@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node API Overview
@section Overview of the Guile API

Guile's application programming interface (@dfn{API}) makes
functionality available that an application developer can use in either
C or Scheme programming.  The interface consists of @dfn{elements} that
may be macros, functions or variables in C, and procedures, variables,
syntax or other types of object in Scheme.

Many elements are available to both Scheme and C, in a form that is
appropriate.  For example, the @code{assq} Scheme procedure is also
available as @code{scm_assq} to C code.  These elements are documented
only once, addressing both the Scheme and C aspects of them.

The Scheme name of an element is related to its C name in a regular
way.  Also, a C function takes its parameters in a systematic way.

Normally, the name of a C function can be derived given its Scheme name,
using some simple textual transformations:

@itemize @bullet

@item
Replace @code{-} (hyphen) with @code{_} (underscore).

@item
Replace @code{?} (question mark) with @code{_p}.

@item
Replace @code{!} (exclamation point) with @code{_x}.

@item
Replace internal @code{->} with @code{_to_}.

@item
Replace @code{<=} (less than or equal) with @code{_leq}.

@item
Replace @code{>=} (greater than or equal) with @code{_geq}.

@item
Replace @code{<} (less than) with @code{_less}.

@item
Replace @code{>} (greater than) with @code{_gr}.

@item
Prefix with @code{scm_}.

@end itemize

@c Here is an Emacs Lisp command that prompts for a Scheme function name and
@c inserts the corresponding C function name into the buffer.

@c @example
@c (defun insert-scheme-to-C (name &optional use-gh)
@c   "Transforms Scheme NAME, a string, to its C counterpart, and inserts it.
@c Prefix arg non-nil means use \"gh_\" prefix, otherwise use \"scm_\" prefix."
@c   (interactive "sScheme name: \nP")
@c   (let ((transforms '(("-"  . "_")
@c                       ("?"  . "_p")
@c                       ("!"  . "_x")
@c                       ("->" . "_to_")
@c                       ("<=" . "_leq")
@c                       (">=" . "_geq")
@c                       ("<"  . "_less")
@c                       (">"  . "_gr")
@c                       ("@@"  . "at"))))
@c     (while transforms
@c       (let ((trigger (concat "\\(.*\\)"
@c                              (regexp-quote (caar transforms))
@c                              "\\(.*\\)"))
@c             (sub (cdar transforms))
@c             (m nil))
@c         (while (setq m (string-match trigger name))
@c           (setq name (concat (match-string 1 name)
@c                              sub
@c                              (match-string 2 name)))))
@c       (setq transforms (cdr transforms))))
@c   (insert (if use-gh "gh_" "scm_") name))
@c @end example

A C function always takes a fixed number of arguments of type
@code{SCM}, even when the corresponding Scheme function takes a
variable number.

For some Scheme functions, some last arguments are optional; the
corresponding C function must always be invoked with all optional
arguments specified.  To get the effect as if an argument has not been
specified, pass @code{SCM_UNDEFINED} as its value.  You can not do
this for an argument in the middle; when one argument is
@code{SCM_UNDEFINED} all the ones following it must be
@code{SCM_UNDEFINED} as well.

Some Scheme functions take an arbitrary number of @emph{rest}
arguments; the corresponding C function must be invoked with a list of
all these arguments.  This list is always the last argument of the C
function.

These two variants can also be combined.

The type of the return value of a C function that corresponds to a
Scheme function is always @code{SCM}.  In the descriptions below,
types are therefore often omitted but for the return value and for the
arguments.