When I started programming with C, it was pretty easy to get code to compile on my first try.

Now, when I use C...
_POSIX_C_SOURCE >= 199309L ... was in the man page, but didn't work... _GNU_SOURCE works.
Read somewhere to just use -std=gnu17 and remove _GNU_SOURCE and nice, it works.
Create another project with -std=gnu17, but using another function and now it doesn't work. Need to add _GNU_SOURCE. Why?