38.1 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 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, 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. The value of the Unix errno variable is available in the data passed by the exception.

It can be extracted with the function system-error-errno:

     (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))))