warning: incompatible implicit declaration of built-in function ‘strstr’

Problem :

When I try to compile c program, I met the following error message:
warning: incompatible implicit declaration of built-in function ‘strstr’

Analysis :

This means that no header file contains the strstr function that I use in the program.

Solution : 

Add the header file  according to my function (strstr)

#include <string.h>