These interfaces provide access to operating system facilities. They provide a simple wrapping around the underlying C interfaces to make usage from Scheme more convenient. They are also used to implement the Guile port of scsh (see The Scheme shell (scsh)).
Generally there is a single procedure for each corresponding Unix
facility. There are some exceptions, such as procedures implemented for
speed and convenience in Scheme with no primitive Unix equivalent,
e.g. copy-file
.
The interfaces are intended as far as possible to be portable across different versions of Unix. In some cases procedures which can’t be implemented on particular systems may become no-ops, or perform limited actions. In other cases they may throw errors.
General naming conventions are as follows:
recv!
.
#t
or #f
) have question marks
appended, e.g., access?
.
primitive-fork
.
EPERM
or R_OK
are converted
to Scheme variables of the same name (underscores are not replaced
with hyphens).
Unexpected conditions are generally handled by raising exceptions.
There are a few procedures which return a special value if they don’t
succeed, e.g., getenv
returns #f
if it the requested
string is not found in the environment. These cases are noted in
the documentation.
For ways to deal with exceptions, see Exceptions.
Errors which the C library would report by returning a null pointer or
through some other means are reported by raising a system-error
exception with scm-error
(see Procedures for Signaling Errors). The
data parameter is a list containing the Unix errno
value
(an integer). For example,
(define (my-handler key func fmt fmtargs data) (display key) (newline) (display func) (newline) (apply format #t fmt fmtargs) (newline) (display data) (newline)) (catch 'system-error (lambda () (dup2 -123 -456)) my-handler) -| system-error dup2 Bad file descriptor (9)
Return the errno
value from a list which is the arguments to an
exception handler. If the exception is not a system-error
,
then the return is #f
. For example,
(catch 'system-error (lambda () (mkdir "/this-ought-to-fail-if-I'm-not-root")) (lambda stuff (let ((errno (system-error-errno stuff))) (cond ((= errno EACCES) (display "You're not allowed to do that.")) ((= errno EEXIST) (display "Already exists.")) (#t (display (strerror errno)))) (newline))))