implicit declaration of function usleep

Question

gcc (GCC) 4.6.3
c89

I am trying to use usleep. However, I keep getting the following warning:

implicit declarat

Answer 1

Add the following to the top of your code:

// For `nanosleep()`:
#include 

#define __USE_POSIX199309
#define _POSIX_C_SOURCE 199309L

And then use nanosleep() instead, to create your own sleep_us() function to sleep a set number of microseconds:

void sleep_us(unsigned long microseconds)
{
    struct timespec ts;
    ts.tv_sec = microseconds / 1e6;             // whole seconds
    ts.tv_nsec = (microseconds % 1e6) * 1e3;    // remainder, in nanoseconds
    nanosleep(&ts, NULL);
}

For compiling and running on Linux Ubuntu, I created a sleep_test.c file and used:

gcc -Wall -g3 -std=c11 -o sleep_test sleep_test.c && ./sleep_test

References:

1. (This is intentionally a circular reference: see my comments under this answer): Is there an alternative sleep function in C to milliseconds?
2. http://man7.org/linux/man-pages/man2/nanosleep.2.html