@node ax_prototype
@unnumberedsec ax_prototype

@majorheading Synopsis

@smallexample
AX_PROTOTYPE(function, includes, code, TAG1, values1 [, TAG2, values2 [...]])
@end smallexample

@majorheading Description

Try all the combinations of <TAG1>, <TAG2>... to successfully compile
<code>. <TAG1>, <TAG2>, ... are substituted in <code> and <include> with
values found in <values1>, <values2>, ... respectively. <values1>,
<values2>, ... contain a list of possible values for each corresponding
tag and all combinations are tested. When AC_TRY_COMPILE(include, code)
is successful for a given substitution, the macro stops and defines the
following macros: FUNCTION_TAG1, FUNCTION_TAG2, ... using AC_DEFINE()
with values set to the current values of <TAG1>, <TAG2>, ... If no
combination is successful the configure script is aborted with a
message.

Intended purpose is to find which combination of argument types is
acceptable for a given function <function>. It is recommended to list
the most specific types first. For instance ARG1, [size_t, int] instead
of ARG1, [int, size_t].

Generic usage pattern:

1) add a call in configure.ac

@smallexample
 AX_PROTOTYPE(...)
@end smallexample

2) call autoheader to see which symbols are not covered

3) add the lines in config.h

@smallexample
 /* Type of Nth argument of function */
 #undef FUNCTION_ARGN
@end smallexample

4) Within the code use FUNCTION_ARGN instead of an hardwired type

Complete example:

1) configure.ac

@smallexample
 AX_PROTOTYPE(getpeername,
 [
  #include <sys/types.h>
  #include <sys/socket.h>
 ],
 [
  int a = 0;
  ARG2 * b = 0;
  ARG3 * c = 0;
  getpeername(a, b, c);
 ],
 ARG2, [struct sockaddr, void],
 ARG3, [socklen_t, size_t, int, unsigned int, long unsigned int])
@end smallexample

2) call autoheader

@smallexample
 autoheader: Symbol `GETPEERNAME_ARG2' is not covered by ./config.h
 autoheader: Symbol `GETPEERNAME_ARG3' is not covered by ./config.h
@end smallexample

3) config.h

@smallexample
 /* Type of second argument of getpeername */
 #undef GETPEERNAME_ARG2
@end smallexample

@smallexample
 /* Type of third argument of getpeername */
 #undef GETPEERNAME_ARG3
@end smallexample

4) in the code

@smallexample
  ...
  GETPEERNAME_ARG2 name;
  GETPEERNAME_ARG3 namelen;
  ...
  ret = getpeername(socket, &name, &namelen);
  ...
@end smallexample

Implementation notes: generating all possible permutations of the
arguments is not easily done with the usual mixture of shell and m4,
that is why this macro is almost 100% m4 code. It generates long but
simple to read code.

@majorheading Source Code

Download the
@uref{http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_prototype.m4,latest
version of @file{ax_prototype.m4}} or browse
@uref{http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=history;f=m4/ax_prototype.m4,the
macro's revision history}.

@majorheading License

@w{Copyright @copyright{} 2009 Loic Dachary @email{loic@@senga.org}}

This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2 of the License, or (at your
option) any later version.

This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.

You should have received a copy of the GNU General Public License along
with this program. If not, see <https://www.gnu.org/licenses/>.

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