[libvirt] [PATCH] build: silence stupid gcc warning on STREQ_NULLABLE

[Daniel P. Berrange]

On Mon, Aug 20, 2012 at 01:51:49PM -0600, Eric Blake wrote:
> Our existing STRNEQ_NULLABLE() triggered a warning in gcc 4.7 when
> used with a literal NULL argument:
> 
> qemumonitorjsontest.c: In function 'testQemuMonitorJSONGetMachines':
> qemumonitorjsontest.c:289:5: error: null argument where non-null required (argument 1) [-Werror=nonnull]
> 
> even though the strcmp is provably dead when a null argument is
> present.  Squelch the warning by refactoring things so that gcc
> never sees strcmp() called with NULL arguments (we still compare
> NULL as not equal to "", this rewrite merely aids gcc).
> 
> Next, gcc has a valid warning about a literal NULLSTR(NULL):
> 
> qemumonitorjsontest.c:289:5: error: invalid application of 'sizeof' to a void type [-Werror=pointer-arith]
> 
> Of course, you'd never write NULLSTR(NULL) directly, but it is
> handy to use through macros.  And since we only really need the
> code to be warning-free when developing with modern gcc, and
> merely compiled correctly elsewhere, we can rely on gcc extensions
> to avoid dereferencing NULL even inside a sizeof operation.
> 
> * src/internal.h (STREQ_NULLABLE, STRNEQ_NULLABLE): Avoid gcc 4.7
> stupidity.
> (NULLSTR): Allow passing compile-time constants via macros.
> ---
>  src/internal.h | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)

Not sure I entirely understand the GCC black magic, but if it works
for you I'll ACK

Daniel
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