```
gcc (GCC) 4.6.3
c89

```

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

> 
> implicit declaration of function usleep
> 
> 
> 

I have included the `unistd.h` header file.

The man pages mentions something about this. But I am not sure I understand by it:

```
usleep():
   Since glibc 2.12:
       _BSD_SOURCE ||
           (_XOPEN_SOURCE >= 500 ||
               _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
           !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
   Before glibc 2.12:
       _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

```

But not sure what I a to do with the above?