Re: Undefined use of weak symbols in gnulib

[Bruno Haible <bruno@clisp.org>]

Florian Weimer wrote on 2021-04-28:
> However, you should really remove those weak symbol
> hacks.  They won't have any effect for glibc 2.34

Done as follows. Tested on a Fedora Rawhide from today.
Thanks for the suggestion.


2021-07-17  Bruno Haible  <bruno@clisp.org>

	Don't use '#pragma weak' for thread functions in Linux/glibc>=2.34.
	Suggested by Florian Weimer <fweimer@redhat.com> in
	<https://lists.gnu.org/archive/html/bug-gnulib/2021-04/msg00211.html>.
	* m4/threadlib.m4 (gl_PTHREADLIB_BODY): Set gl_pthread_in_glibc and set
	LIBPMULTITHREAD accordingly.
	(gl_STDTHREADLIB_BODY): Update comments.
	(gl_THREADLIB_BODY): Define USE_POSIX_THREADS_FROM_LIBC.
	* lib/glthread/thread.h (c11_threads_in_use): Define to 1 if all POSIX
	thread functions are in libc.
	* lib/glthread/lock.h (c11_threads_in_use): Likewise.
	* lib/glthread/cond.h (c11_threads_in_use): Likewise.
	* lib/glthread/tls.h (c11_threads_in_use): Likewise.

diff --git a/lib/glthread/cond.h b/lib/glthread/cond.h
index 9858425..fbab08c 100644
--- a/lib/glthread/cond.h
+++ b/lib/glthread/cond.h
@@ -56,7 +56,9 @@
 #include "glthread/lock.h"
 
 #if !defined c11_threads_in_use
-# if HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
+# if HAVE_THREADS_H && USE_POSIX_THREADS_FROM_LIBC
+#  define c11_threads_in_use() 1
+# elif HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
 #  include <threads.h>
 #  pragma weak thrd_exit
 #  define c11_threads_in_use() (thrd_exit != NULL)
diff --git a/lib/glthread/lock.h b/lib/glthread/lock.h
index 624ce67..cc4c519 100644
--- a/lib/glthread/lock.h
+++ b/lib/glthread/lock.h
@@ -81,7 +81,9 @@
 #include <stdlib.h>
 
 #if !defined c11_threads_in_use
-# if HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
+# if HAVE_THREADS_H && USE_POSIX_THREADS_FROM_LIBC
+#  define c11_threads_in_use() 1
+# elif HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
 #  include <threads.h>
 #  pragma weak thrd_exit
 #  define c11_threads_in_use() (thrd_exit != NULL)
diff --git a/lib/glthread/thread.h b/lib/glthread/thread.h
index 3e84599..44f05f3 100644
--- a/lib/glthread/thread.h
+++ b/lib/glthread/thread.h
@@ -74,7 +74,9 @@
 #include <stdlib.h>
 
 #if !defined c11_threads_in_use
-# if HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
+# if HAVE_THREADS_H && USE_POSIX_THREADS_FROM_LIBC
+#  define c11_threads_in_use() 1
+# elif HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
 #  include <threads.h>
 #  pragma weak thrd_exit
 #  define c11_threads_in_use() (thrd_exit != NULL)
diff --git a/lib/glthread/tls.h b/lib/glthread/tls.h
index 9c2a280..aaaa4b6 100644
--- a/lib/glthread/tls.h
+++ b/lib/glthread/tls.h
@@ -47,7 +47,9 @@
 #include <stdlib.h>
 
 #if !defined c11_threads_in_use
-# if HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
+# if HAVE_THREADS_H && USE_POSIX_THREADS_FROM_LIBC
+#  define c11_threads_in_use() 1
+# elif HAVE_THREADS_H && USE_POSIX_THREADS_WEAK
 #  include <threads.h>
 #  pragma weak thrd_exit
 #  define c11_threads_in_use() (thrd_exit != NULL)
diff --git a/m4/threadlib.m4 b/m4/threadlib.m4
index 8fc3dfd..37b797c 100644
--- a/m4/threadlib.m4
+++ b/m4/threadlib.m4
@@ -1,4 +1,4 @@
-# threadlib.m4 serial 30
+# threadlib.m4 serial 31
 dnl Copyright (C) 2005-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -212,6 +212,27 @@ AC_DEFUN([gl_PTHREADLIB_BODY],
         LIBS=$save_LIBS
         test $gl_pthread_api = yes && break
       done
+      echo "$as_me:__oline__: gl_pthread_api=$gl_pthread_api" >&AS_MESSAGE_LOG_FD
+      echo "$as_me:__oline__: LIBPTHREAD=$LIBPTHREAD" >&AS_MESSAGE_LOG_FD
+
+      gl_pthread_in_glibc=no
+      # On Linux with glibc >= 2.34, libc contains the fully functional
+      # pthread functions.
+      case "$host_os" in
+        linux*)
+          AC_EGREP_CPP([Lucky user],
+            [#include <features.h>
+             #ifdef __GNU_LIBRARY__
+              #if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 34) || (__GLIBC__ > 2)
+               Lucky user
+              #endif
+             #endif
+            ],
+            [gl_pthread_in_glibc=yes],
+            [])
+          ;;
+      esac
+      echo "$as_me:__oline__: gl_pthread_in_glibc=$gl_pthread_in_glibc" >&AS_MESSAGE_LOG_FD
 
       # Test for libpthread by looking for pthread_kill. (Not pthread_self,
       # since it is defined as a macro on OSF/1.)
@@ -219,18 +240,22 @@ AC_DEFUN([gl_PTHREADLIB_BODY],
         # The program links fine without libpthread. But it may actually
         # need to link with libpthread in order to create multiple threads.
         AC_CHECK_LIB([pthread], [pthread_kill],
-          [LIBPMULTITHREAD=-lpthread
-           # On Solaris and HP-UX, most pthread functions exist also in libc.
-           # Therefore pthread_in_use() needs to actually try to create a
-           # thread: pthread_create from libc will fail, whereas
-           # pthread_create will actually create a thread.
-           # On Solaris 10 or newer, this test is no longer needed, because
-           # libc contains the fully functional pthread functions.
-           case "$host_os" in
-             solaris | solaris2.[1-9] | solaris2.[1-9].* | hpux*)
-               AC_DEFINE([PTHREAD_IN_USE_DETECTION_HARD], [1],
-                 [Define if the pthread_in_use() detection is hard.])
-           esac
+          [if test $gl_pthread_in_glibc = yes; then
+             LIBPMULTITHREAD=
+           else
+             LIBPMULTITHREAD=-lpthread
+             # On Solaris and HP-UX, most pthread functions exist also in libc.
+             # Therefore pthread_in_use() needs to actually try to create a
+             # thread: pthread_create from libc will fail, whereas
+             # pthread_create will actually create a thread.
+             # On Solaris 10 or newer, this test is no longer needed, because
+             # libc contains the fully functional pthread functions.
+             case "$host_os" in
+               solaris | solaris2.[1-9] | solaris2.[1-9].* | hpux*)
+                 AC_DEFINE([PTHREAD_IN_USE_DETECTION_HARD], [1],
+                   [Define if the pthread_in_use() detection is hard.])
+             esac
+           fi
           ])
       elif test $gl_pthread_api != yes; then
         # Some library is needed. Try libpthread and libc_r.
@@ -246,6 +271,7 @@ AC_DEFUN([gl_PTHREADLIB_BODY],
              LIBPMULTITHREAD=-lc_r])
         fi
       fi
+      echo "$as_me:__oline__: LIBPMULTITHREAD=$LIBPMULTITHREAD" >&AS_MESSAGE_LOG_FD
     fi
     AC_MSG_CHECKING([whether POSIX threads API is available])
     AC_MSG_RESULT([$gl_pthread_api])
@@ -311,7 +337,8 @@ AC_DEFUN([gl_STDTHREADLIB_BODY],
           dnl glibc >= 2.29 has thrd_create in libpthread.
           dnl FreeBSD >= 10 has thrd_create in libstdthreads; this library depends
           dnl on libpthread (for the symbol 'pthread_mutexattr_gettype').
-          dnl AIX >= 7.1 and Solaris >= 11.4 have thrd_create in libc.
+          dnl glibc >= 2.34, AIX >= 7.1, and Solaris >= 11.4 have thrd_create in
+          dnl libc.
           AC_CHECK_FUNCS([thrd_create])
           if test $ac_cv_func_thrd_create = yes; then
             LIBSTDTHREAD=
@@ -481,7 +508,10 @@ AC_DEFUN([gl_THREADLIB_BODY],
           gl_threads_api=posix
           AC_DEFINE([USE_POSIX_THREADS], [1],
             [Define if the POSIX multithreading library can be used.])
-          if test -n "$LIBMULTITHREAD" || test -n "$LTLIBMULTITHREAD"; then
+          if test -z "$LIBMULTITHREAD" && test -z "$LTLIBMULTITHREAD"; then
+            AC_DEFINE([USE_POSIX_THREADS_FROM_LIBC], [1],
+              [Define if references to the POSIX multithreading library are satisfied by libc.])
+          else
             if case "$gl_cv_have_weak" in *yes) true;; *) false;; esac; then
               AC_DEFINE([USE_POSIX_THREADS_WEAK], [1],
                 [Define if references to the POSIX multithreading library should be made weak.])
@@ -576,7 +606,9 @@ dnl                    flavours   option      weak       result
 dnl ---------------    ---------  ---------   --------   ---------
 dnl Linux 2.4/glibc    posix      -lpthread       Y      OK
 dnl
-dnl GNU Hurd/glibc     posix
+dnl Linux/glibc 2.34   posix                      Y      OK
+dnl
+dnl GNU Hurd/glibc     posix      -lpthread       Y      OK
 dnl
 dnl Ubuntu 14.04       posix      -pthread        Y      OK
 dnl