Demonstration of using a pointer to alter the operation of another function in C.

safercopy_smash.c

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

#define MAXLINE 10 // in the book this is 1000

void safercopy(size_t to_length, char to[], size_t from_length, char from[])
{
    int i = 0;

    // if you're butthurt I put this if-statement here you can remove it to show me how to 
    // break the for-loop and make it run forever
    if(to != NULL && from != NULL && (int)to_length > 0 && (int)from_length > 0) {
        for(i = 0; i < to_length && i < from_length && from[i] != '\0'; i++) {
            printf("i: %d, %x, %x\n", i, to[i], from[i]);
            to[i] = from[i];
        }
    } else {
        // normally you'd then have an error here, but I'm keeping the function call
        // the same as in the book for the challenge
    }
}

int main(int argc, char *argv[])
{
    int offset = atoi(argv[1]);
    char input[] = { 1, 1, 1 };
    char *output = input + offset;
    safercopy(3, output, 3, input);
    
    return 0;
}

for posterity: https://news.ycombinator.com/item?id=8835529