Destructive XOR

rev.c

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

#define MAX_CHARS 64

void swap(char* a, char* b) {
  *a = *a ^ *b;
  *b = *a ^ *b;
  *a = *a ^ *b;
}

void rev(char* text, size_t size) {
  for (
    size_t left = 0, right = size - 1;
    left <= right;
    left++, right--
  ) {
    swap(text + left, text + right);
  }
}

int main(int argc, char* argv[]) {
  if (argc < 2) {
    fprintf(stderr, "No input param given :o\n");
    exit(EXIT_FAILURE);
  }

  char text[MAX_CHARS];

  strncpy(text, argv[1], MAX_CHARS);
  rev(text, strlen(text));

  printf("Given: %s\n", argv[1]);
  printf("Revved: %s\n", text);
}

At function rev line 15:
value of variable right will underflow when size is 0.

At functionmain line 32:
result of strlen call may be undefined since the '\0' character maybe was not copied by strncpy.