Why is GCC inconsistent about emitting warnings for undeclared functions?

Question

The following test.c program

int main() {
   dummySum(1, 2);
   return 0;
}

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

...doesn't generate any warning when compiled with gcc -o test test.c , whereas the following one does:

int main() {
   dummySum(1, 2);
   return 0;
}

void dummySum(int a, int b) {
   a + b;
}

Why?

Answer 1

When faced with an undeclared function, the compiler assumes a function that accepts the given number of arguments (I think) and returns int (that part I'm sure of) . Your second one doesn't, and so you get the redefinition warning.

I believe, based on a very quick scan of the foreward, that C99 ( PDF link ) removed this. No great surprise that GCC still allows them (with a warning), though; I can't imagine how much code would start failing to compile...

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Recommend using -Wall (turning on all warnings) so you get a huge amount of additional information (you can turn off specific warnings when you have a really good reason for whatever you're doing that generates them if need be).

Answer 2

A function cannot be used before it has been declared. When a function declaration is not visible, the implementation assumes in C89 that the function:

• takes an unspecified (but fixed) number of parameters

• returns an int

This is called an implicit function declaration .

In C99, implicit declarations of function have been removed of the language and the implementation is free to refuse to translate the source code.