What does returning zero by convention mean?

Question

This is likely a stupid question but I always find myself wondering which is the standard.

In most (not to say all) C++ first examples you may see the main function returning 0 value. This means the operation went ok or not?

• 0 --> OK
• 1 --> No OK.
• Other --> ?

Which is the standard way of doing it?

By the way, is it better to return an integer or a boolean in this case?

Thank you guys!

Answer 1

0 or EXIT_SUCCESS means success. EXIT_FAILURE means failure. Any other value is implementation defined, and is not guaranteed to be supported. In particular, std::exit(1) or return 1; are not actually guaranteed to indicate failure, although on most common systems they will.

EXIT_SUCCESS and EXIT_FAILURE are defined in <cstdlib>.

Edit: I suppose it might be useful to give a system-specific example:

The GNU make utility returns an exit status to the operating system:

• 0: The exit status is zero if make is successful.
• 2: The exit status is two if make encounters any errors. It will print messages describing the particular errors.
• 1: The exit status is one if you use the `-q' flag and make determines that some target is not already up to date.

The ability to set multiple different values of failure means you can specify exactly how your program failed. There are two caveats, however:

• There is no convention for failure status codes, afaik
• This will work on "normal" OSes (windows, os x, unix), but it's not guaranteed by the C++ standard; so it might not work if you tried porting to VMS or some embedded system.

Answer 2

0 is ok, other values are an error code. The main function should return an int and nothing else according to the standard. The question has been discussed before at What should main() return in C and C++?