Command Line Arguments

"Real Programmers always confuse Christmas and Halloween because OCT 31 == DEC 25" -- Andrew Rutherford

Command Line Arguments

Usually, we see main prototyped as:

void main(void);

int main(void);

However, main is sort of overloaded to take parameters as well:

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

As we've seen with arrays as parameters, the declarations above are equivalent.

argc - The number of arguments passed to main (the program).
argv - An array of pointers to NUL-terminated strings (the arguments).
The caller passes the arguments; the caller of main is the C runtime system.
The first element in argv (argv[0]) is the name of the program.

Note that the symbols argc and argv are just conventions. As with any function you implement, you can name the parameters anything you want. These are all the same:

int main(int argc, char **argv);
int main(int foo, char **bar);
int main(int fred, char **barney);
int main(int argv, char **argc);   // You're just asking for trouble here!

This trivial program simply prints out each argument:

void main(int argc, char *argv[])
{
  int i;
  for (i = 0; i < argc; i++)
    printf("arg%i = %s\n", i, argv[i]);
}

If our program was named foo.exe and we were to invoke the program like this:

foo one two three 911

we would see something like this printed out:

foo
one
two
three
911

Another example:

foo one "two three" four 911

foo
one
two three
four
911

Within Visual Studio we see:

E:\Data\Courses\Summer2004\CS220\Code\Chapter16\RTL\Debug\foo.exe
one
two three
four
911

because the IDE invokes the program using the entire path.

A more compact way of printing the arguments using pointers instead of subscripts:

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

Diagram of the arguments when invoked as:

foo one "two three" four 911

Note: Because argv is an array of pointers to characters (strings), you can only pass strings to a program. If you want to use the parameter as a number, you will have to convert it from a string to a number yourself. See the Data Conversion section in the C Runtime Library.

Example driver code for the FileCopy project:


#include <stdio.h>  /* printf */
#include <stdlib.h> /* atoi   */
#include <time.h>   /* time_t, time */

#include "FileCopy.h"

int main(int argc, char **argv)
{
  unsigned bufsize = 100; /* default size if not provided */
  int reads;
  clock_t start, end;

    /* If less than 2 arguments are provided, remind the user */
    /* how to use the program.                                */
  if (argc < 3)
  {
    printf("\n");
    printf("Usage: FileCopy {source} {destination} [buffersize]\n");
    printf("       Default buffer size is 100 bytes.\n\n");
    return -1;
  }

    /* If the user provides a size, use it */
  if (argc > 3)
    bufsize = atoi(argv[3]);

  start = clock();
  reads = FileCopy(argv[1], argv[2], bufsize);
  end = clock();

  if (reads == E_BAD_SOURCE)
    printf("Unable to open file for read: %s\n", argv[1]);
  else if (reads == E_BAD_DESTINATION)
    printf("Unable to open file for write: %s\n", argv[2]);
  else if (reads == E_NO_MEMORY)
    printf("Unable to allocate buffer of size %u\n", bufsize);
  else
    printf("With bufsize of %i, number of reads: %i. Total time: %i ms.\n", bufsize, reads, end - start);

  return 0;
}

Command Line Arguments

Command Line Arguments

Example driver code for the FileCopy project:

Command Line Paramters for Some Game Engines