[lttng-dev] [PATCH urcu] Fix CPU_{SET, ZERO} configure checks when using -Werror

[Simon Marchi]

I noticed a build failure in the tests when building with
CFLAGS="-Werror".

  In file included from test_urcu.c:38:0:
  ../../tests/common/cpuset.h:38:0: error: "CPU_ZERO" redefined [-Werror]
   # define CPU_ZERO(cpuset) do { *(cpuset) = 0; } while(0)
   ^
  In file included from /usr/include/pthread.h:23:0,
                   from test_urcu.c:26:
  /usr/include/sched.h:82:0: note: this is the location of the previous definition
   # define CPU_ZERO(cpusetp)  __CPU_ZERO_S (sizeof (cpu_set_t), cpusetp)
   ^

The reason is that the configure check for CPU_ZERO (and CPU_SET) fails,
because of a redefinition of _GNU_SOURCE:

  configure:14733: checking whether CPU_ZERO works
  configure:14749: gcc -std=gnu99 -c -Werror -pthread -D_GNU_SOURCE conftest.c >&5
  conftest.c:61:0: error: "_GNU_SOURCE" redefined [-Werror]
     #define _GNU_SOURCE
   ^
  conftest.c:24:0: note: this is the location of the previous definition
   #define _GNU_SOURCE 1
   ^
  cc1: all warnings being treated as errors

_GNU_SOURCE is defined 3 times in total:

 1. On the command line with -D_GNU_SOURCE (which is equivalent to
    #define _GNU_SOURCE 1):

      CFLAGS="$CFLAGS -D_GNU_SOURCE"

 2. By autoconf through AC_USE_SYSTEM_EXTENSIONS, introduced recently

      https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Posix-Variants.html#Posix-Variants

 3. In the actual test code:

      #define _GNU_SOURCE
      #include <sched.h>
      int main()
      {
              cpu_set_t foo; CPU_ZERO(&foo);
              return 0;
      }

Possible fix #1:

Change "#define _GNU_SOURCE" to "#define _GNU_SOURCE 1" in the test
code.  Because redefinitions of macros with the same value are allowed,
the error would disappear and everybody would be happy.

Possible fix #2:

Remove "#define _GNU_SOURCE" from the test code.  It is redundant to add
it to CFLAGS for the lifetime of these checks, and also define it in the
code.  That would also make everybody happy.

Possible fix #3:

Remove both -D_GNU_SOURCE from CFLAGS and "#define _GNU_SOURCE" from the
test code.  AC_USE_SYSTEM_EXTENSIONS is made for exactly that, setting
the macros that enable the quirks specific to each platform.  On non-GNU
platforms, the macro is irrelevant anyway.  This would make everybody
happy as well.

Personally, I prefer #3.  That allows to remove the saved_CFLAGS thing,
which is error-prone.

This change was tested on Ubuntu 14.04 x86-64, with autoconf 2.69-6.
It should ideally be tested on non-GNU platforms (which I don't easily
have access to).  However, not defining _GNU_SOURCE on those platforms
shouldn't break too many things.
---
 configure.ac | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index e7e300b..0921874 100644
--- a/configure.ac
+++ b/configure.ac
@@ -264,8 +264,8 @@ AS_IF([test "x$def_smp_support" = "xyes"], [AC_DEFINE([CONFIG_RCU_SMP], [1])])
 # Since we define _GNU_SOURCE in the sources, must do so too in the
 # autoconf tests, as defining _GNU_SOURCE or not exposes
 # sched_setaffinity bits differently.
-saved_CFLAGS=$CFLAGS
-CFLAGS="$CFLAGS -D_GNU_SOURCE"
+#
+# This is done through the autoconf macro AC_USE_SYSTEM_EXTENSIONS.
 
 AC_CHECK_TYPES([cpu_set_t],
 	[have_cpu_set_t="yes"],
@@ -276,7 +276,6 @@ AC_CHECK_TYPES([cpu_set_t],
 AC_MSG_CHECKING([whether CPU_ZERO works])
 AH_TEMPLATE([HAVE_CPU_ZERO], [Defined to 1 if we have CPU_ZERO and it works])
 AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
-		#define _GNU_SOURCE
 		#include <sched.h>
 		int main()
 		{
@@ -295,7 +294,6 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 AC_MSG_CHECKING([whether CPU_SET works])
 AH_TEMPLATE([HAVE_CPU_SET], [Defined to 1 if we have CPU_SET and it works])
 AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
-		#define _GNU_SOURCE
 		#include <sched.h>
 		int main()
 		{
-- 
2.5.1