What does "implicit declaration of function" mean?

Question

#include <stdio.h>

int main()
{
    int a = 4;
    int b = 3;
    addNumbers(a, b);
}

int addNumbers(int a, int b)
{
    return a + b;
}

Why does this not compile, I get a message saying implicit declaration of function addNumbers()?

Answer 1

Either define the function before main() or provide its prototype before main().

So either do this:

#include <stdio.h>

int addNumbers(int a, int b)
{ //definition
}

int main()
{ //Code in main
  addNumbers(a, b);
}

or this:

#include <stdio.h>

int addNumbers(int, int);
int main()
{ //Code in main
  addNumbers(a, b);
}

int addNumbers(int a, int b)
{ // definition
}