Where does the returned value for 'main' function go?

Question

In C, a function always returns its value to the calling function and never to itself (if return type is not void). Like,

int main()

But since 'main' function is called by the Operating System, which is not a function. So, whom does the 'main' function returns its value? Where does the value go, when its returned using the expression

return(0);

at the end of the program?

Answer 1

This is completely OS-specific, but usually the OS invokes a program by

1. Setting up the program's address space,
2. Creating a record of the new process somewhere in the OS internals,
3. Launching its own custom function, which calls main and stores the return value in the OS internals.

The function I'm referring to in (3) is often a good old fashioned C function that just jumps into main so it can capture the return value from main as if it were an ordinary C function (which it in fact is). Afterwards, it stores that value somewhere in OS-land so that processes that need to read the return value can do so.

Hope this helps!

Answer 2

As already said, this is OS specific. In UNIX systems it looks something like this:

When you compile a program with gcc, it wraps a startup routine around your main() function. This routine calls your main() function and saves its return value. It then calls the exit() function (which your program might call as well), that does some general clean up. This function then again calls _exit(), which is a system call that tells the OS to save the returned value of your main() function in the process table (where meta information about your process is saved). As soon as another process calls wait() on your process id (PID), your returned value is given to the calling process and your process is removed from the table.

Look at this resource for more information: http://www.johnloomis.org/ece537/notes/Processes/process_environment.html