[libvirt] [PATCH] build: allow building with newer glibc-headers and -O0

[Christophe Fergeau]

Hey,

On Mon, Jun 04, 2012 at 12:08:20PM -0600, Eric Blake wrote:
> glibc 2.15 (on Fedora 17) coupled with explicit disabling of
> optimization during development dies a painful death:
> 
> In file included from /usr/include/limits.h:27:0,
>                  from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/limits.h:169,
>                  from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/syslimits.h:7,
>                  from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/limits.h:34,
>                  from util/bitmap.c:26:
> /usr/include/features.h:314:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
> cc1: all warnings being treated as errors
> 
> Work around this by only conditionally defining _FORTIFY_SOURCE,
> in the case where glibc can actually use it.  The trick is using
> AH_VERBATIM instead of AC_DEFINE.
> 
> * m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Squelch
> _FORTIFY_SOURCE when needed to avoid glibc #warnings.
> ---
> 
> Pushing under the build-breaker rule.
> 
>  m4/virt-compile-warnings.m4 |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4
> index 5527bff..a91d69f 100644
> --- a/m4/virt-compile-warnings.m4
> +++ b/m4/virt-compile-warnings.m4
> @@ -102,8 +102,12 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
>      # Silence certain warnings in gnulib, and use improved glibc headers
>      AC_DEFINE([lint], [1],
>        [Define to 1 if the compiler is checking for lint.])
> -    AC_DEFINE([_FORTIFY_SOURCE], [2],
> -      [enable compile-time and run-time bounds-checking, and some warnings])
> +    AH_VERBATIM([FORTIFY_SOURCE],
> +    [/* Enable compile-time and run-time bounds-checking, and some warnings. */
> +     #if __OPTIMIZE__
> +     # define _FORTIFY_SOURCE 2
> +     #endif
> +    ])

This is not causing an actual issue while compiling libvirt, but
__OPTIMIZE__ is not defined when using -O0, which causes a warning
with -Wundef
I've tested this with

$ gcc --version
gcc (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5)

$ gcc -c -Wundef -O0 test.c
#if __OPTIMIZE__
#warning if __OPTIMIZE__
#endif
#ifdef __OPTIMIZE__
#warning ifdef __OPTIMIZE
#endif

Using a #ifdef would be more correct:

diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4
index a91d69f..0ea7276 100644
--- a/m4/virt-compile-warnings.m4
+++ b/m4/virt-compile-warnings.m4
@@ -104,7 +104,7 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
       [Define to 1 if the compiler is checking for lint.])
     AH_VERBATIM([FORTIFY_SOURCE],
     [/* Enable compile-time and run-time bounds-checking, and some
warnings. */
-     #if __OPTIMIZE__
+     #ifdef __OPTIMIZE__
      # define _FORTIFY_SOURCE 2
      #endif
     ])
[

(or doing #ifdef __OPTIMIZE__ #if __OPTIMIZE .... if __OPTIMIZE__ was set
to 0 with -O0 with some gcc/libc versions)

Christophe
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