A simple version of copying via string lengths then breaking it on purpose

safercopy.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++) {
            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 i = 0;

    // use heap memory as many modern systems do
    char *line = malloc(MAXLINE);
    char *longest = malloc(MAXLINE);

    assert(line != NULL && longest != NULL && "memory error");

    // initialize it but make a classic "off by one" error
    for(i = 0; i < MAXLINE; i++) {
        line[i] = 'a';
    }

    // check for various defects
    safercopy(MAXLINE, longest, MAXLINE, line);

    // BUGS ON PURPOSE: lie about sizes to cause overflows, but still for-loop exits
    

    // main way to break it
    safercopy(MAXLINE+1000, longest, MAXLINE+1000, line);

    // lie about size
    safercopy(MAXLINE+1000, longest, MAXLINE-10000, line);
    // really lie about size with invalid size_t variable
    safercopy((size_t)-1, longest, (size_t)-1, line);
    // pass in null pointers
    safercopy((size_t)-1, NULL, (size_t)-1, line);
    safercopy((size_t)-1, longest, (size_t)-1, NULL);
    safercopy((size_t)-1, NULL, (size_t)-1, NULL);

    free(line);
    free(longest);

    return 0;
}