Can provide arguments to the program on the command line:

   $ prog1  4  => 2 args (argc is 2)
                   argv[0] => "prog1" is the name of the program
                   argv[1] => "4"     is an integer that the program can use

   First line of code in program becomes:

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

    - argc is an abbreviation for "argument count"
      argc
         simply the count of args
         INCLUDES name of program
         
    - argv is an abbreviation for "argument vector" 
      argv  
         name for pointer to an array of string pointers passed to main function
         the second argument passed to main
         argv[0] always points to the name of the command itself.


   EXAMPLE:
   ========
   int main ( int argc, char *argv[] )
   {
     int n; // number of reps
   
     // check # of args
     if ( argc != 2 )
      { 
        // ----
        error msg...
        return error...
      }     
   
     // get int argument
     n = atoi( argv[1] ); // first arg (program name is argv[0])
     
     ...
     
 Example:
 =======
   $ mine  -c  10  2.0
   
   int main ( int argc, char *argv[] )


---------------------------------
   argc is 4 (4 arguments)
   argv[] is an array of pointers
      there are 4 pointers in the array [0-3] and a null terminator [4]
      each pointer points to what??
      and each argument is actually just an array of what?
      and each of those arrays is terminated by what?


EXAMPLE: error detection and report
========
   forkProg 7
   
  // check for correct # args
  if( argc != 2 || atoi( argv[1]) > 5 )
  {
     cout << "Usage: " << argv[0] << " numChildren (5 or less)" << endl;
     return 1;
  }   
  else
     numChild = atoi( argv[1] ); // args ok



EXAMPLE: error detection and report (better)
========
  generateRandom 2000 100

  --> argc = 3 
  --> argv[0] = "generateRandom"
  --> argv[1] = "2000"
  --> argv[2] = "100"


 //check # of command-line args which should be 3: program, num, range
 if( argc != 3 ) // ERROR
 {
   // in C
   fprintf( stderr, "Usage: %s number of nums, max size(-1)\n", argv[0] );

   // in C++
   cout << "\nUsage: " << argv[0]
                       << " number of nums, max size(-1)" << endl;

   return 1; // <-- we're done...
 }

 // get the args
 limit = atoi( argv[1] ); // get the number of nums from user
 range = atoi( argv[2] ); // max size -1



EXAMPLE   hello  n
=======
// prog1.cpp
// repeats "Hello World!" n times - n specified on the command line

#include <iostream>
using namespace std;

int main ( int argc, char *argv[] )
{
   int n;
   
   // check for valid number of command-line arguments 
   if( argc != 2 ) // must be 2 args INCLUDING program name
   {  
      // error message tells HOW to run this program
      cout << "Usage: " << argv[0] 
           << " Number of reps (integer)" << endl;
      return 1; // error back to OS
   }
   
   // get the supplied argument
   n = atoi( argv[1] );
   
   // loop and greet n times
   for( int i = 1; i <= n; i++ )
      cout << i << ": Hello World!" << endl;
        
   // back to OS - ok     
   return 0;
}

EXAMPLE 
=======
// prog2.cpp
// repeats supplied message n times
// n specified on the command line
// message supplied on command line

#include <iostream>
using namespace std;

int main( int argc, char *argv[] )
{
   int n;
   string s;

   // check for valid number of command-line arguments
   if( argc != 3 ) // must be 3 args INCLUDING program name
   {
      // error message tells HOW to run this program
      cout << "Usage: " << argv[0]
           << " Repetitions"
           << " Message" << endl;
      return 1; // error back to OS
   }

   // get the supplied argument
   n = atoi( argv[1] );
   s = argv[2]; // could just use argv[2]

   // loop and greet n times
   for( int i = 1; i <= n; i++ )
       cout << i << ": " << s << endl;

   // back to OS - ok
   return 0;
}

   EXAMPLE RUNS:
   =============
         /home/wyatt/private/Code$  prog2  5
         Usage: prog2 Repetitions Message
         
         /home/wyatt/private/Code$  prog2  5  yippeee!  x
         Usage: prog2 Repetitions Message
         
         /home/wyatt/private/Code$  prog2  5  yippeee!
         1: yippeee!
         2: yippeee!
         3: yippeee!
         4: yippeee!
         5: yippeee!

If Windows and Linux are both running on the SAME Intel architecture, why
can't programs run on each other's machine?

1. Will the machine code of the data be the same?

2. Will the machine code of the instructions be the same?
      
3. Will the calls to the OS be the same?
   Interrupts? (21h vs 80h)
   Arguments?
   
4. Supporting libraries the same?
   APIs?
   
5. Executable file structure the same? 
PE COFF / ELF?

How can you explore a PE/COFF file format?


PE Format

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