Iterating over argv[] produces segmentation fault

Question

I'm writing a program that reads "flags" from the command line as well as the program name. I want the program to read several flags (-i, -c and -n) and execute different functions when one or a combination of flags is called.

This is the code I started to write:

  printf("Argv 0: %s\n", argv[0]);
  printf("Argv 1: %s\n", argv[1]);
  printf("Argv 2: %s\n", argv[2]);

  for (int i = 1; i <= argc + 1; i++) {
    if (strcmp("-i", argv[i]) == 0) {
      printf("%s\n", "found -i"); 
    }
    else{
      printf("%s\n", "did not find -i");
    }

  }

Just experimenting with one flag (-i), but I want it the read one or several flags at the same time and it to call the corresponding functions.

When I execute the program:

./program-name test -i
Argv 0: test
Argv 1: -i
Argv 2: (null)
found -i
Segmentation fault 

Answer 1

Don't redo old code that is proven solid. It wastes time and duplicates effort. As a long time C coder, I always (when I remember) go and look at code bases for something first before I try to start from scratch. I repurpose first then create. I'm smart enough to know there were lots of smarterer coders that went before me.

Use the free and working getopt() and getopt_long() functions to parse command line arguments for you. There are lots of examples in code bases if you google search "getopt example" but here is the GNU C tutorial.

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main (int argc, char **argv)
{
  int aflag = 0;
  int bflag = 0;
  char *cvalue = NULL;
  int index;
  int c;

  opterr = 0;

  while ((c = getopt (argc, argv, "abc:")) != -1)
    switch (c)
      {
      case 'a':
        aflag = 1;
        break;
      case 'b':
        bflag = 1;
        break;
      case 'c':
        cvalue = optarg;
        break;
      case '?':
        if (optopt == 'c')
          fprintf (stderr, "Option -%c requires an argument.\n", optopt);
        else if (isprint (optopt))
          fprintf (stderr, "Unknown option `-%c'.\n", optopt);
        else
          fprintf (stderr,
                   "Unknown option character `\\x%x'.\n",
                   optopt);
        return 1;
      default:
        abort ();
      }

  printf ("aflag = %d, bflag = %d, cvalue = %s\n",
          aflag, bflag, cvalue);

  for (index = optind; index < argc; index++)
    printf ("Non-option argument %s\n", argv[index]);
  return 0;
}