int main() returns?

There are actually two approaches. First is platform-specific, just an example from man page, what does UNIX grep returns.

EXIT STATUS
     The following exit values are returned:

     0              One or more matches were found.
     1              No matches were found.
     2              Syntax errors or inaccessible files (even  if
                    matches were found).

So if you really need to differentiate between error codes, you can try to find out similar commands/programs and check what do they return.

Now, second part (and it would be good, if it is enough for you) is C++-laguage specific.
The standard requires cstdlib header files to contain two macros EXIT_SUCCESS and EXIT_FAILURE. So, maybe the easiest way for you is to return EXIT_FAILURE in case of any error, note also if your program does not return anything, it is equivalent to
return 0; // EXIT_SUCCESS
MSVC produces erroneous warning in this case.

Hope it helps

>> erroneous warning
Indeed, main /must/ return an integer so it should be an error message :)

3.6.1:
2 An implementation shall not predefine the main function.   This  func-
  tion  shall  not  be  overloaded.  It shall have a return type of type
  int, but otherwise its type is implementation-defined.  
...

>> special meanings (in DOS) for values returned by main(), but it
>> doesn't actually specify what means what

There are no standardized meanings, though errorlevel 0 is indeed often used as 'no-error-occured'

KangaRoo:

3.6.1.

-5-
A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the effect is that of executing

return 0;

-----------------------

Sometimes it is useful to read till the end ;-))

:-\

Borland compilers allow you to compile the main method with a return type of void, as well, although I tend to avoid this now.

Thanks for the feedback folks, I'll grade this at home later.