Geoff Nixon Geoff Nixon - 1 year ago 49
C Question

Is ‘int main;’ a valid C/C++ program?

I ask because my compiler seems to think so, even though I don’t.

echo 'int main;' | cc -x c - -Wall

echo 'int main;' | c++ -x c++ - -Wall

Clang issues no warning or error with this, and gcc issues only the meek warning:
'main' is usually a function [-Wmain]
, but only when compiled as C. Specifying a
doesn’t seem to matter.

Otherwise, it compiles and links fine. But on execution, it terminates immediately with
(for me).

Reading through the (excellent) answers at What should main() return in C and C++? and a quick grep through the language specs, it would certainly seem to me that a main function is required. But the verbiage from gcc’s
(‘main’ is usually a function) (and the dearth of errors here) seems to possibly suggest otherwise.

But why? Is there some strange edge-case or “historical” use for this? Anyone know what gives?

My point, I suppose, is that I really think this should be an error in a hosted environment, eh?

Answer Source

Since the question is double-tagged as C and C++, the reasoning for C++ and C would be different:

  • C++ uses name mangling to help linker distinguish between textually identical symbols of different types, e.g. a global variable xyz and a free-standing global function xyz(int). However, the name main is never mangled.
  • C does not use mangling, so it is possible for a program to confuse linker by providing a symbol of one kind in place of a different symbol, and have the program successfully link.

That is what's going on here: the linker expects to find symbol main, and it does. It "wires" that symbol as if it were a function, because it does not know any better. The portion of runtime library that passes control to main asks linker for main, so linker gives it symbol main, letting the link phase to complete. Of course this fails at runtime, because main is not a function.

Here is another illustration of the same issue:

file x.c:

#include <stdio.h>
int foo(); // <<== main() expects this
int main(){
    printf("%p\n", (void*)&foo);
    return 0;

file y.c:

int foo; // <<== external definition supplies a symbol of a wrong kind


gcc x.c y.c

This compiles, and it would probably run, but it's undefined behavior, because the type of the symbol promised to the compiler is different from the actual symbol supplied to the linker.

As far as the warning goes, I think it is reasonable: C lets you build libraries that have no main function, so the compiler frees up the name main for other uses if you need to define a variable main for some unknown reason.