source.c:29:5: error: implicit declaration of function 'fwrite' is invalid in C99 [-Werror,-Wimplicit-function-declaration] fwrite("No !\n", 1, 5, stderr); code example

Example: error: implicit declaration of function 'write' is invalid in C99[-Werror,-Wimplicit-function-declaration]write(1, &c, 1);

implicit declaration of function 'read'
implicit declaration of function 'write'

//include this directive to make it work
#include <unistd.h>