Hello world! Rather cliché isn’t it? Is there anything interesting about Hello World? Most would say no, but I am sure we can discover a few interesting bits of information about this mundane code. Let’s analyze the most ubiquitous program ever written.

#include <iostream>
#include <cstdlib>
 
int main()
{
  std::cout << "Hello World!" << std::endl;
  return EXIT_SUCCESS;
}

Boring right? Maybe not!

Int or Void

Let’s start with the basics, the return value. Returning an int allows the program to let the operating system know if the program executed successfully. And while many compilers support a void return value but it is not spec. The only valid definitions for main are:

3.6.1.2

int main() { /* … / } and int main(int argc, char argv[]) { /* … */ }

What to Return

Any integer value can be returned from main. For success, return EXIT_SUCCESS or 0. For failure, return EXIT_FAILURE. If you need multiple failure states, any non-zero integer can be used. Just remember to note what each failure state means.

It is also possible to exit the program at any point by calling std::exit(EXIT_SUCCESS) or std::exit(EXIT_FAILURE). However, using std::exit should be done with caution because objects of local scope, thread local scope, or static scope will not have their destructors executed, as the spec explains:

18.5.3

The program is terminated without executing destructors for objects of automatic, thread, or static storage duration and without calling functions passed to atexit()

It is also important to know that main is the only function that does not require a return statement. When no return statement is defined, 0 is automatically returned by main. Some compilers allow omitting the return statement for any function. To understand why this might be a problem let’s look at how one cpu architecture handles return values. On x86, the return value if stored in the eax register. The eax register is also used for temporary storage because registers are fast and there are a limited number of them. This means that if the return value has not been set, then the value in eax remains undefined. So the function will also return an undefined value. This is not good if you want other people to use your functions.

The History of Return Values

So why is returning zero so common? Originally, C did not have a void type. Not having void meant that every function needed to define some return value. A common practice was to use an int return value when none was needed. And as we learned before, if we have a return value, we need to return a value, or noone will want to use our code. This lead to programmers adopting the convention of explicitly using return 0; on any function that did not require a return value. As time went on eventually 0 introduced the void type but it is still very common to see functions return 0;.

Summation

If you made it here, I applaud you. Understanding specs is not the most exciting thing but hopefully you learned something and can use the information to become a better programmer.

See you next time!