6.48 Interface to errno

Most C language system functions report errors by setting a the special variable errno to some predefined integer constant, (libc)errno. The Scheme code accessing errno values should be agnostic with respect to the actual error codes; for this to happen we should use the following facilities along with bindings from (vicare platform errno), (vicare-libs)More features for errno for details.

Values for the C language variable errno are encoded as negated fixnum values: if EPERM is defined to 1 at the C language level, Vicare defines it as the fixnum -1; at present all the known error codes fit into fixnums range. One syntax identifier binding for each known errno value is exported by the library (vicare platform errno) and reexported by the library (vicare platform constants).

If some error codes are not defined on a platform: the corresponding binding evaluates to #t. Notice that if no error occurred errno is set to the fixnum zero.

The following bindings are exported by the libraries (vicare) and (vicare system $foreign) and reexported by the library (vicare ffi).

Function: errno

Function: errno code

Interface to the C language level errno variable to be used with the foreign–functions interface (FFI).

When called with no values return the value of the C variable errno right after the last call to a C function performed by the current process through the FFI. When called with one argument: set the C variable errno of the current process to code, appropriately handling the coding of the value.

Notice that bindings exported by (vicare posix), (vicare glibc) and (vicare linux) do not use this mechanism; rather they raise an exception whenever an error happen.

As special cases: when the C variable errno is set to zero, errno returns #f; when code is #f, the C variable errno is set to zero; when code is #t, the C variable errno is set to EFAULT.

The mechanism is such that the following form:

(let ((rv (call-foreign-function)))
  (values rv (errno)))

performs a foreign–function call and evaluates to two values: the return value from the foreign function and the value of the C variable errno right after the foreign call.