Fix implicit function declarations with clang

[dl1jbe]

Will be counted as errors starting with CLANG-16

[zcsahok]

What are the offending functions? Maybe we could just replace them with the "correct" ones instead of adding a magic define.

[dl1jbe]

Sorry, missed to give that information.
It's 'timegm()' in both cases. See man page.

[zcsahok]

I see. Indeed GNU libc manual (https://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html) says:

timegm is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the TZ environment variable to UTC, call mktime, then set TZ back.

We have timegm in log_utils.c and writecabrillo.c too. Couldn't we just set _DEFAULT_SOURCE (and _XOPEN_SOURCE) globally in src/Makefile.am? (When compiling with gcc-10.2.1 both are set automagically.)

[dl1jbe]

We have timegm in log_utils.c and writecabrillo.c too. Couldn't we just set _DEFAULT_SOURCE (and _XOPEN_SOURCE) globally in src/Makefile.am?

Just looked why we get no problems from clang reported there: We have _GNU_SOURCE already defined in that files. 'man feature_test_macros' says about it:

"Since glibc 2.19, defining _GNU_SOURCE also has the effect of implicitly defining _DEFAULT_SOURCE.
Before glibc 2.20, defining _GNU_SOURCE also had the effect of implicitly defining _BSD_SOURCE and _SVID_SOURCE."

One of the problems is that the meaning and use of the macros is slowly changing.

(When compiling with gcc-10.2.1 both are set automagically.)

Unluckily that seems to be handled differently between different compilers.

Wrt adding it to src/Makefile.am. It sure needs some test but i am open for it. But maybe we should add _GNU_SOURCE instead.

[zcsahok]

Defining say _GNU_SOURCE globally would be more manageable in my opinion than various defines scattered around in sources files. Provided of course that this covers all cases.

[dl1jbe]

To bring these to an end. I did some checks for the use of _GNU_SOURCE via Makefile.am. It looks promising. All implicit declarations are gone for actual gcc-13 and also clang 16 and 17.
Will provide an updated PR tomorrow.

[zcsahok]

Thanks!