10.2. Arguments to main

For those writing programs which will run in a hosted environment, arguments to main provide a useful opportunity to give parameters to programs. Typically, this facility is used to direct the way the program goes about its task. It's particularly common to provide file names to a program through its arguments.

The declaration of main looks like this:

int main(int argc, char *argv[]);

This indicates that main is a function returning an integer. In hosted environments such as DOS or UNIX, this value or exit status is passed back to the command line interpreter. Under UNIX, for example, the exit status is used to indicate that a program completed successfully (a zero value) or some error occurred (a non-zero value). The Standard has adopted this convention; exit(0) is used to return ‘success’ to its host environment, any other value is used to indicate failure. If the host environment itself uses a different numbering convention, exit will do the necessary translation. Since the translation is implementation-defined, it is now considered better practice to use the values defined in <stdlib.h>: EXIT_SUCCESS and EXIT_FAILURE.

There are at least two arguments to main: argc and argv. The first of these is a count of the arguments supplied to the program and the second is an array of pointers to the strings which are those arguments—its type is (almost) ‘array of pointer to char’. These arguments are passed to the program by the host system's command line interpreter or job control language.

The declaration of the argv argument is often a novice programmer's first encounter with pointers to arrays of pointers and can prove intimidating. However, it is really quite simple to understand. Since argv is used to refer to an array of strings, its declaration will look like this:

char *argv[]

Remember too that when it is passed to a function, the name of an array is converted to the address of its first element. This means that we can also declare argv as char **argv; the two declarations are equivalent in this context.

Indeed, you will often see the declaration of main expressed in these terms. This declaration is exactly equivalent to that shown above:

int main(int argc, char **argv);

When a program starts, the arguments to main will have been initialized to meet the following conditions:

• argc is greater than zero.
• argv[argc] is a null pointer.
• argv[0] through to argv[argc-1] are pointers to strings whose meaning will be determined by the program.
• argv[0] will be a string containing the program's name or a null string if that is not available. Remaining elements of argv represent the arguments supplied to the program. In cases where there is only support for single-case characters, the contents of these strings will be supplied to the program in lower-case.

To illustrate these points, here is a simple program which writes the arguments supplied to main on the program's standard output.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
        while(argc--)
                printf("%s\n", *argv++);
        exit(EXIT_SUCCESS);
}

If the program name is show_args and it has arguments abcde, text, and hello when it is run, the state of the arguments and the value of argv can be illustrated like this:

Figure 10.1. Arguments to a program

Each time that argv is incremented, it is stepped one item further along the array of arguments. Thus after the first iteration of the loop, argv will point to the pointer which in turn points to the abcde argument. This is shown in Figure 10.2.

Figure 10.2. Arguments to a program after incrementing argv

On the system where this program was tested, a program is run by typing its name and then the arguments, separated by spaces. This is what happened (the $ is a prompt):

$ show_args abcde text hello
show_args
abcde
text
hello
$