Omitting return is allowed in main.

On Linux/Unix you will get in a (somewhat) random exit value, so if you run the program from a "set -e" script it will randomly fail. I guess that didn't bother whoever "allowed" that, but I consider it a bug.

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.

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

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.

C++ standard 3.6.2[5] since like forever...

A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std::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;