteer to help you with new ports, so you can send the information to them.
To do the port yourself, you’ll definitely need to modify the
libtool.m4
macros to make platform-specific changes to
the configuration process. You should search that file for the
PORTME
keyword, which will give you some hints on what you’ll
need to change. In general, all that is involved is modifying the
appropriate configuration variables (see libtool
script contents).
Your best bet is to find an already-supported system that is similar to
yours, and make your changes based on that. In some cases, however,
your system will differ significantly from every other supported system,
and it may be necessary to add new configuration variables, and modify
the ltmain.in
script accordingly. Be sure to write to the
mailing list before you make changes to ltmain.in
, since they may
have advice on the most effective way of accomplishing what you want.
Since version 1.2c, libtool has re-introduced the ability to do inter-library dependency on some platforms, thanks to a patch by Toshio Kuratomi badger@prtr-13.ucsc.edu. Here’s a shortened version of the message that contained his patch:
The basic architecture is this: in libtool.m4, the person who writes libtool makes sure ‘$deplibs’ is included in ‘$archive_cmds’ somewhere and also sets the variable ‘$deplibs_check_method’, and maybe ‘$file_magic_cmd’ when ‘deplibs_check_method’ is file_magic.
‘deplibs_check_method’ can be one of five things:
looks in the library link path for libraries that have the right
libname. Then it runs ‘$file_magic_cmd’ on the library and checks
for a match against the extended regular expression regex. When
file_magic_test_file
is set by libtool.m4, it is used as an
argument to ‘$file_magic_cmd’ to verify whether the
regular expression matches its output, and warn the user otherwise.
will pass everything without any checking. This may work on platforms where code is position-independent by default and inter-library dependencies are properly supported by the dynamic linker, for example, on DEC OSF/1 3 and 4.
It causes deplibs to be reassigned ‘deplibs=""’. That way ‘archive_cmds’ can contain deplibs on all platforms, but not have deplibs used unless needed.
is the default for all systems unless overridden in libtool.m4. It is the same as ‘none’, but it documents that we really don’t know what the correct value should be, and we welcome patches that improve it.
Then in ltmain.in we have the real workhorse: a little initialization and postprocessing (to setup/release variables for use with eval echo libname_spec etc.) and a case statement that decides the method that is being used. This is the real code… I wish I could condense it a little more, but I don’t think I can without function calls. I’ve mostly optimized it (moved things out of loops, etc.) but there is probably some fat left. I thought I should stop while I was ahead, work on whatever bugs you discover, etc. before thinking about more than obvious optimizations.
This table describes when libtool was last known to be tested on platforms where it claims to support shared libraries:
------------------------------------------------------- canonical host name compiler libtool results (tools versions) release ------------------------------------------------------- alpha-dec-osf5.1 cc 1.3e ok (1.910) alpha-dec-osf4.0f gcc 1.3e ok (1.910) alpha-dec-osf4.0f cc 1.3e ok (1.910) alpha-dec-osf3.2 gcc 0.8 ok alpha-dec-osf3.2 cc 0.8 ok alpha-dec-osf2.1 gcc 1.2f NS alpha*-unknown-linux-gnu gcc 1.3b ok (egcs-1.1.2, GNU ld 2.9.1.0.23) hppa2.0w-hp-hpux11.00 cc 1.2f ok hppa2.0-hp-hpux10.20 cc 1.3.2 ok hppa1.1-hp-hpux10.20 gcc 1.2f ok hppa1.1-hp-hpux10.20 cc 1.3c ok (1.821) hppa1.1-hp-hpux10.10 gcc 1.2f ok hppa1.1-hp-hpux10.10 cc 1.2f ok hppa1.1-hp-hpux9.07 gcc 1.2f ok hppa1.1-hp-hpux9.07 cc 1.2f ok hppa1.1-hp-hpux9.05 gcc 1.2f ok hppa1.1-hp-hpux9.05 cc 1.2f ok hppa1.1-hp-hpux9.01 gcc 1.2f ok hppa1.1-hp-hpux9.01 cc 1.2f ok i*86-*-beos gcc 1.2f ok i*86-*-bsdi4.0.1 gcc 1.3c ok (gcc-2.7.2.1) i*86-*-bsdi4.0 gcc 1.2f ok i*86-*-bsdi3.1 gcc 1.2e NS i*86-*-bsdi3.0 gcc 1.2e NS i*86-*-bsdi2.1 gcc 1.2e NS i*86-pc-cygwin gcc 1.3b NS (egcs-1.1 stock b20.1 compiler) i*86-*-dguxR4.20MU01 gcc 1.2 ok i*86-*-freebsd4.3 gcc 1.3e ok (1.912) i*86-*-freebsdelf4.0 gcc 1.3c ok (egcs-1.1.2) i*86-*-freebsdelf3.2 gcc 1.3c ok (gcc-2.7.2.1) i*86-*-freebsdelf3.1 gcc 1.3c ok (gcc-2.7.2.1) i*86-*-freebsdelf3.0 gcc 1.3c ok i*86-*-freebsd3.0 gcc 1.2e ok i*86-*-freebsd2.2.8 gcc 1.3c ok (gcc-2.7.2.1) i*86-*-freebsd2.2.6 gcc 1.3b ok (egcs-1.1 & gcc-2.7.2.1, native ld) i*86-*-freebsd2.1.5 gcc 0.5 ok i*86-*-netbsd1.5 gcc 1.3e ok (1.901) (egcs-1.1.2) i*86-*-netbsd1.4 gcc 1.3c ok (egcs-1.1.1) i*86-*-netbsd1.4.3A gcc 1.3e ok (1.901) i*86-*-netbsd1.3.3 gcc 1.3c ok (gcc-2.7.2.2+myc2) i*86-*-netbsd1.3.2 gcc 1.2e ok i*86-*-netbsd1.3I gcc 1.2e ok (egcs 1.1?) i*86-*-netbsd1.2 gcc 0.9g ok i*86-*-linux-gnu gcc 1.3e ok (1.901) (Red Hat 7.0, gcc "2.96") i*86-*-linux-gnu gcc 1.3e ok (1.911) (SuSE 7.0, gcc 2.95.2) i*86-*-linux-gnulibc1 gcc 1.2f ok i*86-*-openbsd2.5 gcc 1.3c ok (gcc-2.8.1) i*86-*-openbsd2.4 gcc 1.3c ok (gcc-2.8.1) i*86-*-solaris2.7 gcc 1.3b ok (egcs-1.1.2, native ld) i*86-*-solaris2.6 gcc 1.2f ok i*86-*-solaris2.5.1 gcc 1.2f ok i*86-ncr-sysv4.3.03 gcc 1.2f ok i*86-ncr-sysv4.3.03 cc 1.2e ok (cc -Hnocopyr) i*86-pc-sco3.2v5.0.5 cc 1.3c ok i*86-pc-sco3.2v5.0.5 gcc 1.3c ok (gcc 95q4c) i*86-pc-sco3.2v5.0.5 gcc 1.3c ok (egcs-1.1.2) i*86-sco-sysv5uw7.1.1 gcc 1.3e ok (1.901) (gcc-2.95.2, SCO linker) i*86-UnixWare7.1.0-sysv5 cc 1.3c ok i*86-UnixWare7.1.0-sysv5 gcc 1.3c ok (egcs-1.1.1) m68k-next-nextstep3 gcc 1.2f NS m68k-sun-sunos4.1.1 gcc 1.2f NS (gcc-2.5.7) m88k-dg-dguxR4.12TMU01 gcc 1.2 ok m88k-motorola-sysv4 gcc 1.3 ok (egcs-1.1.2) mips-sgi-irix6.5 gcc 1.2f ok (gcc-2.8.1) mips-sgi-irix6.4 gcc 1.2f ok mips-sgi-irix6.3 gcc 1.3b ok (egcs-1.1.2, native ld) mips-sgi-irix6.3 cc 1.3b ok (cc 7.0) mips-sgi-irix6.2 gcc 1.2f ok mips-sgi-irix6.2 cc 0.9 ok mips-sgi-irix5.3 gcc 1.2f ok (egcs-1.1.1) mips-sgi-irix5.3 gcc 1.2f NS (gcc-2.6.3) mips-sgi-irix5.3 cc 0.8 ok mips-sgi-irix5.2 gcc 1.3b ok (egcs-1.1.2, native ld) mips-sgi-irix5.2 cc 1.3b ok (cc 3.18) mips-sni-sysv4 cc 1.3.5 ok (Siemens C-compiler) mips-sni-sysv4 gcc 1.3.5 ok (gcc-2.7.2.3, GNU assembler 2.8.1, native ld) mipsel-unknown-openbsd2.1 gcc 1.0 ok powerpc-apple-darwin6.4 gcc 1.5 ok (apple dev tools released 12/2002) powerpc-ibm-aix4.3.1.0 gcc 1.2f ok (egcs-1.1.1) powerpc-ibm-aix4.2.1.0 gcc 1.2f ok (egcs-1.1.1) powerpc-ibm-aix4.1.5.0 gcc 1.2f ok (egcs-1.1.1) powerpc-ibm-aix4.1.5.0 gcc 1.2f NS (gcc-2.8.1) powerpc-ibm-aix4.1.4.0 gcc 1.0 ok powerpc-ibm-aix4.1.4.0 xlc 1.0i ok rs6000-ibm-aix4.1.5.0 gcc 1.2f ok (gcc-2.7.2) rs6000-ibm-aix4.1.4.0 gcc 1.2f ok (gcc-2.7.2) rs6000-ibm-aix3.2.5 gcc 1.0i ok rs6000-ibm-aix3.2.5 xlc 1.0i ok sparc-sun-solaris2.8 gcc 1.3e ok (1.913) (gcc-2.95.3 & native ld) sparc-sun-solaris2.7 gcc 1.3e ok (1.913) (gcc-2.95.3 & native ld) sparc-sun-solaris2.6 gcc 1.3e ok (1.913) (gcc-2.95.3 & native ld) sparc-sun-solaris2.5.1 gcc 1.3e ok (1.911) sparc-sun-solaris2.5 gcc 1.3b ok (egcs-1.1.2, GNU ld 2.9.1 & native ld) sparc-sun-solaris2.5 cc 1.3b ok (SC 3.0.1) sparc-sun-solaris2.4 gcc 1.0a ok sparc-sun-solaris2.4 cc 1.0a ok sparc-sun-solaris2.3 gcc 1.2f ok sparc-sun-sunos4.1.4 gcc 1.2f ok sparc-sun-sunos4.1.4 cc 1.0f ok sparc-sun-sunos4.1.3_U1 gcc 1.2f ok sparc-sun-sunos4.1.3C gcc 1.2f ok sparc-sun-sunos4.1.3 gcc 1.3b ok (egcs-1.1.2, GNU ld 2.9.1 & native ld) sparc-sun-sunos4.1.3 cc 1.3b ok sparc-unknown-bsdi4.0 gcc 1.2c ok sparc-unknown-linux-gnulibc1 gcc 1.2f ok sparc-unknown-linux-gnu gcc 1.3b ok (egcs-1.1.2, GNU ld 2.9.1.0.23) sparc64-unknown-linux-gnu gcc 1.2f ok Notes: - "ok" means "all tests passed". - "NS" means "Not Shared", but OK for static libraries
Note: The vendor-distributed HP-UX sed
(1) programs are horribly
broken, and cannot handle libtool’s requirements, so users may report
unusual problems. There is no workaround except to install a working
sed
(such as GNU sed
) on these systems.
Note: The vendor-distributed NCR MP-RAS cc
programs emits
copyright on standard error that confuse tests on size of
conftest.err. The workaround is to specify CC
when run configure
with CC='cc -Hnocopyr'.
This section is dedicated to the sanity of the libtool maintainers. It describes the programs that libtool uses, how they vary from system to system, and how to test for them.
Because libtool is a shell script, it can be difficult to understand just by reading it from top to bottom. This section helps show why libtool does things a certain way. Combined with the scripts themselves, you should have a better sense of how to improve libtool, or write your own.
The following is a list of valuable documentation references:
The only compiler characteristics that affect libtool are the flags needed (if any) to generate PIC objects. In general, if a C compiler supports certain PIC flags, then any derivative compilers support the same flags. Until there are some noteworthy exceptions to this rule, this section will document only C compilers.
The following C compilers have standard command line options, regardless of the platform:
gcc
This is the GNU C compiler, which is also the system compiler for many free operating systems (FreeBSD, GNU/Hurd, GNU/Linux, Lites, NetBSD, and OpenBSD, to name a few).
The -fpic or -fPIC flags can be used to generate position-independent code. -fPIC is guaranteed to generate working code, but the code is slower on m68k, m88k, and SPARC chips. However, using -fpic on those chips imposes arbitrary size limits on the shared libraries.
The rest of this subsection lists compilers by the operating system that they are bundled with:
aix3*
aix4*
Most AIX compilers have no PIC flags, since AIX (with the exception of AIX for IA-64) runs on PowerPC and RS/6000 chips. 14
hpux10*
Use ‘+Z’ to generate PIC.
osf3*
Digital/UNIX 3.x does not have PIC flags, at least not on the PowerPC platform.
solaris2*
Use -KPIC to generate PIC.
sunos4*
Use -PIC to generate PIC.
On all known systems, a reloadable object can be created by running ld -r -o output.o input1.o input2.o. This reloadable object may be treated as exactly equivalent to other objects.
On most modern platforms the order where dependent libraries are listed has no effect on object generation. In theory, there are platforms that require libraries that provide missing symbols to other libraries to be listed after those libraries whose symbols they provide.
Particularly, if a pair of static archives each resolve some of the other’s symbols, it might be necessary to list one of those archives both before and after the other one. Libtool does not currently cope with this situation well, since duplicate libraries are removed from the link line by default. Libtool provides the command line option --preserve-dup-deps to preserve all duplicate dependencies in cases where it is necessary.
On all known systems, building a static library can be accomplished by running ar cr libname.a obj1.o obj2.o …, where the .a file is the output library, and each .o file is an object file.
On all known systems, if there is a program named ranlib
, then it
must be used to “bless” the created library before linking against it,
with the ranlib libname.a command. Some systems, like Irix,
use the ar ts
command, instead.
Most build systems support the ability to compile libraries and applications on one platform for use on a different platform, provided a compiler capable of generating the appropriate output is available. In such cross compiling scenarios, the platform where the libraries or applications are compiled is called the build platform, while the platform where the libraries or applications are intended to be used or executed is called the host platform. The GNU Build System in The Automake Manual, of which libtool is a part, supports cross compiling via arguments passed to the configure script: --build=... and --host=.... However, when the build platform and host platform are very different, libtool is required to make certain accommodations to support these scenarios.
In most cases, because the build platform and host platform differ, the cross-compiled libraries and executables can’t be executed or tested on the build platform where they were compiled. The testsuites of most build systems will often skip any tests that involve executing such foreign executables when cross-compiling. However, if the build platform and host platform are sufficiently similar, it is often possible to run cross-compiled applications. Libtool’s own testsuite often attempts to execute cross-compiled tests, but will mark any failures as skipped since the failure might simply be due to the differences between the two platforms.
In addition to cases where the host platform and build platform are extremely similar (e.g. ‘i586-pc-linux-gnu’ and ‘i686-pc-linux-gnu’), there is another case where cross-compiled host applications may be executed on the build platform. This is possible when the build platform supports an emulation or API-enhanced environment for the host platform. One example of this situation would be if the build platform were MinGW, and the host platform were Cygwin (or vice versa). Both of these platforms can actually operate within a single Windows instance, so Cygwin applications can be launched from a MinGW context, and vice versa—provided certain care is taken. Another example would be if the build platform were GNU/Linux on an x86 32bit processor, and the host platform were MinGW. In this situation, the Wine environment can be used to launch Windows applications from the GNU/Linux operating system; again, provided certain care is taken.
One particular issue occurs when a Windows platform such as MinGW, Cygwin, or MSYS is the host or build platform, while the other platform is a Unix-style system. In these cases, there are often conflicts between the format of the file names and paths expected within host platform libraries and executables, and those employed on the build platform.
This situation is best described using a concrete example: suppose the build platform is GNU/Linux with canonical triplet ‘i686-pc-linux-gnu’. Suppose further that the host platform is MinGW with canonical triplet ‘i586-pc-mingw32’. On the GNU/Linux platform there is a cross compiler following the usual naming conventions of such compilers, where the compiler name is prefixed by the host canonical triplet (or suitable alias). (For more information concerning canonical triplets and platform aliases, see Specifying Target Triplets in The Autoconf Manual and Canonicalizing in The Autoconf Manual) In this case, the C compiler is named ‘i586-pc-mingw32-gcc’.
As described in Wrapper executables for uninstalled programs, for the MinGW host platform libtool
uses a wrapper executable to set various environment variables before launching
the actual program executable. Like the program executable, the wrapper
executable is cross-compiled for the host platform (that is, for MinGW). As
described above, ordinarily a host platform executable cannot be executed on
the build platform, but in this case the Wine environment could be used to
launch the MinGW application from GNU/Linux. However, the wrapper executable,
as a host platform (MinGW) application, must set the PATH
variable so
that the true application’s dependent libraries can be located—but the
contents of the PATH
variable must be structured for MinGW. Libtool
must use the Wine file name mapping facilities to determine the correct value
so that the wrapper executable can set the PATH
variable to point to the
correct location.
For example, suppose we are compiling an application in /var/tmp on GNU/Linux, using separate source code and build directories:
/var/tmp/foo-1.2.3/app/ | (application source code) |
/var/tmp/foo-1.2.3/lib/ | (library source code) |
/var/tmp/BUILD/app/ | (application build objects here) |
/var/tmp/BUILD/lib/ | (library build objects here) |
Since the library will be built in /var/tmp/BUILD/lib, the wrapper
executable (which will be in /var/tmp/BUILD/app) must add that
directory to PATH
(actually, it must add the directory named
objdir under /var/tmp/BUILD/lib, but we’ll ignore that detail
for now). However, Windows does not have a concept of Unix-style file or
directory names such as /var/tmp/BUILD/lib. Therefore, Wine provides
a mapping from Windows file names such as C:\Program Files to specific
Unix-style file names. Wine also provides a utility that can be used to map
Unix-style file names to Windows file names.
In this case, the wrapper executable should actually add the value
Z:\var\tmp\BUILD\lib
to the PATH
. libtool contains support for path conversions of this
type, for a certain limited set of build and host platform combinations. In
this case, libtool will invoke Wine’s winepath
utility to ensure that
the correct PATH
value is used. See File name conversion.
In certain situations, libtool must convert file names and paths between formats appropriate to different platforms. Usually this occurs when cross-compiling, and affects only the ability to launch host platform executables on the build platform using an emulation or API-enhancement environment such as Wine. Failure to convert paths (see File Name Conversion Failure) will cause a warning to be issued, but rarely causes the build to fail—and should have no effect on the compiled products, once installed properly on the host platform. For more information, see Cross compiling.
However, file name conversion may also occur in another scenario: when using a Unix emulation system on Windows (such as Cygwin or MSYS), combined with a native Windows compiler such as MinGW or MSVC. Only a limited set of such scenarios are currently supported; in other cases file name conversion is skipped. The lack of file name conversion usually means that uninstalled executables can’t be launched, but only rarely causes the build to fail (see File Name Conversion Failure).
libtool supports file name conversion in the following scenarios:
build platform | host platform | Notes |
---|---|---|
MinGW (MSYS) | MinGW (Windows) | see Native MinGW File Name Conversion |
Cygwin | MinGW (Windows) | see Cygwin/Windows File Name Conversion |
Unix + Wine | MinGW (Windows) | Requires Wine. See Unix/Windows File Name Conversion. |
MinGW (MSYS) | Cygwin | Requires LT_CYGPATH . See LT_CYGPATH. Provided for testing
purposes only. |
Unix + Wine | Cygwin | Requires both Wine and LT_CYGPATH , but does not yet work with
Cygwin 1.7.7 and Wine-1.2.
See Unix/Windows File Name Conversion, and LT_CYGPATH. |
In most cases, file name conversion is not needed or attempted. However, when libtool detects that a specific combination of build and host platform does require file name conversion, it is possible that the conversion may fail. In these cases, you may see a warning such as the following:
Could not determine the host file name corresponding to `... a file name ...' Continuing, but uninstalled executables may not work.
or
Could not determine the host path corresponding to `... a path ...' Continuing, but uninstalled executables may not work.
This should not cause the build to fail. At worst, it means that the wrapper executable will specify file names or paths appropriate for the build platform. Since those are not appropriate for the host platform, the uninstalled executables would not operate correctly, even when the wrapper executable is launched via the appropriate emulation or API-enhancement (e.g. Wine). Simply install the executables on the host platform, and execute them there.
MSYS is a Unix emulation environment for Windows, and is specifically designed such that in normal usage it pretends to be MinGW or native Windows, but understands Unix-style file names and paths, and supports standard Unix tools and shells. Thus, “native” MinGW builds are actually an odd sort of cross-compile, from an MSYS Unix emulation environment “pretending” to be MinGW, to actual native Windows.
When an MSYS shell launches a native Windows executable (as opposed to other
MSYS executables), it uses a system of heuristics to detect any
command-line arguments that contain file names or paths. It automatically
converts these file names from the MSYS (Unix-like) format, to the
corresponding Windows file name, before launching the executable. However,
this auto-conversion facility is only available when using the MSYS runtime
library. The wrapper executable itself is a MinGW application (that is, it
does not use the MSYS runtime library). The wrapper executable must set
PATH
to, and call _spawnv
with, values that have already been
converted from MSYS format to Windows. Thus, when libtool writes the source
code for the wrapper executable, it must manually convert MSYS paths to
Windows format, so that the Windows values can be hard-coded into the wrapper
executable.
Cygwin provides a Unix emulation environment for Windows. As part of that
emulation, it provides a file system mapping that presents the Windows file
system in a Unix-compatible manner. Cygwin also provides a utility
cygpath
that can be used to convert file names and paths between
the two representations. In a correctly configured Cygwin installation,
cygpath
is always present, and is in the PATH
.
Libtool uses cygpath
to convert from Cygwin (Unix-style) file names
and paths to Windows format when the build platform is Cygwin and the host
platform is MinGW.
When the host platform is Cygwin, but the build platform is MSYS or some Unix
system, libtool also uses cygpath
to convert from Windows to Cygwin
format (after first converting from the build platform format to Windows format;
See Native MinGW File Name Conversion, and
Unix/Windows File Name Conversion.) Because the build platform is not
Cygwin, cygpath
is not (and should not be) in the PATH
.
Therefore, in this configuration the environment variable LT_CYGPATH
is
required. See LT_CYGPATH.
Wine provides an interpretation environment for
some Unix platforms where Windows applications can be executed. It provides
a mapping between the Unix file system and a virtual Windows file system used
by the Windows programs. For the file name conversion to work, Wine must be
installed and properly configured on the build platform, and the
winepath
application must be in the build platform’s PATH
. In
addition, on 32bit GNU/Linux it is usually helpful if the binfmt extension is
enabled.
For some cross-compile configurations (where the host platform is Cygwin), the
cygpath
program is used to convert file names from the build platform
notation to the Cygwin form (technically, this conversion is from Windows
notation to Cygwin notation; the conversion from the build platform format
to Windows notation is performed via other means). However, because the
cygpath
program is not (and should not be) in the PATH
on
the build platform, LT_CYGPATH
must specify the full build platform
file name (that is, the full Unix or MSYS file name) of the cygpath
program.
The reason cygpath
should not be in the build platform PATH
is
twofold: first, cygpath
is usually installed in the same directory as
many other Cygwin executables, such as sed
, cp
, etc. If
the build platform environment had this directory in its PATH
, then these
Cygwin versions of common Unix utilities might be used in preference to the
ones provided by the build platform itself, with deleterious effects. Second,
especially when Cygwin-1.7 or later is used, multiple Cygwin installations can
coexist within the same Windows instance. Each installation will have separate
“mount tables” specified in CYGROOT-N/etc/fstab. These
mount tables control how that instance of Cygwin will map Windows file
names and paths to Cygwin form. Each installation’s cygpath
utility
automatically deduces the appropriate /etc/fstab file. Since each
CYGROOT-N/etc/fstab mount table may specify different mappings, it
matters what cygpath
is used.
Note that cygpath
is a Cygwin application; to execute this tool from
Unix requires a working and properly configured Wine installation, as well
as enabling the GNU/Linux binfmt
extension. Furthermore, the Cygwin
setup.exe
tool should have been used, via Wine, to properly install
Cygwin into the Wine file system (and registry).
Unfortunately, Wine support for Cygwin is intermittent. Recent releases of
Cygwin (1.7 and above) appear to require more Windows API support than Wine
provides (as of Wine version 1.2); most Cygwin applications fail to execute.
This includes cygpath
itself. Hence, it is best not to use
the LT_CYGPATH machinery in libtool when performing Unix to Cygwin
cross-compiles. Similarly, it is best not to enable the GNU/Linux binfmt
support in this configuration, because while Wine will fail to execute the
compiled Cygwin applications, it will still exit with status zero. This tends
to confuse build systems and test suites (including libtool’s own testsuite,
resulting in spurious reported failures). Wine support for the older
Cygwin-1.5 series appears satisfactory, but the Cygwin team no longer supports
Cygwin-1.5. It is hoped that Wine will eventually be improved such that
Cygwin-1.7 will again operate correctly under Wine. Until then, libtool will
report warnings as described in see File Name Conversion Failure in these
scenarios.
However, LT_CYGPATH
is also used for the MSYS to Cygwin cross compile
scenario, and operates as expected.
There are actually three different scenarios that could all legitimately be called a “Cygwin to MinGW” cross compile. The current (and standard) definition is when there is a compiler that produces native Windows libraries and applications, but which itself is a Cygwin application, just as would be expected in any other cross compile setup.
However, historically there were two other definitions, which we will refer to as the fake one, and the lying one.
In the fake Cygwin to MinGW cross compile case, you actually use a native MinGW compiler, but you do so from within a Cygwin environment:
export PATH="/c/MinGW/bin:${PATH}" configure --build=i686-pc-cygwin \ --host=mingw32 \ NM=/c/MinGW/bin/nm.exe
In this way, the build system “knows” that you are cross compiling, and the
file name conversion logic will be used. However, because the tools
(mingw32-gcc
, nm
, ar
) used are actually native
Windows applications, they will not understand any Cygwin (that is, Unix-like)
absolute file names passed as command line arguments (and, unlike MSYS, Cygwin
does not automatically convert such arguments). However, so long as only
relative file names are used in the build system, and non-Windows-supported
Unix idioms such as symlinks and mount points are avoided, this scenario should
work.
If you must use absolute file names, you will have to force Libtool to convert file names for the toolchain in this case, by doing the following before you run configure:
export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32
In the lying Cygwin to MinGW cross compile case, you lie to the build system:
export PATH="/c/MinGW/bin:${PATH}" configure --build=i686-pc-mingw32 \ --host=i686-pc-mingw32 \ --disable-dependency-tracking
and claim that the build platform is MinGW, even though you are actually running under Cygwin and not MinGW. In this case, libtool does not know that you are performing a cross compile, and thinks instead that you are performing a native MinGW build. However, as described in (see Native MinGW File Name Conversion), that scenario triggers an “MSYS to Windows” file name conversion. This, of course, is the wrong conversion since we are actually running under Cygwin. Also, the toolchain is expecting Windows file names (not Cygwin) but unless told so Libtool will feed Cygwin file names to the toolchain in this case. To force the correct file name conversions in this situation, you should do the following before running configure:
export lt_cv_to_host_file_cmd=func_convert_file_cygwin_to_w32 export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32
Note that this relies on internal implementation details of libtool, and
is subject to change. Also, --disable-dependency-tracking
is required,
because otherwise the MinGW GCC will generate dependency files that contain
Windows file names. This, in turn, will confuse the Cygwin make
program, which does not accept Windows file names:
Makefile:1: *** target pattern contains no `%'. Stop.
There have also always been a number of other details required for the lying case to operate correctly, such as the use of so-called identity mounts:
# cygwin-root/etc/fstab D:/foo /foo some_fs binary 0 0 D:/bar /bar some_fs binary 0 0 E:/grill /grill some_fs binary 0 0
In this way, top-level directories of each drive are available using identical names within Cygwin.
Note that you also need to ensure that the standard Unix directories
(like /bin, /lib, /usr, /etc) appear in the root
of a drive. This means that you must install Cygwin itself into the C:/
root directory (or D:/, or E:/, etc)—instead of the
recommended installation into C:/cygwin/. In addition, all file names
used in the build system must be relative, symlinks should not be used within
the source or build directory trees, and all -M* options to
gcc
except -MMD must be avoided.
This is quite a fragile setup, but it has been in historical use, and so is documented here.
This topic describes a couple of ways to portably create Windows Dynamic Link Libraries (DLLs). Libtool knows how to create DLLs using GNU tools and using Microsoft tools.
A typical library has a “hidden” implementation with an interface described in a header file. On just about every system, the interface could be something like this:
Example foo.h:
#ifndef FOO_H #define FOO_H int one (void); int two (void); extern int three; #endif /* FOO_H */
And the implementation could be something like this:
Example foo.c:
#include "foo.h" int one (void) { return 1; } int two (void) { return three - one (); } int three = 3;
When using contemporary GNU tools to create the Windows DLL, the above
code will work there too, thanks to its auto-import/auto-export
features. But that is not the case when using older GNU tools or perhaps
more interestingly when using proprietary tools. In those cases the code
will need additional decorations on the interface symbols with
__declspec(dllimport)
and __declspec(dllexport)
depending
on whether the library is built or it’s consumed and how it’s built and
consumed. However, it should be noted that it would have worked also
with Microsoft tools, if only the variable three
hadn’t been
there, due to the fact the Microsoft tools will automatically import
functions (but sadly not variables) and Libtool will automatically export
non-static symbols as described next.
With Microsoft tools, Libtool digs through the object files that make up
the library, looking for non-static symbols to automatically export.
I.e., Libtool with Microsoft tools tries to mimic the auto-export feature
of contemporary GNU tools. It should be noted that the GNU auto-export
feature is turned off when an explicit __declspec(dllexport)
is
seen. The GNU tools do this to not make more symbols visible for projects
that have already taken the trouble to decorate symbols. There is no
similar way to limit what symbols are visible in the code when Libtool
is using Microsoft tools. In order to limit symbol visibility in that
case you need to use one of the options -export-symbols or
-export-symbols-regex.
No matching help with auto-import is provided by Libtool, which is why variables must be decorated to import them from a DLL for everything but contemporary GNU tools. As stated above, functions are automatically imported by both contemporary GNU tools and Microsoft tools, but for other proprietary tools the auto-import status of functions is unknown.
When the objects that form the library are built, there are generally
two copies built for each object. One copy is used when linking the DLL
and one copy is used for the static library. On Windows systems, a pair
of defines are commonly used to discriminate how the interface symbols
should be decorated. The first define is ‘-DDLL_EXPORT’, which is
automatically provided by Libtool when libtool
builds the copy
of the object that is destined for the DLL. The second define is
‘-DLIBFOO_BUILD’ (or similar), which is often added by the package
providing the library and is used when building the library, but not
when consuming the library.
However, the matching double compile is not performed when consuming libraries. It is therefore not possible to reliably distinguish if the consumer is importing from a DLL or if it is going to use a static library.
With contemporary GNU tools, auto-import often saves the day, but see the GNU ld documentation and its --enable-auto-import option for some corner cases when it does not (see Options specific to i386 PE targets in Using ld, the GNU linker).
With Microsoft tools you typically get away with always compiling the code such that variables are expected to be imported from a DLL and functions are expected to be found in a static library. The tools will then automatically import the function from a DLL if that is where they are found. If the variables are not imported from a DLL as expected, but are found in a static library that is otherwise pulled in by some function, the linker will issue a warning (LNK4217) that a locally defined symbol is imported, but it still works. In other words, this scheme will not work to only consume variables from a library. There is also a price connected to this liberal use of imports in that an extra indirection is introduced when you are consuming the static version of the library. That extra indirection is unavoidable when the DLL is consumed, but it is not needed when consuming the static library.
For older GNU tools and other proprietary tools there is no generic way to make it possible to consume either of the DLL or the static library without user intervention, the tools need to be told what is intended. One common assumption is that if a DLL is being built (‘DLL_EXPORT’ is defined) then that DLL is going to consume any dependent libraries as DLLs. If that assumption is made everywhere, it is possible to select how an end-user application is consuming libraries by adding a single flag ‘-DDLL_EXPORT’ when a DLL build is required. This is of course an all or nothing deal, either everything as DLLs or everything as static libraries.
To sum up the above, the header file of the foo library needs to be changed into something like this:
Modified foo.h:
#ifndef FOO_H #define FOO_H #if defined _WIN32 && !defined __GNUC__ # ifdef LIBFOO_BUILD # ifdef DLL_EXPORT # define LIBFOO_SCOPE __declspec (dllexport) # define LIBFOO_SCOPE_VAR extern __declspec (dllexport) # endif # elif defined _MSC_VER # define LIBFOO_SCOPE # define LIBFOO_SCOPE_VAR extern __declspec (dllimport) # elif defined DLL_EXPORT # define LIBFOO_SCOPE __declspec (dllimport) # define LIBFOO_SCOPE_VAR extern __declspec (dllimport) # endif #endif #ifndef LIBFOO_SCOPE # define LIBFOO_SCOPE # define LIBFOO_SCOPE_VAR extern #endif LIBFOO_SCOPE int one (void); LIBFOO_SCOPE int two (void); LIBFOO_SCOPE_VAR int three; #endif /* FOO_H */
When the targets are limited to contemporary GNU tools and Microsoft tools, the above can be simplified to the following:
Simplified foo.h:
#ifndef FOO_H #define FOO_H #if defined _WIN32 && !defined __GNUC__ && !defined LIBFOO_BUILD # define LIBFOO_SCOPE_VAR extern __declspec (dllimport) #else # define LIBFOO_SCOPE_VAR extern #endif int one (void); int two (void); LIBFOO_SCOPE_VAR int three; #endif /* FOO_H */
This last simplified version can of course only work when Libtool is used to build the DLL, as no symbols would be exported otherwise (i.e., when using Microsoft tools).
It should be noted that there are various projects that attempt to relax these requirements by various low level tricks, but they are not discussed here. Examples are FlexDLL and edll.
libtool
script contents ¶Since version 1.4, the libtool
script is generated by
configure
(see Configuring libtool). In earlier versions,
configure
achieved this by calling a helper script called
ltconfig. From libtool version 0.7 to 1.0, this script
simply set shell variables, then sourced the libtool backend,
ltmain.sh
. ltconfig
from libtool version 1.1 through 1.3
inlined the contents of ltmain.sh
into the generated
libtool
, which improved performance on many systems. The tests
that ltconfig used to perform are now kept in libtool.m4
where they can be written using Autoconf. This has the runtime
performance benefits of inlined ltmain.sh
, and improves
the build time a little while considerably easing the amount of raw
shell code that used to need maintaining.
The convention used for naming variables that hold shell commands for
delayed evaluation, is to use the suffix _cmd
where a single
line of valid shell script is needed, and the suffix _cmds
where
multiple lines of shell script may be delayed for later
evaluation. By convention, _cmds
variables delimit the
evaluation units with the ~
character where necessary.
Here is a listing of each of the configuration variables, and how they
are used within ltmain.sh
(see Configuring libtool):
The name of the system library archiver.
The name of the compiler used to configure libtool. This will always contain the compiler for the current language (see Tags).
An echo
program that does not interpret backslashes as an
escape character. It may be given only one argument, so due quoting
is necessary.
The name of the linker that libtool should use internally for reloadable linking and possibly shared libraries.
The name of the C compiler and C compiler flags used to configure libtool.
The name of a BSD- or MS-compatible program that produces listings of
global symbols.
For BSD nm
, the symbols should be in one the following formats:
address C global-variable-name address D global-variable-name address T global-function-name
For MS dumpbin
, the symbols should be in one of the following
formats:
counter size UNDEF notype External | global-var counter address section notype External | global-var counter address section notype () External | global-func
The size of the global variables are not zero and the section of the global functions are not "UNDEF". Symbols in "pick any" sections ("pick any" appears in the section header) are not global either.
Set to the name of the ranlib
program, if any.
The flag that is used by ‘archive_cmds’ to declare that there will be unresolved symbols in the resulting shared library. Empty, if no such flag is required. Set to ‘unsupported’ if there is no way to generate a shared library with references to symbols that aren’t defined in that library.
Whether libtool should automatically generate a list of exported symbols
using export_symbols_cmds
before linking an archive.
Set to ‘yes’ or ‘no’. Default is ‘no’.
Commands used to create shared libraries, shared libraries with -export-symbols and static libraries, respectively.
Specify filename containing input files for AR
.
If the shared library depends on a static library, ‘old_archive_from_new_cmds’ contains the commands used to create that static library. If this variable is not empty, ‘old_archive_cmds’ is not used.
If a static library must be created from the export symbol list to
correctly link with a shared library, ‘old_archive_from_expsyms_cmds’
contains the commands needed to create that static library. When these
commands are executed, the variable soname
contains the name of the
shared library in question, and the ‘$objdir/$newlib’ contains the
path of the static library these commands should build. After executing
these commands, libtool will proceed to link against ‘$objdir/$newlib’
instead of soname
.
Set to ‘yes’ if the extraction of a static library requires locking the library file. This is required on Darwin.
Set to the specified and canonical names of the system that libtool was built on.
Whether libtool should build shared libraries on this system. Set to ‘yes’ or ‘no’.
Whether libtool should build static libraries on this system. Set to ‘yes’ or ‘no’.
Whether the compiler supports the -c and -o options simultaneously. Set to ‘yes’ or ‘no’.
Whether the compiler has to see an object listed on the command line in order to successfully invoke the linker. If ‘no’, then a set of convenience archives or a set of object file names can be passed via linker-specific options or linker scripts.
Whether dlopen
is supported on the platform.
Set to ‘yes’ or ‘no’.
Whether it is possible to dlopen
the executable itself.
Set to ‘yes’ or ‘no’.
Whether it is possible to dlopen
the executable itself, when it
is linked statically (-all-static). Set to ‘yes’ or
‘no’.
List of symbols that should not be listed in the preloaded symbols.
Compiler link flag that allows a dlopened shared library to reference symbols that are defined in the program.
Commands to extract exported symbols from libobjs
to the
file export_symbols
.
Commands to extract the exported symbols list from a shared library. These commands are executed if there is no file ‘$objdir/$soname-def’, and should write the names of the exported symbols to that file, for the use of ‘old_archive_from_expsyms_cmds’.
Determines whether libtool will privilege the installer or the
developer. The assumption is that installers will seldom run programs
in the build tree, and the developer will seldom install. This is only
meaningful on platforms where shlibpath_overrides_runpath
is
not ‘yes’, so fast_install
will be set to ‘needless’ in
this case. If fast_install
set to ‘yes’, libtool will create
programs that search for installed libraries, and, if a program is run
in the build tree, a new copy will be linked on-demand to use the
yet-to-be-installed libraries. If set to ‘no’, libtool will create
programs that use the yet-to-be-installed libraries, and will link
a new copy of the program at install time. The default value is
‘yes’ or ‘needless’, depending on platform and configuration
flags, and it can be turned from ‘yes’ to ‘no’ with the
configure flag --disable-fast-install.
On some systems, the linker always hardcodes paths to dependent libraries
into the output. In this case, fast_install
is never set to ‘yes’,
and relinking at install time is triggered. This also means that DESTDIR
installation does not work as expected.
How to find potential files when deplibs_check_method
is
‘file_magic’. file_magic_glob
is a sed
expression,
and the sed
instance is fed potential file names that are
transformed by the file_magic_glob
expression. Useful when the
shell does not support the shell option nocaseglob
, making
want_nocaseglob
inappropriate. Normally disabled (i.e.
file_magic_glob
is empty).
Commands to tell the dynamic linker how to find shared libraries in a specific directory.
Same as finish_cmds
, except the commands are not displayed.
A pipeline that takes the output of NM
, and produces a listing of
raw symbols followed by their C names. For example:
$ eval "$NM progname | $global_symbol_pipe" D symbol1 C-symbol1 T symbol2 C-symbol2 C symbol3 C-symbol3 ... $
The first column contains the symbol type (used to tell data from code) but its meaning is system dependent.
A pipeline that translates the output of global_symbol_pipe
into
proper C declarations. Since some platforms, such as HP/UX, have
linkers that differentiate code from data, data symbols are declared
as data, and code symbols are declared as functions.
Either ‘immediate’ or ‘relink’, depending on whether shared library paths can be hardcoded into executables before they are installed, or if they need to be relinked.
Set to ‘yes’ or ‘no’, depending on whether the linker
hardcodes directories if a library is directly specified on the command
line (such as ‘dir/libname.a’) when
hardcode_libdir_flag_spec
is specified.
Some architectures hardcode "absolute" library directories that cannot
be overridden by shlibpath_var
when hardcode_direct
is
‘yes’. In that case set hardcode_direct_absolute
to
‘yes’, or otherwise ‘no’.
Whether the platform supports hardcoding of run-paths into libraries. If enabled, linking of programs will be much simpler but libraries will need to be relinked during installation. Set to ‘yes’ or ‘no’.
Flag to hardcode a libdir
variable into a binary, so that the
dynamic linker searches libdir
for shared libraries at runtime.
If it is empty, libtool will try to use some other hardcoding mechanism.
If the compiler only accepts a single hardcode_libdir_flag
, then
this variable contains the string that should separate multiple
arguments to that flag.
Set to ‘yes’ or ‘no’, depending on whether the linker
hardcodes directories specified by -L flags into the resulting
executable when hardcode_libdir_flag_spec
is specified.
Set to ‘yes’ or ‘no’, depending on whether the linker
hardcodes directories by writing the contents of ‘$shlibpath_var’
into the resulting executable when hardcode_libdir_flag_spec
is
specified. Set to ‘unsupported’ if directories specified by
‘$shlibpath_var’ are searched at run time, but not at link time.
Set to the specified and canonical names of the system that libtool was configured for.
List of symbols that must always be exported when using export_symbols
.
Whether the linker adds runtime paths of dependency libraries to the runtime path list, requiring libtool to relink the output when installing. Set to ‘yes’ or ‘no’. Default is ‘no’.
Permission mode override for installation of shared libraries. If the
runtime linker fails to load libraries with wrong permissions, then it
may fail to execute programs that are needed during installation,
because these need the library that has just been installed. In this
case, it is necessary to pass the mode to install
with
-m install_override_mode.
The standard old archive suffix (normally ‘a’).
The format of a library name prefix. On all Unix systems, static libraries are called ‘libname.a’, but on some systems (such as OS/2 or MS-DOS), the library is just called ‘name.a’.
A list of shared library names. The first is the name of the file, the rest are symbolic links to the file. The name in the list is the file name that the linker finds when given -lname.
Whether libtool must link a program against all its dependency libraries. Set to ‘yes’ or ‘no’. Default is ‘unknown’, which is a synonym for ‘yes’.
Linker flag (passed through the C compiler) used to prevent dynamic linking.
The release and revision from which the libtool.m4 macros were
taken. This is used to ensure that macros and ltmain.sh
correspond to the same Libtool version.
The approximate longest command line that can be passed to ‘$SHELL’ without being truncated, as computed by ‘LT_CMD_MAX_LEN’.
Whether we can dlopen
modules without a ‘lib’ prefix.
Set to ‘yes’ or ‘no’. By default, it is ‘unknown’, which
means the same as ‘yes’, but documents that we are not really sure
about it. ‘no’ means that it is possible to dlopen
a
module without the ‘lib’ prefix.
Whether versioning is required for libraries, i.e. whether the dynamic linker requires a version suffix for all libraries. Set to ‘yes’ or ‘no’. By default, it is ‘unknown’, which means the same as ‘yes’, but documents that we are not really sure about it.
Whether files must be locked to prevent conflicts when compiling simultaneously. Set to ‘yes’ or ‘no’.
Specify filename containing input files for NM
.
Compiler flag to disable builtin functions that conflict with declaring
external global symbols as char
.
The flag that is used by ‘archive_cmds’ to declare that there will be no unresolved symbols in the resulting shared library. Empty, if no such flag is required.
The name of the directory that contains temporary libtool files.
The standard object file suffix (normally ‘o’).
Any additional compiler flags for building library object files.
Commands run after installing a shared or static library, respectively.
Commands run after uninstalling a shared or static library, respectively.
Commands necessary for finishing linking programs. postlink_cmds
are executed immediately after the program is linked. Any occurrence of
the string @OUTPUT@
in postlink_cmds
is replaced by the
name of the created executable (i.e. not the wrapper, if a wrapper is
generated) prior to execution. Similarly, @TOOL_OUTPUT@
is
replaced by the toolchain format of @OUTPUT@
. Normally disabled
(i.e. postlink_cmds
empty).
Commands to create a reloadable object. Set reload_cmds
to
‘false’ on systems that cannot create reloadable objects.
The environment variable that tells the linker what directories to hardcode in the resulting executable.
Indicates whether it is possible to override the hard-coded library
search path of a program with an environment variable. If this is set
to no, libtool may have to create two copies of a program in the build
tree, one to be installed and one to be run in the build tree only.
When each of these copies is created depends on the value of
fast_install
. The default value is ‘unknown’, which is
equivalent to ‘no’.
The environment variable that tells the dynamic linker where to find shared libraries.
The name coded into shared libraries, if different from the real name of the file.
Command to strip a shared (striplib
) or static (old_striplib
)
library, respectively. If these variables are empty, the strip flag
in the install mode will be ignored for libraries (see Install mode).
Expression to get the run-time system library search path. Directories that appear in this list are never hard-coded into executables.
Expression to get the compile-time system library search path. This
variable is used by libtool when it has to test whether a certain
library is shared or static. The directories listed in
shlibpath_var
are automatically appended to this list, every time
libtool runs (i.e., not at configuration time), because some linkers use
this variable to extend the library search path. Linker switches such
as -L also augment the search path.
Linker flag (passed through the C compiler) used to generate thread-safe libraries.
If the toolchain is not native to the build platform (e.g. if you are using MSYS to drive the scripting, but are using the MinGW native Windows compiler) this variable describes how to convert file names from the format used by the build platform to the format used by host platform. Normally set to ‘func_convert_file_noop’, libtool will autodetect most cases where other values should be used. On rare occasions, it may be necessary to override the autodetected value (see Cygwin to MinGW Cross).
If the toolchain is not native to the build platform (e.g. if you are using some Unix to drive the scripting together with a Windows toolchain running in Wine) this variable describes how to convert file names from the format used by the build platform to the format used by the toolchain. Normally set to ‘func_convert_file_noop’.
The library version numbering type. One of ‘libtool’, ‘freebsd-aout’, ‘freebsd-elf’, ‘irix’, ‘linux’, ‘osf’, ‘sunos’, ‘windows’, or ‘none’.
Find potential files using the shell option nocaseglob
, when
deplibs_check_method
is ‘file_magic’. Normally set to
‘no’. Set to ‘yes’ to enable the nocaseglob
shell
option when looking for potential file names in a case-insensitive
manner.
Compiler flag to generate shared objects from convenience archives.
The C compiler flag that allows libtool to pass a flag directly to the
linker. Used as: ${wl}some-flag
.
Variables ending in ‘_cmds’ or ‘_eval’ contain a
‘~’-separated list of commands that are eval
ed one after
another. If any of the commands return a nonzero exit status, libtool
generally exits with an error message.
Variables ending in ‘_spec’ are eval
ed before being used by
libtool.
Here are a few tricks that you can use to make maintainership easier:
ltmain.in
, I keep a permanent libtool
script in my
PATH
, which sources ltmain.in
directly.
The following steps describe how to create such a script, where
/home/src/libtool
is the directory containing the libtool source
tree, /home/src/libtool/libtool
is a libtool script that has been
configured for your platform, and ~/bin
is a directory in your
PATH
:
trick$ cd ~/bin trick$ sed 's%^\(macro_version=\).*$%\1@VERSION@%; s%^\(macro_revision=\).*$%\1@package_revision@%; /^# ltmain\.sh/q' /home/src/libtool/libtool > libtool trick$ echo '. /home/src/libtool/ltmain.in' >> libtool trick$ chmod +x libtool trick$ libtool --version ltmain.sh (GNU @PACKAGE@@TIMESTAMP@) @VERSION@ Copyright (C) 2014 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. trick$
The output of the final ‘libtool --version’ command shows that the
ltmain.in
script is being used directly. Now, modify
~/bin/libtool
or /home/src/libtool/ltmain.in
directly in
order to test new changes without having to rerun configure
.
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If you don’t
specify an rpath
, then libtool builds a libtool convenience
archive, not a shared library (see Linking static libraries).
However, you should avoid using -L or -l flags to link against an uninstalled libtool library. Just specify the relative path to the .la file, such as ../intl/libintl.la. This is a design decision to eliminate any ambiguity when linking against uninstalled shared libraries.
And why should we? main.o doesn’t directly depend on -lm after all.
Don’t strip static libraries though, or they will be unusable.
Since GNU Automake 1.5, the flags -dlopen or -dlpreopen (see Link mode) can be employed with the ‘program_LDADD’ variable. Unfortunately, older releases didn’t accept these flags, so if you are stuck with an ancient Automake, we recommend quoting the flag itself, and setting ‘program_DEPENDENCIES’ too:
program_LDADD = "-dlopen" libfoo.la program_DEPENDENCIES = libfoo.la
LT_INIT
requires
that you define the Makefile variable top_builddir
in your
Makefile.in. Automake does this automatically, but Autoconf
users should set it to the relative path to the top of your build
directory (../.., for example).
GNU Image Manipulation Program, for those who haven’t taken the plunge. See http://www.gimp.org/.
We used to recommend __P
,
__BEGIN_DECLS
and __END_DECLS
. This was bad advice since
symbols (even preprocessor macro names) that begin with an underscore
are reserved for the use of the compiler.
LIBPATH
on AIX, and SHLIB_PATH
on HP-UX.
Some platforms, notably Mac OS X,
differentiate between a runtime library that cannot be opened by
lt_dlopen
and a dynamic module that can. For maximum
portability you should try to ensure that you only pass
lt_dlopen
objects that have been compiled with libtool’s
-module flag.
This is used for
the host dependent module loading API – shl_load
and
LoadLibrary
for example
We used to recommend adding the contents of ltdl.m4 to
acinclude.m4, but with aclocal
from a modern
Automake (1.8 or newer) and this release of libltdl that is not only
unnecessary but makes it easy to forget to upgrade acinclude.m4
if you move to a different release of libltdl.
Even if libltdl is installed, ‘LTDL_INIT’ may fail to detect it if libltdl depends on symbols provided by libraries other than the C library.
All code compiled
for the PowerPC and RS/6000 chips (powerpc-*-*
, powerpcle-*-*
,
and rs6000-*-*
) is position-independent, regardless of the operating
system or compiler suite. So, “regular objects” can be used to build
shared libraries on these systems and no special PIC compiler flags are
required.