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@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.
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