7.2.1 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:

• The Scheme name is often identical to the name of the underlying Unix facility.
• Underscores in Unix procedure names are converted to hyphens.
• Procedures which destructively modify Scheme data have exclamation marks appended, e.g., recv!.
• Predicates (returning only #t or #f) have question marks appended, e.g., access?.
• Some names are changed to avoid conflict with dissimilar interfaces defined by scsh, e.g., primitive-fork.
• Unix preprocessor names such as 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 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))
  my-handler)

-|
system-error
dup2
Bad file descriptor
(9)

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,

(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))))