main() return value standard - stupid question

I feel kinda dumb asking this question, but I'm in an argument with someone about whether or not to use "return 0;" at the end of main(). I'm not sure if I'm reading the standard wrong, but shouldn't a program return with a value of zero on success? Maybe I'm being inept at searching tonight, but I didn't find any answers than answered my question.

Not including the "return 0;" at the end of main() would not guarantee this. The actual return value would be fairly unpredictable.

Basically is there a standard that says "return 0;" should appear at the end of main()? Are there good reasons NOT to return a value of zero on a successful program execution?

My copy of C99 states:
"If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified." (emphasis mine)

So, if you do not explicitly return a value from main(), main() will return 0.

On the other hand, I understand that C89 requires that some value be explicitly returned from main(), 0 meaning success.

return 0;
return EXIT_SUCCESS;
return EXIT_FAILURE;
These are the good things to return.
Any other value is implementation specific.
Not returning anything at all is undefined.

In new C++ (only, I think), you can get away with omitting a return statement, and the compiler will add "return 0;" for you. But since this trick only works with main, it seems like a sop to all the void main programmers to make it easier for the poor dears to make their programs compliant by only changing 'void' to 'int'

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apparently, C99 has the same soppyness as well.
</edit>


> Are there good reasons NOT to return a value of zero on a successful program execution?
The grep program is an example.
It returns 0 if something is found, and 1 if nothing is found.
As far as the grep program itself is concerned, it ran to completion successfully. It wasn't its fault that nothing could be found.

Well, 99.9&#37; of the time you want main() to return 0 at the closing brace, so I think it's reasonable to make it happen by default - and taking advantage of this reduces the chance that someone will forget that 0 is the normal return value, and write "return 1;" by mistake. And without the implicit "return 0;", there's no way to ensure that main() always has a well-defined return value - unless the compiler requires an explicit return at the closing brace, but in general there's no way to be sure if the closing brace will be reached, which is probably why compilers like gcc seem to allow functions with a return value but no explicit return statement to compile.

> On the other hand, I understand that C89 requires that some value be explicitly returned from main(), 0
> meaning success.

I believe that C89 doesn't actually require an explicit return statement in main(), but that without one, the return value is undefined - gcc seems to confirm this, since no explicit return leads only to a warning, not an error saying that ISO C90 requires such-and-such.

Ah, makes sense. So it's more just a good idea to include the 'return 0;' but not really required. Not that it's hard to type that one extra nine or so characters. I suppose it just bugs me because it gives unpredictable behavior.

Thanks for helping clarify.

You never know when someone else will check the return value of your program, even if you don't, so it's a good idea to always put it in (in C, anyway - I leave it out in C++).

Quote:

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Originally Posted by robatino

You never know when someone else will check the return value of your program, even if you don't, so it's a good idea to always put it in (in C, anyway - I leave it out in C++).

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Those were my thoughts (well minus the C++ part since I don't know all of the basics of that language). I was explaining to someone why I thought they should always have a defined return value at the end of their program when someone else said only put it in if you have to so I started arguing with the second person about it. Unpredictable behavior just bugs me.