When Lustre is configured with CFLAGS="-D_FORTIFY_SOURCE=2 -O2" on Centos 6, the compilation will fails with errors such as this one:

cacheio.c: In function ‘qword_printhex’:
cacheio.c:174: error: ignoring return value of ‘fwrite’, declared with attribute warn_unused_result

The value is usually ignored because it doesn't matter.

Note that these errors tend to show on more modern distributions, that have _FORTIFY_SOURCE on by default.