Command Line Arguments

• This material is from Chapter 6.6.
• We will want to use this information in most of our programs for this class so I am presenting it now.
• In 130-230 we present main as

   
  int main() {
  ...
  }
      

• This requires interactive input to change the behavior of the program.
• And it is defiantly not the way most system utilities work.
  • g++ -o myprog myprog.C -lmylib
  • ls -alrt *.C
• The examples above employ command line arguments
• One of the jobs of the shell is to parse the command line and pass any arguments to the program to be executed.
• Not everything is a command line argument for
  • ps -aux | less -N
  • ps is a program to run
  • -aux is the command line argument to ps
  • | is a command to the shell (a pipe command)
  • less is a second command to run.
  • -N is a command line argument to less.
• The shell generally takes care of these when it creates the process.
• The new process is handed values from the shell via two parameters to main argc and argv
  • argc is the argument count
    • It is an integer
    • It is an array size.
    • It is always at least 1.
    • It is the number of arguments being passed to the program.
    • I had never done this before but

      #include 
      
      using namespace std;
      
      int main(int argc) {
          cout << "There were " << argc << " arguments " << endl;
      
          return 0;
      }
      	     

      works, but produces a warning.
  • argv is the list of arguments.
    • It is a char ** argv
    • or a char *argv[], I prefer this but it really doesn't matter.
    • argv[0] is always the name of the file that was invoked.

• #include 
  
  using namespace std;
  
  int main(int argc, char * argv[]) {
      int i;
  
      cout << "The program is " << argv[0] << endl;
  
      if (argc > 1) {
          cout << "The command line arguments are: " << endl;
          for(i=1;i<argc;i++) {
  	   cout << "\targument number " << i+1 << "  is \"" 
  	        << argv[i] << "\"" << endl;
          }
      }
  
      return 0;
  }
      

• Actually handling command line arguments is a bit of a mess.
• A few utility routines are in order
  • Since the strings are c style and we will need to compare them we have two choices
    • Convert everything to a C++ style string, too expensive
    • Use C string comparisons.
    • #include <string.h>
    • strcmp
      • int strcmp(const char *s1, const char *s2);
      • Returns 0
        • if they are the same,
        • negative if s1 is less
        • positive if s1 is greater

        • if (strcmp(argv[i], "?") == 0) {
                Usage(argv[0]);
                i++;
          }
          		 

  • Since everything is a string, we need a way to turn it into an int.
    • A standard solution is atoi
    • #include<cstdlib>
    • int atoi(const char *nptr);
    • This is part of a family of functions, (atoi, atof, atol)
    • Return is the value or 0 if it can't convert.
  • I learned a new one today based on the man pages,
    • strtol
    • #include<cstdlib>
    • long int strtol(const char *nptr, char **endptr, int base);
    • endptr points to NULL, it is ignored.
    • if it points to something, a return value is where in the string conversion stopped.
    • base must be between 2 and 36, and will convert the input from that base.
    • It will perform the normal skip white space like the stream extraction operator.
    • here is some example code
  • Here is some example code of a counting program which uses several command line arguments.
  • There is an alternate way to parse command line arguments, getopt.
    • This is covered in appendix B.
    • I may discuss it later.
    • You may learn it and use it if you wish.
  • It appears that the boost library has a solution as well.
    • Look here
    • You may learn and employ this if you wish.
  • The full code for today is here