What do I need to do to make gcc include the declaration of gmtime_r(3) from time.h? Looking at /usr/include/time.h, gmtime_r and localtime_r are inside #ifdef __USE_POSIX.
Do I need to do something to turn __USE_POSIX on, like a command-line option? I'm running with -std=c99 now. I understand that __USE_* macros are not intended to be set directly by user code.
/* Return the `struct tm' representation of *TIMER
in Universal Coordinated Time (aka Greenwich Mean Time). */
extern struct tm *gmtime (const time_t *__timer) __THROW;
/* Return the `struct tm' representation
of *TIMER in the local timezone. */
extern struct tm *localtime (const time_t *__timer) __THROW;
#ifdef __USE_POSIX
/* Return the `struct tm' representation of *TIMER in UTC,
using *TP to store the result. */
extern struct tm *gmtime_r (const time_t *__restrict __timer,
struct tm *__restrict __tp) __THROW;
/* Return the `struct tm' representation of *TIMER in local time,
using *TP to store the result. */
extern struct tm *localtime_r (const time_t *__restrict __timer,
struct tm *__restrict __tp) __THROW;
#endif /* POSIX */
The proper way to do this is:
#define _POSIX_C_SOURCE 200112L
#include <time.h>
This method indicates explicitly the features that a source file requires, making it self-contained. It is also portable to any POSIX system using any compiler. No special compiler flag is needed.
With GCC this code will compile with -std=c89, -std=c99, or any other value. Importantly what -std=gnu?? enables by default might differ depending on version or platform.
See Feature Test Macros for details.