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In C yes, on C++ it is required to return 0 if omited.

C99 also defines it:

>5.1.2.2.3 Program termination [...] reaching the } that terminates the main function returns a value of 0.

Thanks for the heads up, than it sums it up either the OP was using a broken implementation, or the incorrect language version.

There's an implicit return zero in C++.

It shouldn't in C++. Omitting an explicit return (or falling through to the end without hitting another return) should have the same result as if you ended it with `return 0`. If it's returning a random value then that is erroneous behavior. I've also never witnessed this and print non-zero exit codes in my command line prompt (so after execution my prompt looks like `blahblah(EXIT_CODE) $`).

And I just looked it up for C. Since C99, if the return type of `main` is `int` and there is no explicit return (or again you fall through to the end without hitting one) there's an implicit `return 0`. Before C99, undefined.