Answer 1

The return value isn't used by the program itself, but by the operating system. Depending on your operating system, you can see the return value with a statement like "echo $?" (Linux) immediately after you run the program in a console. A return value other than 0 won't stop your program from running properly, but it will signal to the OS that something went wrong.

Answer 2

Anna But what would the OS do with that?

Like, say I create a program and use return 1; or a value other than 0. It should run fine, but it'll signal that something went wrong, like you said. Now what does the OS do?

Answer 3

Daniel Cooper The OS will likely do nothing (it may store the fail in a log file). If the program is part of the system itself then the OS might be programmed​ to know what it means at act accordingly, but not sure how common this would be. Where it can be useful is if you have a program that calls a second program, the first program has access to the return value of the second program.