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

ANSWER TO QUESTION: use #define _BSD_SOURCE or #define _GNU_SOURCE

For those with the error

warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]
 # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
   ^~~~~~~

after using #define _BSD_SOURCE try using

#define _GNU_SOURCE

NOTE: use before you include the header that gives you usleep() i.e. before including unistd.h