C Text conversion program

Question

I am very new to the C language and have been working on a program that takes either a text file or input from the keyboard and either turns all the letters to capitals, lowercase, or rotates them by 13 places depending on the input given by the user which should go something like: ./tconv -u test.txt This should, in theory, turn all the letters in test.txt to uppercase letters. If no file is given, ./tconv -u, then it should take input from the keyboard.

I think I am missing something fairly simple, but when I run it with any -r,-u, or -l arguments, it says it cannot read "-r", "-u", or"-l". What am I missing?

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

FILE*
input_from_args(int argc, const char *argv[])
{
  if (argc==1){
    return stdin;
  }else{
    return fopen(argv[1],"r");
  }
}

int
rot13(c)
{
  int e;
  int ROT;
  ROT = 13;
  if(c>='A' && c <='Z'){
    if((e=c+ROT)<='Z')
      return e;
    else{
      e = c - ROT;
      return e;
    }
  }
  else{
    return c;
  }
}

int
main(int argc, const char*argv[])
{
  FILE *src = input_from_args(argc,argv);
  FILE *dest = stdout;
  if (src == NULL){
    fprintf(stderr, "%s: unable to open %s\n", argv[0], argv[1]);
    exit(EXIT_FAILURE);
  }
  char *rotate = "-r";
  char *lower = "-l";
  char *upper = "-u";
  int i;
  i =0;
  int ch;
while ((ch = fgetc(src))!=EOF){
    if (strcmp(upper,argv[i])==0){
      fprintf(dest,"%c",toupper(ch));
    }
    else if (strcmp(lower,argv[i])==0){
      fprintf(dest,"%c",tolower(ch));
    }
    else if (strcmp(rotate,argv[i])==0){
      fprintf(dest,"%c",rot13(ch));
    }
    else{
      fprintf(dest,"%c",ch);
    }
  }
  fclose(src);
  return EXIT_SUCCESS;
}

Answer 1

If you are using linux platform you can look into getopt_long to parse the command line options. it is provided by GNU for linux systems and it is system dependent. For system independent command line parsing Google gflags

This will help to reduce the overhead of command line parsing and you can concentrate more on program logic.

Here is an example taken from http://www.gnu.org/software/libc/manual/html_node/Getopt-Long-Option-Example.html

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

/* Flag set by ‘--verbose’. */
static int verbose_flag;

int
main (int argc, char **argv)
{
  int c;

  while (1)
    {
      static struct option long_options[] =
        {
          /* These options set a flag. */
          {"verbose", no_argument,       &verbose_flag, 1},
          {"brief",   no_argument,       &verbose_flag, 0},
          /* These options don’t set a flag.
             We distinguish them by their indices. */
          {"add",     no_argument,       0, 'a'},
          {"append",  no_argument,       0, 'b'},
          {"delete",  required_argument, 0, 'd'},
          {"create",  required_argument, 0, 'c'},
          {"file",    required_argument, 0, 'f'},
          {0, 0, 0, 0}
        };
      /* getopt_long stores the option index here. */
      int option_index = 0;

      c = getopt_long (argc, argv, "abc:d:f:",
                       long_options, &option_index);

      /* Detect the end of the options. */
      if (c == -1)
        break;

      switch (c)
        {
        case 0:
          /* If this option set a flag, do nothing else now. */
          if (long_options[option_index].flag != 0)
            break;
          printf ("option %s", long_options[option_index].name);
          if (optarg)
            printf (" with arg %s", optarg);
          printf ("\n");
          break;

        case 'a':
          puts ("option -a\n");
          break;

        case 'b':
          puts ("option -b\n");
          break;

        case 'c':
          printf ("option -c with value `%s'\n", optarg);
          break;

        case 'd':
          printf ("option -d with value `%s'\n", optarg);
          break;

        case 'f':
          printf ("option -f with value `%s'\n", optarg);
          break;

        case '?':
          /* getopt_long already printed an error message. */
          break;

        default:
          abort ();
        }
    }

  /* Instead of reporting ‘--verbose’
     and ‘--brief’ as they are encountered,
     we report the final status resulting from them. */
  if (verbose_flag)
    puts ("verbose flag is set");

  /* Print any remaining command line arguments (not options). */
  if (optind < argc)
    {
      printf ("non-option ARGV-elements: ");
      while (optind < argc)
        printf ("%s ", argv[optind++]);
      putchar ('\n');
    }

  exit (0);
}

Answer 2

Argv[0] is your program, argv[1] is your flag and argv[2] is the file name if you provided one. You are trying to open argv[1] a file named "-r"