[c] Regarding 'main(int argc, char *argv[])'

Possible Duplicates:
What are the arguments to main() for?
What does int argc, char *argv[] mean?

Every program is starting with the main(int argc, char *argv[]) definition.

I don't understand what it means. I would be very glad if somebody could explain why we use these arguments if we don't use them in the program? Why not just: int main()?

Is the name of the program one of the elements of *argv[] and argc is the count of the number of arguments in *argv[]? What are the other arguments sent to *argv[]? How do we send them?

argc is the number of command line arguments and argv is array of strings representing command line arguments.

This gives you the option to react to the arguments passed to the program. If you are expecting none, you might as well use int main.

Those are for passing arguments to your program, for example from command line, when a program is invoked

$ gcc mysort.c -o mysort

$ mysort 2 8 9 1 4 5

Above, the program mysort is executed with some command line parameters. Inside main( int argc, char * argv[]), this would result in

Argument Count, argc = 7 

since there are 7 arguments (counting the program), and

Argument Vector, argv[] = { "mysort", "2", "8", "9", "1", "4", "5" };

Following is a complete example.

$ cat mysort.c
#include <stdio.h>
int main( int argc, char * argv [] ) {
    printf( "argc = %d\n", argc );
    for( int i = 0; i < argc; ++i ) {
        printf( "argv[ %d ] = %s\n", i, argv[ i ] );
    }
}

$ gcc mysort.c -o mysort

$ ./mysort 2 8 9 1 4 5
argc = 7
argv[ 0 ] = ./mysort
argv[ 1 ] = 2
argv[ 2 ] = 8
argv[ 3 ] = 9
argv[ 4 ] = 1
argv[ 5 ] = 4
argv[ 6 ] = 5

[The char strings "2", "8" etc. can be converted to number using some character to number conversion function, e.g. atol() (link)]

You can run your application with parameters such as app -something -somethingelse. int argc represents number of these parameters and char *argv[] is an array with actual parameters being passed into your application. This way you can work with them inside of your application.

With argc (argument count) and argv (argument vector) you can get the number and the values of passed arguments when your application has been launched.

This way you can use parameters (such as -version) when your application is started to act a different way.

But you can also use int main(void) as a prototype in C.

There is a third (less known and nonstandard) prototype with a third argument which is envp. It contains environment variables.

Resources:

• Wikipedia - Main function, C and C++

argc means the number of argument that are passed to the program. char* argv[] are the passed arguments. argv[0] is always the program name itself. I'm not a 100% sure, but I think int main() is valid in C/C++.

The comp.lang.c FAQ deals with the question

"What's the correct declaration of main()?"

argc is the number of command line arguments given to the program at runtime, and argv is an array of arrays of characters (rather, an array of C-strings) containing these arguments. If you know you're not going to need the command line arguments, you can declare your main at taking a void argument, instead:

int main(void) {
    /* ... */ 
}

Those are the only two prototypes defined for main as per the standards, but some compilers allow a return type of void as well. More on this on Wikipedia.