gperf Utility
gperf
gperf
gperf
gperf
This manual documents the GNU gperf perfect hash function generator
utility, focusing on its features and how to use them, and how to report
bugs.
High-Level Description of GNU gperf
Input Format to gperf
Declarations
Invoking gperf
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gperf Utilitygperf perfect hash function generator utility was
written in GNU C++ by Douglas C. Schmidt. The general
idea for the perfect hash function generator was inspired by Keith
Bostic's algorithm written in C, and distributed to net.sources around
1984. The current program is a heavily modified, enhanced, and extended
implementation of Keith's basic idea, created at the University of
California, Irvine. Bugs, patches, and suggestions should be reported
to <bug-gnu-gperf@gnu.org>.
In addition, Adam de Boor and Nels Olson provided many tips and insights
that greatly helped improve the quality and functionality of gperf.
gperf is a perfect hash function generator written in C++. It
transforms an n element user-specified keyword set W into a
perfect hash function F. F uniquely maps keywords in
W onto the range 0..k, where k >= n-1. If k
= n-1 then F is a minimal perfect hash function.
gperf generates a 0..k element static lookup table and a
pair of C functions. These functions determine whether a given
character string s occurs in W, using at most one probe into
the lookup table.
gperf currently generates the reserved keyword recognizer for
lexical analyzers in several production and research compilers and
language processing tools, including GNU C, GNU C++, GNU Java, GNU Pascal,
GNU Modula 3, and GNU indent. Complete C++ source code for gperf is
available from http://ftp.gnu.org/pub/gnu/gperf/.
A paper describing gperf's design and implementation in greater
detail is available in the Second USENIX C++ Conference proceedings
or from http://www.cs.wustl.edu/~schmidt/resume.html.
gperf
A static search structure is an Abstract Data Type with certain
fundamental operations, e.g., initialize, insert,
and retrieve. Conceptually, all insertions occur before any
retrievals. In practice, gperf generates a static array
containing search set keywords and any associated attributes specified
by the user. Thus, there is essentially no execution-time cost for the
insertions. It is a useful data structure for representing static
search sets. Static search sets occur frequently in software system
applications. Typical static search sets include compiler reserved
words, assembler instruction opcodes, and built-in shell interpreter
commands. Search set members, called keywords, are inserted into
the structure only once, usually during program initialization, and are
not generally modified at run-time.
Numerous static search structure implementations exist, e.g., arrays, linked lists, binary search trees, digital search tries, and hash tables. Different approaches offer trade-offs between space utilization and search time efficiency. For example, an n element sorted array is space efficient, though the average-case time complexity for retrieval operations using binary search is proportional to log n. Conversely, hash table implementations often locate a table entry in constant time, but typically impose additional memory overhead and exhibit poor worst case performance.
Minimal perfect hash functions provide an optimal solution for a particular class of static search sets. A minimal perfect hash function is defined by two properties:
For most applications it is far easier to generate perfect hash
functions than minimal perfect hash functions. Moreover,
non-minimal perfect hash functions frequently execute faster than
minimal ones in practice. This phenomena occurs since searching a
sparse keyword table increases the probability of locating a “null”
entry, thereby reducing string comparisons. gperf's default
behavior generates near-minimal perfect hash functions for
keyword sets. However, gperf provides many options that permit
user control over the degree of minimality and perfection.
Static search sets often exhibit relative stability over time. For
example, Ada's 63 reserved words have remained constant for nearly a
decade. It is therefore frequently worthwhile to expend concerted
effort building an optimal search structure once, if it
subsequently receives heavy use multiple times. gperf removes
the drudgery associated with constructing time- and space-efficient
search structures by hand. It has proven a useful and practical tool
for serious programming projects. Output from gperf is currently
used in several production and research compilers, including GNU C, GNU
C++, GNU Java, GNU Pascal, and GNU Modula 3. The latter two compilers are
not yet part of the official GNU distribution. Each compiler utilizes
gperf to automatically generate static search structures that
efficiently identify their respective reserved keywords.
gperfThe perfect hash function generator gperf reads a set of
“keywords” from an input file (or from the standard input by
default). It attempts to derive a perfect hashing function that
recognizes a member of the static keyword set with at most a
single probe into the lookup table. If gperf succeeds in
generating such a function it produces a pair of C source code routines
that perform hashing and table lookup recognition. All generated C code
is directed to the standard output. Command-line options described
below allow you to modify the input and output format to gperf.
By default, gperf attempts to produce time-efficient code, with
less emphasis on efficient space utilization. However, several options
exist that permit trading-off execution time for storage space and vice
versa. In particular, expanding the generated table size produces a
sparse search structure, generally yielding faster searches.
Conversely, you can direct gperf to utilize a C switch
statement scheme that minimizes data space storage size. Furthermore,
using a C switch may actually speed up the keyword retrieval time
somewhat. Actual results depend on your C compiler, of course.
In general, gperf assigns values to the bytes it is using
for hashing until some set of values gives each keyword a unique value.
A helpful heuristic is that the larger the hash value range, the easier
it is for gperf to find and generate a perfect hash function.
Experimentation is the key to getting the most from gperf.
gperfYou can control the input file format by varying certain command-line
arguments, in particular the ‘-t’ option. The input's appearance
is similar to GNU utilities flex and bison (or UNIX
utilities lex and yacc). Here's an outline of the general
format:
declarations
%%
keywords
%%
functions
Unlike flex or bison, the declarations section and
the functions section are optional. The following sections describe the
input format for each section.
It is possible to omit the declaration section entirely, if the ‘-t’ option is not given. In this case the input file begins directly with the first keyword line, e.g.:
january
february
march
april
...
The keyword input file optionally contains a section for including
arbitrary C declarations and definitions, gperf declarations that
act like command-line options, as well as for providing a user-supplied
struct.
structIf the ‘-t’ option (or, equivalently, the ‘%struct-type’ declaration)
is enabled, you must provide a C struct as the last
component in the declaration section from the input file. The first
field in this struct must be of type char * or const char *
if the ‘-P’ option is not given, or of type int if the option
‘-P’ (or, equivalently, the ‘%pic’ declaration) is enabled.
This first field must be called ‘name’, although it is possible to modify
its name with the ‘-K’ option (or, equivalently, the
‘%define slot-name’ declaration) described below.
Here is a simple example, using months of the year and their attributes as input:
struct month { char *name; int number; int days; int leap_days; };
%%
january, 1, 31, 31
february, 2, 28, 29
march, 3, 31, 31
april, 4, 30, 30
may, 5, 31, 31
june, 6, 30, 30
july, 7, 31, 31
august, 8, 31, 31
september, 9, 30, 30
october, 10, 31, 31
november, 11, 30, 30
december, 12, 31, 31
Separating the struct declaration from the list of keywords and
other fields are a pair of consecutive percent signs, ‘%%’,
appearing left justified in the first column, as in the UNIX utility
lex.
If the struct has already been declared in an include file, it can
be mentioned in an abbreviated form, like this:
struct month;
%%
january, 1, 31, 31
...
The declaration section can contain gperf declarations. They
influence the way gperf works, like command line options do.
In fact, every such declaration is equivalent to a command line option.
There are three forms of declarations:
When a declaration is given both in the input file and as a command line option, the command-line option's value prevails.
The following gperf declarations are available.
struct type declaration for generated
code; see above for an example.
gperf to generate code in the language specified by the
option's argument. Languages handled are currently:
#define const to empty
for compilers which don't know about this keyword.
The default is ANSI-C.
struct.
Perfect_Hash.
isalnum and isgraph do
not guarantee that a byte is in this range. Only an explicit
test like ‘c >= 'A' && c <= 'Z'’ guarantees this.)
strcmp.
However, using ‘%compare-lengths’ might greatly increase the size of the
generated C code if the lookup table range is large (which implies that
the switch option ‘-S’ or ‘%switch’ is not enabled), since the length
table contains as many elements as there are entries in the lookup table.
strncmp function to perform
string comparisons. The default action is to use strcmp.
<jjc@ai.mit.edu>.
<string.h>, at the
beginning of the code. By default, this is not done; the user must
include this header file himself to allow compilation of the code.
TOTAL_KEYWORDS,
MIN_WORD_LENGTH, MAX_WORD_LENGTH, and so on. This option
permits the use of two hash tables in the same file, even when the option
‘-E’ (or, equivalently, the ‘%enum’ declaration) is not given or
the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration)
is given.
switch statement scheme,
rather than an array lookup table. This can lead to a reduction in both
time and space requirements for some input files. The argument to this
option determines how many switch statements are generated. A
value of 1 generates 1 switch containing all the elements, a
value of 2 generates 2 tables with 1/2 the elements in each
switch, etc. This is useful since many C compilers cannot
correctly generate code for large switch statements. This option
was inspired in part by Keith Bostic's original C program.
Using a syntax similar to GNU utilities flex and bison, it
is possible to directly include C source text and comments verbatim into
the generated output file. This is accomplished by enclosing the region
inside left-justified surrounding ‘%{’, ‘%}’ pairs. Here is
an input fragment based on the previous example that illustrates this
feature:
%{
#include <assert.h>
/* This section of code is inserted directly into the output. */
int return_month_days (struct month *months, int is_leap_year);
%}
struct month { char *name; int number; int days; int leap_days; };
%%
january, 1, 31, 31
february, 2, 28, 29
march, 3, 31, 31
...
The second input file format section contains lines of keywords and any associated attributes you might supply. A line beginning with ‘#’ in the first column is considered a comment. Everything following the ‘#’ is ignored, up to and including the following newline. A line beginning with ‘%’ in the first column is an option declaration and must not occur within the keywords section.
The first field of each non-comment line is always the keyword itself. It
can be given in two ways: as a simple name, i.e., without surrounding
string quotation marks, or as a string enclosed in double-quotes, in
C syntax, possibly with backslash escapes like \" or \234
or \xa8. In either case, it must start right at the beginning
of the line, without leading whitespace.
In this context, a “field” is considered to extend up to, but
not include, the first blank, comma, or newline. Here is a simple
example taken from a partial list of C reserved words:
# These are a few C reserved words, see the c.gperf file
# for a complete list of ANSI C reserved words.
unsigned
sizeof
switch
signed
if
default
for
while
return
Note that unlike flex or bison the first ‘%%’ marker
may be elided if the declaration section is empty.
Additional fields may optionally follow the leading keyword. Fields
should be separated by commas, and terminate at the end of line. What
these fields mean is entirely up to you; they are used to initialize the
elements of the user-defined struct provided by you in the
declaration section. If the ‘-t’ option (or, equivalently, the
‘%struct-type’ declaration) is not enabled
these fields are simply ignored. All previous examples except the last
one contain keyword attributes.
The optional third section also corresponds closely with conventions
found in flex and bison. All text in this section,
starting at the final ‘%%’ and extending to the end of the input
file, is included verbatim into the generated output file. Naturally,
it is your responsibility to ensure that the code contained in this
section is valid C.
indent.If you want to invoke GNU indent on a gperf input file,
you will see that GNU indent doesn't understand the ‘%%’,
‘%{’ and ‘%}’ directives that control gperf's
interpretation of the input file. Therefore you have to insert some
directives for GNU indent. More precisely, assuming the most
general input file structure
declarations part 1
%{
verbatim code
%}
declarations part 2
%%
keywords
%%
functions
you would insert ‘*INDENT-OFF*’ and ‘*INDENT-ON*’ comments as follows:
/* *INDENT-OFF* */
declarations part 1
%{
/* *INDENT-ON* */
verbatim code
/* *INDENT-OFF* */
%}
declarations part 2
%%
keywords
%%
/* *INDENT-ON* */
functions
gperf
Several options control how the generated C code appears on the standard
output. Two C functions are generated. They are called hash and
in_word_set, although you may modify their names with a command-line
option. Both functions require two arguments, a string, char *
str, and a length parameter, int len. Their default
function prototypes are as follows:
By default, the generated
hashfunction returns an integer value created by adding len to several user-specified str byte positions indexed into an associated values table stored in a local static array. The associated values table is constructed internally bygperfand later output as a static local C array called ‘hash_table’. The relevant selected positions (i.e. indices into str) are specified via the ‘-k’ option when runninggperf, as detailed in the Options section below (see Options).
If str is in the keyword set, returns a pointer to that keyword. More exactly, if the option ‘-t’ (or, equivalently, the ‘%struct-type’ declaration) was given, it returns a pointer to the matching keyword's structure. Otherwise it returns
NULL.
If the option ‘-c’ (or, equivalently, the ‘%compare-strncmp’ declaration) is not used, str must be a NUL terminated string of exactly length len. If ‘-c’ (or, equivalently, the ‘%compare-strncmp’ declaration) is used, str must simply be an array of len bytes and does not need to be NUL terminated.
The code generated for these two functions is affected by the following options:
struct.
switch statement rather than use a large,
(and potentially sparse) static array. Although the exact time and
space savings of this approach vary according to your C compiler's
degree of optimization, this method often results in smaller and faster
code.
If the ‘-t’ and ‘-S’ options (or, equivalently, the
‘%struct-type’ and ‘%switch’ declarations) are omitted, the default
action
is to generate a char * array containing the keywords, together with
additional empty strings used for padding the array. By experimenting
with the various input and output options, and timing the resulting C
code, you can determine the best option choices for different keyword
set characteristics.
By default, the code generated by gperf operates on zero
terminated strings, the usual representation of strings in C. This means
that the keywords in the input file must not contain NUL bytes,
and the str argument passed to hash or in_word_set
must be NUL terminated and have exactly length len.
If option ‘-c’ (or, equivalently, the ‘%compare-strncmp’
declaration) is used, then the str argument does not need
to be NUL terminated. The code generated by gperf will only
access the first len, not len+1, bytes starting at str.
However, the keywords in the input file still must not contain NUL
bytes.
If option ‘-l’ (or, equivalently, the ‘%compare-lengths’
declaration) is used, then the hash table performs binary
comparison. The keywords in the input file may contain NUL bytes,
written in string syntax as \000 or \x00, and the code
generated by gperf will treat NUL like any other byte.
Also, in this case the ‘-c’ option (or, equivalently, the
‘%compare-strncmp’ declaration) is ignored.
The identifiers of the functions, tables, and constants defined by the code
generated by gperf can be controlled through gperf declarations
or the equivalent command-line options. This is useful for three purposes:
For this purpose, just use the available declarations or options at will.
Assume you include code generated by gperf in a library, and to
avoid collisions with other libraries, you want to ensure that all exported
identifiers of this library start with a certain prefix.
By default, the only exported identifier is the lookup function. You can therefore use the option ‘-N’ (or, equivalently, the ‘%define lookup-function-name’ declaration).
When you use the option ‘-L C++’ (or, equivalently, the ‘%language=C++’ declaration), the only exported entity is a class. You control its name through the option ‘-Z’ (or, equivalently, the ‘%define class-name’ declaration).
gperf generated codes in a single compilation unit.
Assume you invoke gperf multiple times, with different input files,
and want the generated code to included from the same source file. In this
case, you have to customize not only the exported identifiers, but also the
names of functions with ‘static’ scope, types, and constants.
By default, you will have to deal with the lookup function, the hash function, and the constants. You should therefore use the option ‘-N’ (or, equivalently, the ‘%define lookup-function-name’ declaration), the option ‘-H’ (or, equivalently, the ‘%define hash-function-name’ declaration), and the option ‘--constants-prefix’ (or, equivalently, the ‘%define constants-prefix’ declaration).
If you use the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration), you will also have to deal with the word array, the length table if present, and the string pool if present. This means: You should use the option ‘-W’ (or, equivalently, the ‘%define word-array-name’ declaration). If you use the option ‘-l’ (or, equivalently, the ‘%compare-lengths’ declaration), you should use the option ‘--length-table-name’ (or, equivalently, the ‘%define length-table-name’ declaration). If you use the option ‘-P’ (or, equivalently, the ‘%pic’ declaration), you should use the option ‘-Q’ (or, equivalently, the ‘%define string-pool-name’ declaration).
gperf is under GPL, but that does not cause the output produced
by gperf to be under GPL. The reason is that the output contains
only small pieces of text that come directly from gperf's source
code – only about 7 lines long, too small for being significant –, and
therefore the output is not a “work based on gperf” (in the
sense of the GPL version 3).
On the other hand, the output produced by gperf contains
essentially all of the input file. Therefore the output is a
“derivative work” of the input (in the sense of U.S. copyright law);
and its copyright status depends on the copyright of the input. For most
software licenses, the result is that the the output is under the same
license, with the same copyright holder, as the input that was passed to
gperf.
gperfThere are many options to gperf. They were added to make
the program more convenient for use with real applications. “On-line”
help is readily available via the ‘--help’ option. Here is the
complete list of options.
The results are written to standard output if no output file is specified or if it is ‘-’.
These options are also available as declarations in the input file (see Gperf Declarations).
struct type declaration for generated
code. Any text before a pair of consecutive ‘%%’ is considered
part of the type declaration. Keywords and additional fields may follow
this, one group of fields per line. A set of examples for generating
perfect hash tables and functions for Ada, C, C++, Pascal, Modula 2,
Modula 3 and JavaScript reserved words are distributed with this release.
gperf generated function.
These options are also available as declarations in the input file (see Gperf Declarations).
gperf to generate code in the language specified by the
option's argument. Languages handled are currently:
#define const to empty
for compilers which don't know about this keyword.
The default is ANSI-C.
gperf. It does not do anything.
gperf. It does not do anything.
Most of these options are also available as declarations in the input file (see Gperf Declarations).
struct.
Perfect_Hash.
isalnum and isgraph do
not guarantee that a byte is in this range. Only an explicit
test like ‘c >= 'A' && c <= 'Z'’ guarantees this.) This was the
default in versions of gperf earlier than 2.7; now the default is
to support 8-bit and multibyte characters.
strcmp.
However, using ‘-l’ might greatly increase the size of the
generated C code if the lookup table range is large (which implies that
the switch option ‘-S’ or ‘%switch’ is not enabled), since the length
table contains as many elements as there are entries in the lookup table.
strncmp function to perform
string comparisons. The default action is to use strcmp.
<jjc@ai.mit.edu>.
<string.h>, at the
beginning of the code. By default, this is not done; the user must
include this header file himself to allow compilation of the code.
TOTAL_KEYWORDS,
MIN_WORD_LENGTH, MAX_WORD_LENGTH, and so on. This option
permits the use of two hash tables in the same file, even when the option
‘-E’ (or, equivalently, the ‘%enum’ declaration) is not given or
the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration)
is given.
switch statement scheme,
rather than an array lookup table. This can lead to a reduction in both
time and space requirements for some input files. The argument to this
option determines how many switch statements are generated. A
value of 1 generates 1 switch containing all the elements, a
value of 2 generates 2 tables with 1/2 the elements in each
switch, etc. This is useful since many C compilers cannot
correctly generate code for large switch statements. This option
was inspired in part by Keith Bostic's original C program.
gperf. It does not do anything.
gperfFor instance, the option ‘-k 1,2,4,6-10,'$'’ generates a hash function that considers positions 1,2,4,6,7,8,9,10, plus the last byte in each keyword (which may be at a different position for each keyword, obviously). Keywords with length less than the indicated byte positions work properly, since selected byte positions exceeding the keyword length are simply not referenced in the hash function.
This option is not normally needed since version 2.8 of gperf;
the default byte positions are computed depending on the keyword set,
through a search that minimizes the number of byte positions.
gperf treats all these keywords as
part of an equivalence class and generates a perfect hash function with
multiple comparisons for duplicate keywords. It is up to you to completely
disambiguate the keywords by modifying the generated C code. However,
gperf helps you out by organizing the output.
Using this option usually means that the generated hash function is no
longer perfect. On the other hand, it permits gperf to work on
keyword sets that it otherwise could not handle.
gperf
jumps by random amounts.
If `generate switch' option ‘-S’ (or, equivalently, ‘%switch’) is not enabled, the maximum associated value influences the static array table size, and a larger table should decrease the time required for an unsuccessful search, at the expense of extra table space.
The default value is 1, thus the default maximum associated value about the same size as the number of keywords (for efficiency, the maximum associated value is always rounded up to a power of 2). The actual table size may vary somewhat, since this technique is essentially a heuristic.
gperf is executing. It is useful both for
maintaining the program and for determining whether a given set of
options is actually speeding up the search for a solution. Some useful
information is dumped at the end of the program when the ‘-d’
option is enabled.
gperfThe following are some limitations with the current release of
gperf:
gperf utility is tuned to execute quickly, and works quickly
for small to medium size data sets (around 1000 keywords). It is
extremely useful for maintaining perfect hash functions for compiler
keyword sets. Several recent enhancements now enable gperf to
work efficiently on much larger keyword sets (over 15,000 keywords).
When processing large keyword sets it helps greatly to have over 8 megs
of RAM.
It should be “relatively” easy to replace the current perfect hash function algorithm with a more exhaustive approach; the perfect hash module is essential independent from other program modules. Additional worthwhile improvements include:
switch
option can minimize the data size, at the expense of slightly longer
lookups (note that the gcc compiler generally produces good code for
switch statements, reducing the need for more complex schemes).
[1] Chang, C.C.: A Scheme for Constructing Ordered Minimal Perfect Hashing Functions Information Sciences 39(1986), 187-195.
[2] Cichelli, Richard J. Author's Response to “On Cichelli's Minimal Perfect Hash Functions Method” Communications of the ACM, 23, 12(December 1980), 729.
[3] Cichelli, Richard J. Minimal Perfect Hash Functions Made Simple Communications of the ACM, 23, 1(January 1980), 17-19.
[4] Cook, C. R. and Oldehoeft, R.R. A Letter Oriented Minimal Perfect Hashing Function SIGPLAN Notices, 17, 9(September 1982), 18-27.
[5] Cormack, G. V. and Horspool, R. N. S. and Kaiserwerth, M. Practical Perfect Hashing Computer Journal, 28, 1(January 1985), 54-58.
[6] Jaeschke, G. Reciprocal Hashing: A Method for Generating Minimal Perfect Hashing Functions Communications of the ACM, 24, 12(December 1981), 829-833.
[7] Jaeschke, G. and Osterburg, G. On Cichelli's Minimal Perfect Hash Functions Method Communications of the ACM, 23, 12(December 1980), 728-729.
[8] Sager, Thomas J. A Polynomial Time Generator for Minimal Perfect Hash Functions Communications of the ACM, 28, 5(December 1985), 523-532
[9] Schmidt, Douglas C. GPERF: A Perfect Hash Function Generator Second USENIX C++ Conference Proceedings, April 1990.
[10] Schmidt, Douglas C. GPERF: A Perfect Hash Function Generator C++ Report, SIGS 10 10 (November/December 1998).
[11] Sebesta, R.W. and Taylor, M.A. Minimal Perfect Hash Functions for Reserved Word Lists SIGPLAN Notices, 20, 12(September 1985), 47-53.
[12] Sprugnoli, R. Perfect Hashing Functions: A Single Probe Retrieving Method for Static Sets Communications of the ACM, 20 11(November 1977), 841-850.
[13] Stallman, Richard M. Using and Porting GNU CC Free Software Foundation, 1988.
[14] Stroustrup, Bjarne The C++ Programming Language. Addison-Wesley, 1986.
[15] Tiemann, Michael D. User's Guide to GNU C++ Free Software Foundation, 1989.
hash: Output Formatin_word_set: Output Formatswitch: Output Detailsswitch: Output Format