Command line arguments in C:

Command line arguments in C:

      It is possible to pass some values from the command line to your C programs when they are executed. These values are called command line arguments and many times they are important for your program specially when you want to control your program from outside instead of hard coding those values inside the code.

The command line arguments are handled using main() function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array which points to each argument passed to the program. 

argc      - Number of arguments in the command line including program name
argv[]   – This is carrying all the arguments

• In real time application, it will happen to pass arguments to the main program itself.  These arguments are passed to the main () function while executing binary file from command line.
• For example, when we compile a program (test.c), we get executable file in the name “test”.
• Now, we run the executable “test” along with 4 arguments in command line like below.

this is a program

Where,

argc             =       5
argv[0]         =       “test”
argv[1]         =       “this”
argv[2]         =       “is”
argv[3]         =       “a”
argv[4]         =       “program”
argv[5]         =       NULL

Example program for argc() and argv() functions in C:

#include <stdio.h>

#include <stdlib.h>

int main(int argc, char *argv[])   //  command line arguments

{

if(argc!=5) 

{

   printf("Arguments passed through command line " \

          "not equal to 5");

   return 1;

}

   printf("\n Program name  : %s \n", argv[0]);

   printf("1st arg  : %s \n", argv[1]);

   printf("2nd arg  : %s \n", argv[2]);

   printf("3rd arg  : %s \n", argv[3]);

   printf("4th arg  : %s \n", argv[4]);

   printf("5th arg  : %s \n", argv[5]);

return 0;

}

 Output:

Program name : test 1st arg : this 2nd arg : is 3rd arg : a 4th arg : program 5th arg : (null)

IN OTHER WORDS:

C provides a fairly simple mechanism for retrieving command line parameters entered by the user. It passes an argv parameter to the main function in the program. argv structures appear in a fair number of the more advanced library calls, so understanding them is useful to any C programmer.

Enter the following code and compile it:

#include <stdio.h>

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

    printf("%d\n",argc);
    for (x=0; x<argc; x++)
        printf("%s\n",argv[x]);
    return 0;
}

In this code, the main program accepts two parameters, argv and argc. The argv parameter is an array of pointers to string that contains the parameters entered when the program was invoked at the UNIX command line. The argc integer contains a count of the number of parameters. This particular piece of code types out the command line parameters. To try this, compile the code to an executable file named aaa and type aaa xxx yyy zzz. The code will print the command line parameters xxx, yyy and zzz, one per line.

The char *argv[] line is an array of pointers to string. In other words, each element of the array is a pointer, and each pointer points to a string (technically, to the first character of the string). Thus, argv[0] points to a string that contains the first parameter on the command line (the program's name), argv[1] points to the next parameter, and so on. The argc variable tells you how many of the pointers in the array are valid. You will find that the preceding code does nothing more than print each of the valid strings pointed to by argv.

Because argv exists, you can let your program react to command line parameters entered by the user fairly easily. For example, you might have your program detect the word help as the first parameter following the program name, and dump a help file to stdout. File names can also be passed in and used in your fopen statements.