Emulates the unix command cat, with a few new tricks added

dog.c

/* Emulates the unix command cat, with a few new tricks added
 * By <anupdhml@gmail.com>
 * April 7, 2010
 *
 * Program arguments
 * -r: reverses the letters of the input
 * -c N, where N is a no between 0 and 25: ciphers the input by adding N to
 *  the ASCII value of each letter and printing the result as a character
*/

/* Credits:
 * http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/
 * http://mazack.org/unix/errno.php
 */

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <stdbool.h>

#define INIT_BUFFERSIZE 80
#define MAXFILECOUNT 1

static size_t read_input(FILE *source, char *buffer[], int buffersize);
static void reverse(char *input[], size_t inputlen);
static void encode(int cipher_val, char *input[], size_t inputlen);
static bool isnum(char arg[]);
static void argparse (int argc, char *argv[], bool *doreverse, bool *doencode,
                      bool *dofileread, int *cipher_val, FILE **input_file);
static void print_output (bool doreverse, bool doencode, bool dofileread,
                     int cipher_val, FILE *input_file);

/* --------------------------------------------------------------------------*/

int main(int argc, char *argv[]) {
  bool doreverse, doencode, dofileread;
  doreverse = doencode = dofileread = false;

  int cipher_val = 0;
  FILE *input_file = NULL;

  argparse(argc, argv, &doreverse, &doencode, &dofileread,
           &cipher_val, &input_file);

  print_output(doreverse, doencode, dofileread, cipher_val, input_file);

  return EXIT_SUCCESS;
}

/* --------------------------------------------------------------------------*/

static void print_output (bool doreverse, bool doencode, bool dofileread,
                     int cipher_val, FILE *input_file) {
  char *str;
  size_t strlength = 0;

  str = (char *) malloc(INIT_BUFFERSIZE * sizeof(char));
  if (str == (char *) NULL) {
    fprintf(stderr, "dog: Cannot allocate memory");
    exit(12);     // With suitable unix error code

  }

  if(dofileread) {
    strlength = read_input (input_file, &str, INIT_BUFFERSIZE);
    fclose(input_file);
  }
  else
    strlength = read_input(stdin, &str, INIT_BUFFERSIZE);

  if(doreverse)
    reverse(&str, strlength);
  if(doencode)
    encode(cipher_val, &str, strlength);

  printf("%s\n", str);

  free(str);
  return;
}


// Determine whether the user asked dog to perform its other tricks.
static void argparse (int argc, char *argv[], bool *doreverse, bool *doencode,
                      bool *dofileread, int *cipher_val, FILE **inputfile) {
  int file_count = 0;

  for (int i = 1; i < argc; i++) {

    // Check for reverse switch
    if (strcmp(argv[i], "-r") == 0)
      *doreverse = true;

    // Check for the cipher option
    else if (strcmp(argv[i], "-c") == 0) {
      if (i + 1 <= argc - 1) {    // Check if an argument follows the switch -c.
        i++;

        if (isnum (argv[i])) {
          *cipher_val = atoi(argv[i]);  // Writing the value string as int
          if (*cipher_val >= 0 && *cipher_val <= 25)
            *doencode = true;
          else {
            fprintf(stderr, "dog: %d: Invalid argument for the switch -c."
                            "Use integers between 0 and 25.\n", *cipher_val);
            exit(33);
          }
        }

        else {
          fprintf(stderr, "dog: %s: Invalid argument for the switch -c. "
                  "Only numerical values  are allowed.\n", argv[i]);
          exit(22);
        }
      }

      else {
        fprintf(stderr, "dog: Missing argument for the switch -c\n");
        exit(22);
      }
    }

    // Check if filenames were provided as arguments.
    else {
      *inputfile = fopen(argv[i], "r");

      if (*inputfile == NULL) {
        fprintf(stderr, "dog: %s: No such file\n", argv[i]);
        exit(2);
      }

      else {
        file_count++;
        if (file_count <= MAXFILECOUNT)
          *dofileread = true;
        else {
          fprintf(stderr, "dog: Can't open more than %d file\n", MAXFILECOUNT);
          fclose(*inputfile);
          exit(7);
        }
      }
    }

  }
}


// Returns true if the given string is numeric, false otherwise.
static bool isnum(char arg[]) {
  int i;

  for (i = 0; i < strlen(arg); i++)
    if (!isdigit(arg[i])) {
      return false;
      break;
    }

  return true;
}


// Reverse the given string.
static void reverse(char *input[], size_t inputlen) {
  size_t i,j;
  char temp;

  for (i = 0, j = inputlen - 1 ; i <= j; i++, j--) {
    temp = (*input)[i];
    (*input)[i] = (*input)[j];
    (*input)[j] = temp;
  }
  return;
}


// A basic rotation cipher.
static void encode(int cipher_val, char *input[], size_t inputlen) {
  size_t i;
  char input_char;
  char shifted_char;

  for (i = 0; i < inputlen; i++) {
    input_char = (*input)[i];

    if(input_char < 'A' || input_char > 'z');

    else {
      shifted_char = input_char + cipher_val;
      if(isupper(input_char))
        (*input)[i] = ((shifted_char - 'A') % ('Z' - 'A') ) + 'A';
      else
        (*input)[i] = ((shifted_char - 'a') % ('z' - 'a') ) + 'a';
    }
  }

  return;
}


// Reads the given input until EOF marker is encounterd.
// So at the moment, must press EOF marker to see the output of the program.
static size_t read_input(FILE *source, char *buffer[], int buffersize) {
  size_t chars_read = 0;
  char c;

  while ( (c = fgetc(source)) != EOF ) {
    if (chars_read == buffersize) {
      buffersize *= 2;

      *buffer = (char *) realloc(*buffer, buffersize);

      if (*buffer == (char *) NULL) {
        fprintf(stderr, "dog: Cannot allocate memory");
        exit(12);
      }
    }

    (*buffer)[chars_read] = c;
    chars_read++;
  }

  return chars_read;
}