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Re: _AC_PROG_CXX_EXIT_DECLARATION: don't `#include <stdlib.h>'

From: Paul Eggert
Subject: Re: _AC_PROG_CXX_EXIT_DECLARATION: don't `#include <stdlib.h>'
Date: 22 May 2003 13:07:27 -0700
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3

Jim Meyering <address@hidden> writes:

> This change makes it so that when running configure with e.g., CC=aCC,
> you'll now get this in confdefs.h:
>   #ifdef __cplusplus
>   extern "C" void exit (int);
>   #endif
> rather than this:
>   #ifdef __cplusplus
>   #include <stdlib.h>
>   #endif

Thanks for looking into this.  While we're on this subject I have
three related questions.

First, why is the "#ifdef __cplusplus" needed at all?  Since the test
is done without "#ifdef __cplusplus", I don't see why the confdefs.h
code has it.

Second, this code:

     _AC_COMPILE_IFELSE([AC_LANG_PROGRAM(address@hidden:@include <stdlib.h>

seems backwards to me.  $ac_declaration should appear before <stdlib.h>
in the test, since that's how the actual code will do things.

Third, the current Autoconf code does not work with pedantic C99,
which (like C++) requires that functions must be declared before they
are used.

How about the following revised patch instead?

2003-05-22  Jim Meyering  <address@hidden>
            and Paul Eggert <address@hidden>

        * lib/autoconf/c.m4 (_AC_PROG_C_EXIT_DECLARATION):
        Renamed from _AC_PROG_CXX_EXIT_DECLARATION, since it applies
        to C99 as well as to C++.  Remove `#include <stdlib.h>' from
        the list; we should never make confdefs.h include <stdlib.h>
        or <cstdlib>, because the resulting namespace pollution would
        cause other tests to fail.  Configure scripts run with some
        older versions of g++ and HP's aCC would fail due to such an
        #include.  Problems reported by Matthew Mueller in
        <> and by Keith Bostic in
        Prefer 'void exit (int);' to nothing, since it's required
        by C99 (and strictly speaking is necessary even for C89).
        Remove '#ifdef __cplusplus' from confdefs.h, since the code
        is not specific to C++.

--- lib/autoconf/c.m4.~1.178.~  Thu May 22 12:09:14 2003
+++ lib/autoconf/c.m4   Thu May 22 12:57:52 2003
@@ -476,7 +476,7 @@ _AC_PROG_CC_STDC
 _AC_COMPILE_IFELSE(address@hidden:@ifndef __cplusplus
   choke me
-                  [_AC_PROG_CXX_EXIT_DECLARATION])
+                  [_AC_PROG_C_EXIT_DECLARATION])
@@ -685,7 +685,7 @@ m4_expand_once([_AC_COMPILER_OBJEXT])[]d
 GXX=`test $ac_compiler_gnu = yes && echo yes`
@@ -721,21 +721,20 @@ fi[]dnl
 # -----------------------------
 # Find a valid prototype for exit and declare it in confdefs.h.
 [for ac_declaration in \
-   ''\
-   '#include <stdlib.h>' \
+   'void exit (int);' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
    'extern "C" void exit (int);' \
-   'void exit (int);'
+   ''
-  _AC_COMPILE_IFELSE([AC_LANG_PROGRAM(address@hidden:@include <stdlib.h>
address@hidden:@include <stdlib.h>],
                                      [exit (42);])],
@@ -745,11 +744,9 @@ $ac_declaration],
 rm -f conftest*
 if test -n "$ac_declaration"; then
-  echo '#ifdef __cplusplus' >>confdefs.h
   echo $ac_declaration      >>confdefs.h
-  echo '#endif'             >>confdefs.h

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