On closer inspection, termbox2.h does include signal.h itself[1], and additionally defines _XOPEN_SOURCE[2] and _DEFAULT_SOURCE, so the inclusion of signal.h can't be escaped.
My testing has shown that when -std=c99 is specified, it is as if that switch
explicitly undefines _DEFAULT_SOURCE/_XOPEN_SOURCE **defined inside the header
file** (this is the weird part). If -D_DEFAULT_SOURCE is given as an argument,
or without -std=c99, when gcc is called directly rather than through the
wrapper, c99, there are no warnings:
$ c99 -E edit.c | grep fileno
if (fstat(fileno(fp), &st) != 0) {
$ gcc -E edit.c | grep fileno
int fileno(FILE *);
int fileno_unlocked(FILE *);
if (fstat(fileno(fp), &st) != 0) {
$ c99 -D_DEFAULT_SOURCE -E edit.c | grep fileno
int fileno(FILE *);
int fileno_unlocked(FILE *);
if (fstat(fileno(fp), &st) != 0) {
$
I use Alpine Linux, thus musl libc. Searching the web yielded the answer[3] on
Stack Overflow, though:
> If you want this to cleanly compile with -std=c99, you must consider the
> inclusion of the _DEFAULT_SOURCE feature test macro
I'm not sure about why you wouldn't want POSIX compliance though (To compile on
Plan9? But it has APE[4]).
[1]:
https://github.com/arthur-jacquin/edit/blob/41cd580d80459d1f13ef923c812cf6a858b240f0/termbox2.h#L40
[2]:
https://github.com/arthur-jacquin/edit/blob/41cd580d80459d1f13ef923c812cf6a858b240f0/termbox2.h#L29
[3]: https://stackoverflow.com/a/10435860
[4]: https://9p.io/wiki/plan9/Porting_alien_software_to_Plan_9/index.html
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