got compile error when use clock_gettime in c99

Question

when I use clock_gettime in my code snippet, and compile with flag -std=c99, I got a error like this:

warning: implicit declaration of function 'clock_gettime'
error: 'CLOCK_REALTIME' undeclared (first use in this function)

and I have included the file 'time.h'. anyone knows how to fix it.

Answer 1

in your original code with -std=c99, try adding

#define _POSIX_C_SOURCE >= 199309L

the man page for clock_gettime indicates this is a necessary feature test macro requirement.