Question

My compiler (GCC) is giving me the warning:

warning: implicit declaration of function

Why is it coming?

Answer 1

You are using a function for which the compiler has not seen a declaration ("prototype") yet.

For example:

int main() {
    fun(2, "21"); /* The compiler has not seen the declaration. */       
    return 0;
}

int fun(int x, char *p) {
    /* ... */
}

You need to declare your function before main, like this, either directly or in a header:

int fun(int x, char *p);