Command Line Arguments in C

In C, we can supply arguments to ‘main’ function. Every C program has at least one function main(). The programs that we have discussed so far don’t pass any arguments to the main(). A question that immediately comes to one mind is that, can we pass any argument(s) to main()?
Yes, it is possible to pass arguments to main() on many systems like DOS and UNIX from the operating system’s prompt. These arguments passed to the main () by the user at the command prompt are known as command-line arguments. Command-line arguments are parameters to main when the program starts. These arguments are supplied at the time of invoking the program.

An expanded version of main() that receives command-line arguments specified by the user at the command prompt is of the form,

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

Here argc and argv are the two built-in formal arguments in main() that can receive command-line arguments. The first argument, argc (argument counter), specifies the number of arguments passed in the command line. It always has a count of at least one. The second argument, argv (argument vector), is an array of char type pointers. There is always at least one char pointer in the array pointed to by argv, i.e., argv [0]. The argv [0] points to the name of the program’s executable file. Each element in the array of pointer points to the command-line argument that is treated as a string. If argc is greater than 1, then the argv parameters, argv [1], argv[2], and so on point to the command line arguments as they were entered on the command line, and argv [argc] is a NULL pointer. White spaces separate these arguments.

Now let us suppose the fruit.c program is compiled and linked. So an executable file fruit.exe will exist. If you run the executable file of the program (fruit.exe) file from the DOS prompt as follows

C:\TC> fruit.exe first.txt second.txt

Then value received by argc will be 3 (two arguments plus one program name).
The argv[0] points to the “fruit.exe”, argv[1] to “first.txt” and argv[2] points to “second.txt” and argv[3] is a null pointer.

So argv[0] = fruit.exe
   argv[1] = first.txt
   argv[2] = second.txt
   argv[3] = NULL pointer

The primary use of the command line argument is while implementing DOS commands type, copy, etc. There is no need to recompile the program every time we want to use this utility.

How to Run a Program using Command Line Arguments

Following are the steps to execute a program containing command-line arguments.

1. After typing the program, we compile the program using ALT+ F9 keys from the keyboard.
2. Then we link the program by moving to Compile > Link and then press Enter from the keyboard. As a result, an executable file is created.
3. Quit the editor by going into the File menu bar and selecting Quit or going to the OS Shell option of the File menu bar and pressing Enter. Now type the following at the prompt.

4. C:\TC>ABC data1 data2

5. When you are done, type exit at the prompt to return to the program. Now let us consider a program to show how command-line arguments work.

Program to print strings entered as command line arguments.

#include<stdio.h>
int main(int argc, char *argv[]) {
 int n;
 printf("Following are the arguments entered in the command line");
 for (n = 0; n < argc; n++) {
  printf("\n %s", argv[n]);
}
 printf("\n Number of arguments entered are\n %d\n", argc);
 return 0;
}

Explanation: After typing the program, we compile the program using Alt + F9 keys from the keyboard. Then we link the program by moving to Compile> Link and then press Enter from the keyboard. As a result, an executable file will be created. Suppose the program name is fruit.C, then its executable file would be fruit.exe.Now go to file> DOS shell and press the Enter key from the keyboard. As a result, the DOS prompt will appear on the screen.

Now type

C:\ TC>fruit apple mango

Output:

Following are the arguments entered in the command line

C:\fruit.exe
apple
mango

Number of arguments entered are 3

After this, type exit at the prompt to return to the program.