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Command Line Argument: 'Main' Prototype

Command line arguments allow a program's main function to receive input from the operating system or another program. The main function prototype int main(int argc, char* argv[]) defines argc as the number of arguments and argv as an array of string pointers containing the argument values. For example, if a program is run with "./program arg1 arg2", argv[0] would be "./program", argv[1] would be "arg1", and argv[2] would be "arg2". The atoi and atof functions can be used to convert command line arguments from strings to integer and float values.

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146 views2 pages

Command Line Argument: 'Main' Prototype

Command line arguments allow a program's main function to receive input from the operating system or another program. The main function prototype int main(int argc, char* argv[]) defines argc as the number of arguments and argv as an array of string pointers containing the argument values. For example, if a program is run with "./program arg1 arg2", argv[0] would be "./program", argv[1] would be "arg1", and argv[2] would be "arg2". The atoi and atof functions can be used to convert command line arguments from strings to integer and float values.

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Geay Peter
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Command Line Argument

Command line arguments are arguments for the main function. main is basically a function
It can receive arguments like other functions. The ‘calling function’ in this case is the operating
system, or another program

'main' prototype
int main(int argc, char* argv[])
When we want main to accept command line arguments, we must define it like this

• argc holds the number of arguments that were entered by the caller
• argv is an array of pointers to char – an array of strings – holding the text values of the
arguments. The first argument is always the program’s name

Consider the following program


/* This program displays its command-line arguments */
#include <stdio.h>

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


{
int i;
printf("The program's command line arguments are: \n");
for (i = 0; i < argc; ++i) {
printf("%s\n", argv[i]);}
return 0;
}

If this program is compiled and run (using command prompt)

$ gedit cmdProgram.c
$ gcc -o cmdProgram cmdProgram.c
$ ./cmdProgram I love C program

======output=========

The program's command line arguments are:


./cmdProgram
I
love
C
program
$

So the first argument argv[ ], that is argv[0] is always the program name, the other
arguments followed from argv[1], argv[2], etc, argc giving the total number of arguments

In windows you can specify these arguments directly, by using the Windows console (Start
Run…, then type ‘cmd’ and drag the executable into the window. Then type the arguments
and <Enter>)

Helper Functions: atoi/atof


int atoi(char s[]);
double atof(char s[]);
Command line arguments are received in the form of strings
These functions are used when we want to transform them into numbers
For example – atof(“13.5”) returns the number 13.5. To use atoi you must include :
#include <stdlib.h>

Example
Write a program that accepts two numbers as command line arguments, representing a
rectangle’s height and width (as floating-point numbers). The program should display the
rectangle’s area and perimeter

Solution
int main(int argc, char* argv[])
{
double width, height;

if (argc != 3) {
printf("Wrong number of arguments!\n");
return 1;
}

width = atof(argv[1]);
height = atof(argv[2]);

printf("The rectangle's area is %g\n", width * height);


printf("The rectangle's perimeter is %g\n",2 * (width + height));
return 0;
}

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