I understand why C requires a main function to begin the execution of a program, but of the now three books I've read entirely or in portion, none has explained why programs begin by declaring main as int, or with an argument of void:
int main(void)
can someone tell me what the purpose of this is?
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.
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 (e.g., 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.
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.
Like any other function in C, main is also a function. Thus it has a return type and can
accept arguments.
int main(void)
Here int is the return type of main. Many people still use 'void' because they do not
update themselves with the current language standards. haccks's answer mentions about
the latest standard specifying the signature of main function.
Its general and good practice to have int as main's return type as it tells the parent process of main about the termination (success / failure) of the program.
Like any other function main is also capable of accepting arguments, but with an exception, i.e. the arguments to main are given before the execution of program starts. These are called "command line arguments".
main can accept arguments in two ways :
1. int main(void)
or
int main()
2. int main(int argc, char *argv[])
or
int main(int argc, char **argv)
The first one says that main is not expecting any arguments where as the second declara-
-tion expects the user to provide command line arguments to main.
Note : main should take either 0 or 2 arguments. If you try to give any number of
arguments other than these then it gives the following warning when you compile your code
warning: ‘main’ takes only zero or two arguments [-Wmain]
Edit : The above warning is displayed if you are using gcc.
