C function defined as int but having no return statement in the body still compiles

Question

Say you have a C code like this:

#include 

int main(){
    printf("Hello, world!\\n");
    printf("%d\\n", f());    
}

in         


        

Answer 1

18 is the return value of the first print statement. (the number of characters printed)

This value is stored in stack memory.

In second printf function the stack value is 'returned' by function f. But f just left the value that printf created in that slot on the stack.

for example, in this code:

#include 

int main(){
    printf("Hello, world!1234\n");
    printf("%d\n", f());    
}

int f(){

}

Which compiles fine with gcc, and the output (on my system) is:

Hello, world!

18

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