C: defining function below main - why does this compile?

Question

struct ack {
    char a, b, c;
};

main()
{
   struct ack bad;

   foo(bad);
}

foo(c)
   struct ack c;
{

}

This is a test case in gcc. When I try to compile it using gcc4.8, it compiles without problem. However, I have learned that you have to declare your functions before main . Why does this even compile?

Answer 1

Try to compile with

-std=c11 -pedantic-errors

and you'll get the required diagnostics.

By default gcc compiles with -std=gnu89 which is c89 + GNU extensions. The c89 rule for implicit declarations has been removed in c99.

Answer 2

Currently, in general, when the compiler finds the call to foo(bad) in main() and foo is NOT defined, then the compiler assumes it exists and will return int .

This punts the problem to the linker who will complain if foo() is NOT defined somewhere in the source file.