Simple random string generator

rand.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <string.h>

void usage(char *file, int status) {
  printf("Generates a string of random characters "
      "(using numbers and upper/lowercase letters, unless otherwise specified).\n");
  printf("usage: %s length [options]\n\n", file);
  printf("Options:\n");
  printf("  -n\tuse numbers\n");
  printf("  -a\tuse lowercase letters\n");
  printf("  -A\tuse uppercase letters\n");
  printf("  -s\tuse symbols\n");
  printf("  -c\tdisplay the character set in use\n");
  exit(status);
}

int main(int argc, char* argv[]) {
  srand(time(NULL) * (int) getpid());
  char *file = argv[0];
  if(argc < 2) usage(file, EXIT_SUCCESS);

  int length = atoi(argv[1]);
  if(length <= 0) { 
    printf("%s: invalid length -- %s\n\n", file, argv[1]); 
    usage(file, EXIT_FAILURE); 
  }

  //Correct args so getopt() doesn't get confused
  argv[1] = file;
  argv++;
  argc--;
  
  int opt;
  int useNumbers   = 0;
  int useLowercase = 0;
  int useUppercase = 0;
  int useSymbols   = 0;
  int showCharset  = 0;

  while((opt = getopt(argc, argv, "naAsc")) != -1) {
    switch(opt) {
      case 'n' : useNumbers   = 1; break;
      case 'a' : useLowercase = 1; break;
      case 'A' : useUppercase = 1; break;
      case 's' : useSymbols   = 1; break;
      case 'c' : showCharset  = 1; break;
      default  : printf("\n"); usage(file, EXIT_FAILURE);
    }
  }

  const char *num     = "0123456789";
  const char *lower   = "abcdefghijklmnopqrstuvwxyz";
  const char *upper   = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  const char *symbols = "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~";
  char *charset = malloc(95 * sizeof(char)); 

  if(!useNumbers && !useLowercase && !useUppercase && !useSymbols) useNumbers = useLowercase = useUppercase = 1;
  if(useNumbers)   strncat(charset, num, strlen(num));
  if(useLowercase) strncat(charset, lower, strlen(lower));
  if(useUppercase) strncat(charset, upper, strlen(upper));
  if(useSymbols)   strncat(charset, symbols, strlen(symbols));
  if(showCharset)  printf("Using character set: %s\n", charset);

  size_t i;
  for(i = 0; i < length; i++) {
    size_t index = (double) rand() / RAND_MAX * strlen(charset);
    printf("%c", charset[index]);
  }
  printf("\n");
  free(charset);
  exit(EXIT_SUCCESS);
}