implicit return value in C with no explicit return in a function that should return non-void

Question

I have some legacy C code that I compiled with gcc version 4.9.2 on Linux with return-type warning on [-Wreturn-type]. I have a function as below:

int somefn() {
   // .. do something ..
   // no explicit return statement
}

and the caller had the call as below:

if (somefn()){
   // handling of success
}
else {
  // handling of failure
}

When warnings were suppressed, compilation+linking went all ok, and at runtime, we may get surprises, What could go wrong?

Answer 1

I compiled with gcc version 4.9.2 on Linux with return-type warning on [-Wreturn-type] The caller expected a true (in C, interpreted as non-zero) for success and 0 for failure. This is what was happening. Since a function that returns non-void, when called, space is created on the stack for the return value, then context is switched to the callee, and finally, when the control comes back to the caller, context is switched back popping all local variables from stack and keeping the return value there on stack to be popped by the caller after return from callee. Thus, having no explicit return statement caused some value to be returned. That is equivalent to having explicit return statement as ‘return 1;’ or 'return 0;' as appropriate. So, better (of course) would be to have explicit return statement e.g.

int somefn() {
   // .. do something ..
   if (..somecond..)
    return 0; // failure
   else
    return 1; // success
}

To avoid surprises, I would say, compilers should flag 'no return statement in function returning non-void' as error.