modify_reg changes the default values of a particular register.

---- 8< ---- 8< ----

// example.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void exit_if_password_incorrect(const char* const password) {
  if (strcmp("seasame", password) != 0) {
    puts("Password incorrect");
    exit(1);
  }
}

void success() {
  puts("Success!");
}

int main(int argc, char* argv[]) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s <password>\n", argv[0]);
    return 1;
  }
  exit_if_password_incorrect(argv[1]);
  success();
  return 0;
}

$ gcc -Wall -Werror -o example example.c
$ ./example foo
Password incorrect
$ ../modify_reg ./example eip 00001fb0
< eip: 00001e74
> eip: 00001fb0
$ ./example foo
Success!
Segmentation fault

---- 8< ---- 8< ----

This example crashes with a seg fault in __dyld_misaligned_stack_error after
the program exits because modifying the instruction pointer skips some
initialization.  This will be more useful either by either modifying more
registers to setup a sane environment, or do some other code motification.