Writing C main function

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What should main() return in C and C++? 19 answers

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The return value of main() is used to indicate success or failure to its parent process. More generally, it can be used to communicate back specific statuses as well, though C doesn't define those.

If main() returns 0 or EXIT_SUCCESS , then the program was successful. EXIT_FAILURE or non-zero, then it failed.

The void in the parameter list simply says that it takes no arguments. This is because of a (mis)feature of C which allows you to declare a function without fully specifying the paramters it takes. A function declared int func(); can be called with any number of parameters, but int func(void); can only becalled with zero.

Example

on linux,

two trivial programs:

$ cat ret0.c
int main (void) { return 0; }
$ cat ret42.c
int main (void) { return 42; }

Then in `bash` we can look at
$ ./ret0 ; echo $?
0
$ ./ret42 ; echo $?
42

So it's possible to use that status when calling your program.

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The int return is there to give an error indicator back to the OS. return 0 means no error, all other codes (typically return 1 ) indicates the program could not finish successfully. Other programs (eg, shell scripts) can use this error code to determine if your program executed its task, or ran into a problem.

void just means no arguments. It's the same as

int main()
{
    /* program */
}

but more explicit.

A program can take command line arguments, in which case main must be defined as

    int main(int argc /* number of arguments */, char *argv[] /* arguments)
    {
        /* program
    }

Any good book on C should explain this.

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First off let us forget about main. In C(not C++) if you define a function with no parameters like this

int f(){ return 0;}

It is legal to call such a function with any number of arguments:

int a = f(); /* legal */
int a = f("help", 1, 2.0); /* legal */

If you want your function f to only work with exactly no arguments you can amend it like this:

int f(void){return 0;}
int a = f(); /* legal */
int a = f("help", 1, 2.0); /* not legal as it has too many parameters */

The same thing applies to main() and main(void) . In most cases in the reasonable world most people would never care however I have encountered legal code that calls main within the program.

So declaring main like:

int main() {
     /* Do a bunch of stuff here */
}

Allows for code elsewhere in your program to do this:

main(); 
main(1,2,3,4);

By declaring main(void) you add a compiler check that prevents the latter example main(1,2,3,4) from compiling.

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