The main function and exit code

A program is often launched not by a person but by another program, such as a script or a system that checks assignments. The launching program needs to know whether the launched one succeeded, and parsing its output for that is awkward.

Picture a courier back from a delivery. The dispatcher does not need the story of the trip: a mark on the delivery note is enough—delivered or not. A program has the same kind of mark, only it is a number. So main returns a number:

#include <iostream>

int main() {
    std::cout << "Done\n";  // Done
    return 0;
}

Let's go through the body of main line by line:

1. std::cout << "Done\n"; prints Done.
2. return 0; ends main and hands the number 0 to whoever launched the program.

The exit code

The int before main is the return type: the function returns an integer. The statement return 0; ends main, and 0 becomes the program's exit code. Whoever launched the program receives it. By convention, 0 means success and any other number means failure.

This is where the delivery-note picture stops working: the mark has no words, only a number, and the program's author decides what each number other than 0 means.

The exit code does not go to the output: return 0; prints nothing. You can see it this way:

• in Visual Studio, when the program ends, the console window shows a line that contains exited with code 0;
• in a Linux or macOS terminal, the command echo $? shows it.

return takes effect at once: the statements that come after it in main never run.

Ending without return

Of the functions that return a value, only main can do without return. If execution reaches the closing brace of main, the program returns 0, as after return 0;. That is how the Hello, world program works: it has no return, and its exit code is 0. That is why return 0; at the end of main is often left out.

In any other function that returns a value, ending this way is undefined behavior: the C++ standard does not say what happens. The compiler issues a warning in that case.

Common mistake

Here main is declared with void—"no result"—instead of int:

#include <iostream>

void main() {  // ✗
    std::cout << "Done\n";
}

error: 'main' must return 'int'

The C++ standard requires main to return int, so clang and GCC do not compile this program.

Key points

• main returns an int, the program's exit code; by convention 0 means success.
• return ends the function at once: the statements after it do not run.
• If main reaches its closing brace without return, the program returns 0.
• In any other function that returns a value, ending without return is undefined behavior.