Revisiting the main() Function

Revisiting the main() Function

As you've learned, each C program should have one and only one main() function. The execution of a program starts and ends at its main() function.

As with other functions in C, you can pass arguments to a main() function. So far, I've been using the void keyword in the definition of the main() function to indicate that there are no arguments passed to the function. Now, the question is how to do it if you want to pass information to the main() function.
Command-Line Arguments

Because each C program starts at its main() function, information is usually passed to the main() function via command-line arguments.

A command-line argument is a parameter that follows a program's name when the program is invoked from the operating system's command line. For instance, given a C program, test.c, whose executable file is called test.exe, if you run the program from a DOS prompt like this,

C:\app>test.exe argument1 argument2 argument3


argument1, argument2, and argument3 are called command-line arguments to the main() function in the test.c program.

Of course, you can simply omit an executable file's extension, .exe, when you run it from a DOS prompt.

The next subsection teaches you how to receive command-line arguments.
Receiving Command-Line Arguments

There are two built-in arguments in the main() function that can be used to receive command-line arguments. Usually, the name of the first argument is argc, and it is used to store the number of arguments on the command line. The second argument is called argv and is a pointer to an array of char pointers. Each element in the array of pointers points to a command-line argument that is treated as a string.

In order to use argc and argv, you have to declare them in the main() function in your program like this:

data_type_specifier main(int argc, char *argv[])
{
   . . .
}

Here data_type_specifier specifies the data type returned by the main() function. By default, the data type returned by the main() function is int. If the main() does not return any value, you should put the void keyword in front of the main() function definition.

Let's continue to use the example shown in the last section. Suppose that the main() function defined in the test.c program looks like this:

void main(int argc, char *argv[])
{
   . . .
}

 

If you run the executable file of the program from a DOS prompt,

C:\app>test.exe argument1 argument2 argument3

the value received by argc is 4, because the name of the program itself is counted as the first command-line argument. Accordingly, argv[0] holds the string of the path and program name C:\app\test.exe, and argv[1], argv[2], and argv[3] contain the strings of argument1, argument2, and argument3, respectively. (Note that C:\app\ is the path to the executable file on my machine.)

The program in Listing 18.5 is another example of passing command-line arguments to the main() function.
 

TYPE
Listing 18.5. Passing command-line arguments to the main() function.

1:  /* 18L05.c: Command-line arguments */
2:  #include <stdio.h>
3:
4:  main (int argc, char *argv[])
5:  {
6:     int i;
7:
8:     printf("The value received by argc is %d.\n", argc);
9:     printf("There are %d command-line arguments passed to main().\n",
10:           argc);
11:
12:    printf("The first command-line argument is: %s\n", argv[0]);
13:    printf("The rest of the command-line arguments are:\n");
14:    for (i=1; i<argc; i++)
15:       printf("%s\n", argv[i]);
16:
17:    return 0;
18: }

After the executable, 18L05.exe, is executed and passed with several command-line arguments, the following output is displayed on the screen:

OUTPUT

C:\app>18L05 Hello, world!
The value received by argc is 3.
There are 3 command-line arguments passed to main().
The first command-line argument is: C:\app\18L05.EXE
The rest of the command-line arguments are:
Hello,
world!
C:\app>

ANALYSIS

The purpose of the program in Listing 18.5 is to show you how to check the number of command-line arguments and print out the strings that hold the arguments entered by the user.

Note that there are two arguments, argc and argv, that are declared in line 4 for the main() function. Then, the statements in lines 8 and 9 print out the value of the total number of arguments held by argc. If there is no command-line argument entered by the user, argc contains the default value of 1 because the name of the program itself is counted as the first argument.

Line 12 prints out the first string saved in the memory location pointed to by argv[0]. As you can see from the output, the content of the first string is the executable file name of the program in Listing 18.5, plus the path to the executable file.

The for loop in lines 14 and 15 displays the rest of the strings that contain the command-line arguments entered by the user. In this example, I enter two command-line argument strings, "Hello," and "world!", which are shown back on the screen after the execution of the for loop.

NOTE

    argc and argv are normally used as the two built-in arguments in the main() function, but you can use other names to replace them in their declarations.
    In addition to these two arguments, some compilers may support another argument to the main() function. The third argument is a pointer to an array of pointers that are used to point to memory locations containing the environmental parameters, such as the paths, the Windows boot directory name, the temporary directory name, and soon.