Given a string (provided as parameter), randomly flip ONE BIT in it.

flip.c

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

void flip(char * str, int len) {
  int byte, bit, bias;
  char mask;

  /* mmm, seeeds */
  srand(time(NULL));

  /* We want some bias towards the middle of the string. */
  bias = (len / 3);
  byte = rand() % (len - (bias * 2));

  /* Construct a nice mask for this... */
  bit = rand() % (sizeof(char) * 8);
  mask = 2 << bit;
  str[byte] = str[byte] ^ mask;
}

int main(int argc, char ** argv) {
  int i, cnt, offset, len;
  char * str;

  cnt = 0;
  for(i = 1; i < argc; i++) {
    cnt += strlen(argv[i]);
    cnt++; // Space or end of line character.
  }

  /* Null-terminated, of course. */
  str = (char *)malloc(sizeof(char) * cnt);
  memset(str, '\0', sizeof(char) * cnt);

  offset = 0;

  for(i = 1; i < argc; i++) {
    len = strlen(argv[i]);
    memcpy(str+offset, argv[i], len);

    if(i < argc-1) {
      str[offset+len] = ' ';
    }

    offset += len+1;
  }

  /* Randomly flip one of the bits in one of the characters. */
  flip(str, cnt);
  printf("%s\n", str);

  free(str);

  return(0);
}