Writing C-programs which can be run with additional arguments from command line

Question

I'm learning C-programming and have got stuck on a problem to which I can't find any answer anywhere.

What I want to do is to write a C-program which I can run with additional arguments directly from the terminal, e.g.

cat -n input.txt - nosuchfile.txt input.txt

What I want to know is how I can write any function so I can run it as above (after compilation), so what the program does is perhaps not very important, but for the sake of completeness, cat takes a list of input-files and prints them to stdout. It has full error handling (hence the file nosuchfile.txt), and can also include line numbering (-n) and take input from the standard input (-).

For clarification, I have previously written programs where I can compile the source files, and run the program with e.g. ./cat, and if input is required this has been acquired after this command to start running the program. Thus, the terminal has looked something like this:

gcc ...
./cat
-n input.txt - nosuchfile.txt input.txt

I want to know how to be able to run the program like this

gcc...
cat -n input.txt - nosuchfile.txt input.txt

Thank you very much!

You are asking about argc and argv. You can also look up getopt. However, this is the sort of question that isn't really helpful to others, so I am also voting to close. — Heath Hunnicutt, Mar 14 '13 at 05:50
possible duplicate of [Handling command line flags in C/C++](http://stackoverflow.com/questions/14737957/handling-command-line-flags-in-c-c) — abelenky, Mar 14 '13 at 06:00

Aki Suihkonen · Answer 1 · 2013-03-14T07:31:19.403

There are 2 or 3 well defined parameters for main in most systems:

#include <stdio.h>
int main(int ac, char **av) { printf("%d %s\n", ac, av[0]); return 0; }

would print the number of parameters(+1) and the name of the program. av[1] would contain a pointer to a string containing the first parameter (if ac>1) etc.

The third possible parameter , char **env) (under some systems) would contain a pointer to environment variables.

EDIT The gnu getopt library helps parsing the command lines just as used in unix / gnu utilities in general.

score 0 · Accepted Answer · answered Mar 14 '13 at 05:53

You can use command line arguments:

#include <stdio.h>

int main( int argc, char *argv[] ) // argc is the (c)ount of arguments, argv is the (v)alues
{
  printf( "\nCommand-line arguments:\n" );

  for( int count = 0 ; count < argc ; count++ )
  {
    printf( "  argument %d = %s\n", count, argv[count] );
  }

  return 0;
}

Writing C-programs which can be run with additional arguments from command line

2 Answers2