Why can't we just always use this form?

Should be safe as long as we put something like

#if defined(_GNU_SOURCE) || _POSIX_C_SOURCE < 200112L
#  error "We might be getting GNU strerror_r here"
#endif

Well I get the GNU form here by default...
It would be possible to define _POSIX_C_SOURCE in the configure script, but there is no telling what impact that will have on other functions. Detecting what form to use is the safer way.

Ah, the documentation clarifies. The GNU version may return something else than buf... puke

Exactly, in that case buf will remain empty :(

Couldn't you write just char buf[256] = 0; and it would zero the first array element?

But that version is more likely to confuse people. I prefer laanwj's more obvious version, even though it's one line longer.

@Diapolo That would zero out the entire buffer, which is not necessary here (according to the documentation FormatMessage will always return a null-terminated string or nothing at all).

http://msdn.microsoft.com/en-us/library/windows/desktop/ms679351%28v=vs.85%29.aspx

I see though that we don't check the output of FormatMessageA - which can fail, itself. We should probably set a default message (like 'Unknown') in case it fails. Edit: fixed