argc/argv
P.J. Drongowski
17 March 2005

********************************************
Command line arguments
********************************************

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

  * The operating system and C++ runtime environment set-up the arguments
    to the function main
  * By convention, the arguments are called "argc" and "argv"
    + argc: Number of command line arguments including the program name
    + argv: Contains pointers to the individual arguments
  * The individual arguments are represented as strings; argv is a one
    dimensional array of char pointers where each pointer points to an
    individual argument
  * Example: g++ -c file.cpp
      argc = 3
      argv[0] -> "g++"
      argv[1] -> "-c"
      argv[2] -> "file.cpp
  * UNIX conventions
    + Options: '-' followed by a single character
    + An option may take a take a parameter which follows the option character
    + Options usually appear before other arguments on command line, but this
      is not always required
  * The main function usually processes the command line
  * Idiom (from Practical C++ Programming)

        while (argc > 1) {
          if (argv[1][0] == '-') {
            switch( argv[1][1]) {
              case 'v': {
                ...
                break ;
              }
              case 'u': {
                ...
                break ;
              }
              default: {
                cerr << "Unknown option: " << argv[1][1] << endl ;
                break ;
              }
            }
          }
          ++argv ;
          --argc ;
        }

  * The standard library includes a C language routine called "getopt"
    which assists command line processing (See "man 3 getopt")
  * There is also a C++ version of the GNU getopt function called "GetOpt"


***************************************************
simple.cpp
***************************************************

#include <cstdlib>
#include <fstream>
#include <iostream>
using namespace std ;

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

  if (argc != 2) {
    cerr << "Too few arguments" << endl ;
    cerr << "Usage: simple file" << endl ;
    exit(EXIT_FAILURE) ;
  }

  inputFile.open(argv[1], ios::in) ;
  if (! inputFile) {
    cerr << "*error* Couldn't open file: " << argv[1] << endl ;
    cerr << "Usage: simple file\n" ;
    exit(EXIT_FAILURE) ;
  }

  inputFile.close() ;

  exit(EXIT_SUCCESS) ;
}


***************************************************
options.cpp
***************************************************


#include <cstdlib>
#include <fstream>
#include <iostream>
using namespace std ;

void printUsage()
{
  cerr << "Usage: options [-aio] file" << endl ;
}

int main(int argc, char *argv[])
{
  ifstream inputFile ;
  bool a_option = false,
       i_option = false,
       o_option = false ;
  char *option ;
  int count ;

  if (argc < 2) {
    cerr << "Too few arguments" << endl ;
    printUsage() ;
    exit(EXIT_FAILURE) ;
  }

  for (count = 1 ; count < argc ; count++) {
    if (argv[count][0] == '-') {
      switch( argv[count][1] ) {
        case 'a' : {
          cout << "Option a\n" ;
          a_option = true ;
          break ;
	}
        case 'i' : {
          cout << "Option i\n" ;
          i_option = true ;
          break ;
	}
        case 'o' : {
          cout << "Option o\n" ;
          o_option = true ;
          break ;
	}
        default : {
          cerr << "Unknown option: " << argv[count] << endl ;
          break ;
	}
      }
    } else {
      break ;
    }
  }

  if (count == argc) {
    cerr << "Missing file name" << endl ;
    printUsage() ;
    exit(EXIT_FAILURE) ;
  }

  inputFile.open(argv[count], ios::in) ;
  if (! inputFile) {
    cerr << "*error* Couldn't open file: " << argv[count] << endl ;
    printUsage() ;
    exit(EXIT_FAILURE) ;
  }

  inputFile.close() ;

  exit(EXIT_SUCCESS) ;
}