Why does clock_gettime not compile when using C99?

Why does this C code compile in C99? What should I read to learn more?

I can't post unless I add more text so here's some nonsense text because I don't think there's anything else to say

$ cat m.c
#include <stdio.h>
#include <time.h>
int main() {
    struct timespec time;
    int res = clock_gettime(CLOCK_REALTIME, &time);
    printf("%d %ld %ld\n", res, time.tv_sec, time.tv_nsec);
    return 0;
}


$ clang m.c && ./a.out && rm ./a.out
0 1631386905 774654955


$ clang -std=c99 m.c && ./a.out && rm ./a.out
m.c:4:18: error: variable has incomplete type 'struct timespec'
        struct timespec time;
                        ^
m.c:4:9: note: forward declaration of 'struct timespec'
        struct timespec time;
            ^
m.c:5:12: warning: implicit declaration of function 'clock_gettime' is invalid in C99 [-Wimplicit-function-declaration]
        int res = clock_gettime(CLOCK_REALTIME, &time);
                ^
m.c:5:26: error: use of undeclared identifier 'CLOCK_REALTIME'
        int res = clock_gettime(CLOCK_REALTIME, &time);
                                ^
1 warning and 2 errors generated.

Solution

It’s because using the -std=c99 option defines a macro that causes the declaration of clock_gettime to be hidden.

The GNU C library can expose different versions of its functions, or hide them altogether, depending on what is expected by user code. User code declares those expectations by defining certain specially-named macros (called feature test macros, though this is something of a misnomer) with pre-determined values recognised by the GNU C library headers. One such macro is __STRICT_ANSI__, which is implicitly defined if any of the -std=cXX options are used.

Quoting feature_test_macros(7):

__STRICT_ANSI__

ISO Standard C. This macro is implicitly defined by gcc(1) when invoked with, for example, the -std=c99 or -ansi flag.

[…]

If any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc 2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc 2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_SOURCE are not defined by default.

Using -std=gnuXX option instead of -std=cXX does not cause __STRICT_ANSI__ to be defined; it also enables GNU extensions to the C language. If you want to keep those language extensions disabled, you can instead put #define _DEFAULT_SOURCE 1 at the very top of the file to expose all the symbols the C library otherwise would expose (or perhaps another feature test macro to adjust the library interface to your liking). An explicit feature test macro will take precedence over __STRICT_ANSI__.