Is there a default return value of C functions

Question

I'm amazed that a file with the following lines of code is properly built and the linker generates an executable. 

Except for main, I believed all functions had to include a proper return statement unless the return type was void.

int foo(){}
double bar(){}
int main(){}

What am I missing?

Answer 1

They really must.

If they don't, you'll receive undefined behaviour.

If the function is sufficiently sophisticated, the compiler may find it difficult or impossible to determine if all return pathways produce a value; hence, the compiler is not obliged to diagnose the mistake.

If you enable warnings, most compilers will produce warnings in many circumstances.