Implicit declaration of timersub() function in Linux - what must I define? - c

Question

I am trying to compile timersub() function in linux but i always get:
test.c: In function ‘main’:
test.c:27:2: warning: implicit declaration of function ‘timersub’ [-Wimplicit-function-declaration]
timersub(&now, &then, &diff);
^
/tmp/ccLzfLsl.o: In function `main':
test.c:(.text+0x55): undefined reference to `timersub'
collect2: error: ld returned 1 exit status
this is just a simple code of the function with all the library that i use..
#define _XOPEN_SOURCE
#define _POSIX_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include "openflow.h"
#include "cbench.h"
#include "fakeswitch.h"
#include <unistd.h>
int main()
{
struct timeval now, then, diff;
gettimeofday(&then,NULL);
sleep(1);
gettimeofday(&now, NULL);
timersub(&now, &then, &diff);
return 0;
}
i am compiling it with:
gcc --std=c99 -Wall -DTRACE -o test test.c

Answer 1

See the manual. It's not in POSIX but in BSD function. So you need _BSD_SOURCE.
Define it at the top:
#define _BSD_SOURCE
or alternatively compile with:
gcc --std=c99 -Wall -DTRACE -D_BSD_SOURCE -o test test.c
Since Glibc 2.20, the macro _BSD_SOURCE has been deprecated and has been superseded by _DEFAULT_SOURCE. From feature test macros:
_DEFAULT_SOURCE (since glibc 2.19)
This macro can be defined to
ensure that the "default" definitions are provided even when the
defaults would otherwise be disabled, as happens when individual
macros are explicitly defined, or the compiler is invoked in one of
its "standard" modes (e.g., cc -std=c99). Defining _DEFAULT_SOURCE
without defining other individual macros or invoking the compiler in
one of its "stan‐ dard" modes has no effect.
The "default" definitions comprise those required by POSIX.1-2008 and
ISO C99, as well as various definitions originally derived from BSD
and System V. On glibc 2.19 and earlier, these defaults were
approximately equivalent to explicitly defining the following:
cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809
But if you are using an older Gblic, you'd still need to use _BSD_SOURCE.

Answer 2

Basically i just deleted #define _XOPEN_SOURCE and #define _POSIX_SOURCE and compiled it only with gcc -Wall -DTRACE and it worked