XOR two strings from argv in C

Question

Want to XOR two strings fetched from argv. I checked this question How to xor two string in C? but it could not solve it for me.

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

int main(int argc, char const *argv[]) {
  char output[]="";

  int i;
  for (i=0; i<strlen(argv[1]); i++){
      char temp = argv[1][i]^argv[2][i];
      output[i]=  temp;

  }
  output[i] = '\0';
  printf("XOR: %s\n",output);
  return 0;
}

When I use lldb to debug my output ("(lldb) print output") it is /a/x16/t/x13 but it can not be printed by printf(). I know that it is not a string anymore. Can you help me how to make it able to be printf:ed. The text that is printed in the terminal is "XOR: "

Answer 1

There's some memory bugs in your code. Perhaps the following would work better:

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

#define min(i, j) ((i) < (j) ? (i) : (j))

int main(int argc, char const *argv[])
  {
  char *output;
  int i;

  /* Allocate a buffer large enough to hold the smallest of the two strings
   * passed in, plus one byte for the trailing NUL required at the end of
   * all strings. 
   */

  output = malloc(min(strlen(argv[1]), strlen(argv[2])) + 1);

  /* Iterate through the strings, XORing bytes from each string together
   * until the smallest string has been consumed. We can't go beyond the
   * length of the smallest string without potentially causing a memory
   * access error.
   */

  for(i = 0; i < min(strlen(argv[1]), strlen(argv[2])) ; i++)
      output[i] = argv[1][i] ^ argv[2][i];

  /* Add a NUL character on the end of the generated string. This could
   * equally well be written as
   *
   *   output[min(strlen(argv[1]), strlen(argv[2]))] = 0;
   *
   * to demonstrate the intent of the code.
   */

  output[i] = '\0';

  /* Print the XORed string. Note that if characters in argv[1]
   * and argv[2] with matching indexes are the same the resultant byte
   * in the XORed result will be zero, which will terminate the string.
   */

  printf("XOR: %s\n", output);

  return 0;
  }

As far as printf goes, keep in mind that x ^ x = 0 and that \0 is the string terminator in C.

Best of luck.