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Possible Duplicate:
What should main() return in C/C++? 

#include<stdio.h>
int main()
{
    return 0;
}

In the code snippet given above, where does the return 0 returned by main go? Or in other words which function called the main function in the beginning.

Community
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Dref D
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4 Answers4

13

main is called by some startup function in the C runtime library. The C language standard says that returning from main is equivalent to calling the exit function, so most C runtimes look something like this:

void _start(void)  /* Exact function signature may vary */
{
    /* Platform-specifi startup (e.g. fetch argc and argv from the OS) */
    ...
    int status = main(argc, argv);
    exit(status);
    /* Never reached */
}

The exit status gets passed back to the operating system, and then what happens from there is OS-dependent.

When you compile and link your program, the executable file format (e.g. PE or ELF) contains a start address, which is the virtual address at which execution begins. That function is typically part of the C runtime library (like the example _start above). That function has to end by calling a system call such as exit, since if it just returned, it would have nowhere to go to: it would just pop an address off the stack and jump to that location, which would be garbage.

Depending on how the OS loader initializes processes, the program arguments argc, argv, and other data (such as the environment) might either come in as function parameters (either through registers or the stack), or they might require system calls (e.g. GetCommandLine on Windows) to retrieve them. But dealing with all of that is the job of the C runtime, and unless you're explicitly going out of your way to avoid using the C runtime, you needn't worry about those details.

Adam Rosenfield
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    If you are guessing, please at least do it correctly - on most hosting environments, I saw `_start()` having the same signature as `main()`. –  Nov 02 '12 at 18:27
  • I don't think "main" is explicitly searched for. Main is just a convention used by compilers. A program begins after it's file format header ends. The OS scans the format header to understand specifics about the file including (I think) where the the program begins or it may understand this implicitly based on other provided header information. –  Nov 03 '12 at 05:46
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    @annoying_squid no, main is just a symbol like every other, and the startup stub calls it –  Nov 03 '12 at 06:34
3

Your compiler targets a certain platform, which includes operating-system specific mechanisms for starting processes. Part of that platform specific code contains the return value of main. When you link your program into an executable, there is an OS-specific piece of binary code that your linker adds which takes care of calling main and reporting the return value back to the operating system.

Kerrek SB
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2

The return value goes to the hosted environment. Typically, operating system calls main and gets exits status of the program.

P.P
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0

where does the return 0 returned by main go? Or in other words which function called the main function in the beginning.

It is called by the C startup library, a stub function that is called (almost) directly by the kernel. For example, on Linux and OS X, it's a function named _start. It has the same signature as main() and the operating system itself uses its return value.