Hi,

this is what I did in terms of implementing Jon's and Jakub's suggestions: many thanks to both of you!

The patch should be in general quite conservative and safe, in particular, the gthr-posix.h changes, which I cannot approve myself, touch bits only used by libstdc++-v3 + the macro avoiding the unconditional inclusion of <unistd.h>, which, according to Jakub's analysis, should be required only by objc.

I built and tested c++ and its lib and built all languages.  Ok?

Thanks,
Paolo.

///////////////////////
/gcc
2011-07-15  Paolo Carlini  <paolo.carl...@oracle.com>
            Jakub Jelinek  <ja...@redhat.com>
            Jonathan Wakely  <jwakely....@gmail.com>

        PR libstdc++/49745
        * gthr-posix.h: Do not include <unistd.h> unconditionally; use
        _GTHREADS_ASSUME_POSIX_TIMEOUTS instead of _POSIX_TIMEOUTS.

/libstdc++-v3
2011-07-15  Paolo Carlini  <paolo.carl...@oracle.com>
            Jakub Jelinek  <ja...@redhat.com>

        PR libstdc++/49745
        * acinclude.m4 ([GLIBCXX_CHECK_GTHREADS]): Check separately for
        _POSIX_TIMEOUTS and define _GTHREADS_ASSUME_POSIX_TIMEOUTS.
        * libstdc++-v3/libsupc++/guard.cc: Include <unistd.h>.
        * testsuite/17_intro/headers/c++1998/49745.cc: New.
        * configure: Regenerate.
        * config.h.in: Likewise.
Index: libstdc++-v3/libsupc++/guard.cc
===================================================================
--- libstdc++-v3/libsupc++/guard.cc     (revision 176310)
+++ libstdc++-v3/libsupc++/guard.cc     (working copy)
@@ -35,6 +35,7 @@
     && defined(_GLIBCXX_ATOMIC_BUILTINS_4) && 
defined(_GLIBCXX_HAVE_LINUX_FUTEX)
 # include <climits>
 # include <syscall.h>
+# include <unistd.h>
 # define _GLIBCXX_USE_FUTEX
 # define _GLIBCXX_FUTEX_WAIT 0
 # define _GLIBCXX_FUTEX_WAKE 1
Index: libstdc++-v3/testsuite/17_intro/headers/c++1998/49745.cc
===================================================================
--- libstdc++-v3/testsuite/17_intro/headers/c++1998/49745.cc    (revision 0)
+++ libstdc++-v3/testsuite/17_intro/headers/c++1998/49745.cc    (revision 0)
@@ -0,0 +1,22 @@
+// { dg-do compile }
+
+// Copyright (C) 2011 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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 3, or (at your option)
+// any later version.
+
+// This library 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 library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// libstdc++/49745
+#include <iostream>
+int truncate = 0;
Index: libstdc++-v3/acinclude.m4
===================================================================
--- libstdc++-v3/acinclude.m4   (revision 176310)
+++ libstdc++-v3/acinclude.m4   (working copy)
@@ -3155,6 +3155,22 @@
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -fno-exceptions -I${toplevel_srcdir}/gcc"
 
+  AC_MSG_CHECKING([for _POSIX_TIMEOUTS >= 0 in <unistd.h>])
+
+  AC_TRY_COMPILE([#include <unistd.h>],
+    [
+      #if !defined(_POSIX_TIMEOUTS) || _POSIX_TIMEOUTS < 0
+      #error
+      #endif
+    ], [ac_assume_posix_timeouts=yes], [ac_assume_posix_timeouts=no])
+
+  AC_MSG_RESULT([$ac_assume_posix_timeouts])
+
+  if test x"$ac_assume_posix_timeouts" = x"yes"; then
+    AC_DEFINE(_GTHREADS_ASSUME_POSIX_TIMEOUTS, 1,
+             [Define if _POSIX_TIMEOUT is defined >= 0 by <unistd.h>.])
+  fi
+
   target_thread_file=`$CXX -v 2>&1 | sed -n 's/^Thread model: //p'`
   case $target_thread_file in
     posix)
@@ -3163,7 +3179,10 @@
 
   AC_MSG_CHECKING([for gthreads library])
 
-  AC_TRY_COMPILE([#include "gthr.h"],
+  AC_TRY_COMPILE([
+                  #include "gthr.h"
+                 #include <unistd.h>
+                 ],
     [
       #ifndef __GTHREADS_CXX0X
       #error
Index: gcc/gthr-posix.h
===================================================================
--- gcc/gthr-posix.h    (revision 176310)
+++ gcc/gthr-posix.h    (working copy)
@@ -1,7 +1,7 @@
 /* Threads compatibility routines for libgcc2 and libobjc.  */
 /* Compile this one with gcc.  */
 /* Copyright (C) 1997, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
-   2008, 2009, 2010 Free Software Foundation, Inc.
+   2008, 2009, 2010, 2011 Free Software Foundation, Inc.
 
 This file is part of GCC.
 
@@ -39,7 +39,10 @@
 #endif
 
 #include <pthread.h>
+
+#if defined(_LIBOBJC) || defined(_LIBOBJC_WEAK)
 #include <unistd.h>
+#endif
 
 typedef pthread_t __gthread_t;
 typedef pthread_key_t __gthread_key_t;
@@ -100,11 +103,9 @@
 
 __gthrw3(pthread_mutex_lock)
 __gthrw3(pthread_mutex_trylock)
-#ifdef _POSIX_TIMEOUTS
-#if _POSIX_TIMEOUTS >= 0
+#ifdef _GTHREADS_ASSUME_POSIX_TIMEOUTS
 __gthrw3(pthread_mutex_timedlock)
 #endif
-#endif /* _POSIX_TIMEOUTS */
 __gthrw3(pthread_mutex_unlock)
 __gthrw3(pthread_mutex_init)
 __gthrw3(pthread_mutex_destroy)
@@ -131,11 +132,9 @@
 
 __gthrw(pthread_mutex_lock)
 __gthrw(pthread_mutex_trylock)
-#ifdef _POSIX_TIMEOUTS
-#if _POSIX_TIMEOUTS >= 0
+#ifdef _GTHREADS_ASSUME_POSIX_TIMEOUTS
 __gthrw(pthread_mutex_timedlock)
 #endif
-#endif /* _POSIX_TIMEOUTS */
 __gthrw(pthread_mutex_unlock)
 __gthrw(pthread_mutex_init)
 __gthrw(pthread_mutex_destroy)
@@ -753,8 +752,7 @@
     return 0;
 }
 
-#ifdef _POSIX_TIMEOUTS
-#if _POSIX_TIMEOUTS >= 0
+#ifdef _GTHREADS_ASSUME_POSIX_TIMEOUTS
 static inline int
 __gthread_mutex_timedlock (__gthread_mutex_t *__mutex,
                           const __gthread_time_t *__abs_timeout)
@@ -765,7 +763,6 @@
     return 0;
 }
 #endif
-#endif
 
 static inline int
 __gthread_mutex_unlock (__gthread_mutex_t *__mutex)
@@ -811,8 +808,7 @@
   return __gthread_mutex_trylock (__mutex);
 }
 
-#ifdef _POSIX_TIMEOUTS
-#if _POSIX_TIMEOUTS >= 0
+#ifdef _GTHREADS_ASSUME_POSIX_TIMEOUTS
 static inline int
 __gthread_recursive_mutex_timedlock (__gthread_recursive_mutex_t *__mutex,
                                     const __gthread_time_t *__abs_timeout)
@@ -820,7 +816,6 @@
   return __gthread_mutex_timedlock (__mutex, __abs_timeout);
 }
 #endif
-#endif
 
 static inline int
 __gthread_recursive_mutex_unlock (__gthread_recursive_mutex_t *__mutex)

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