warning: implicit declaration of function

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);

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