Why and how does GCC compile a function with a missing return statement?

Question

#include 

char toUpper(char);

int main(void)
{
    char ch, ch2;
    printf(\"lowercase input : \");
    ch = getchar();
    ch2 = toUpper(ch);
             


        

Answer 1

It depends on the Application Binary Interface and which registers are used for the computation.

E.g. on x86, the first function parameter and the return value is stored in EAX and so gcc is most likely using this to store the result of the calculation as well.