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6.2.1 POSIX Interface Conventions

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:

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 Error Reporting). 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))
     Bad file descriptor

— Function: system-error-errno arglist

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,

           (lambda ()
             (mkdir "/this-ought-to-fail-if-I'm-not-root"))
           (lambda stuff
             (let ((errno (system-error-errno stuff)))
                ((= errno EACCES)
                 (display "You're not allowed to do that."))
                ((= errno EEXIST)
                 (display "Already exists."))
                 (display (strerror errno))))