C: Exception thrown at 0x0FFB0BA0 (ucrtbased.dll)

Question

I am working on a program to ask a user to enter a word and the number of letters the user wants copied from that word. The program works when I use Compile Online, but when I run the program in Micrsoft Visual Studio, the program freezes after I enter the word I want copied. I ran the debugger and found the error shown below. I take it, from googling, that I am writing past the amount of memory set aside for my array? Should I use malloc to fix that? Posted below are the errors and the code (link to original stackoverflow thread.

Exception thrown at 0x0FFB0BA0 (ucrtbased.dll) in lab1113problem7.exe: 0xC0000005: Access violation writing location 0x00B80000.

Unhandled exception at 0xFEFEFEFE in lab1113problem7.exe: 0xC00001A5: An invalid exception handler routine has been detected (parameters: 0x00000003).

The program '[7196] lab1113problem7.exe' has exited with code 0 (0x0).

Program:

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

char *copywords(char *dest, const char *source, size_t n);

int main(void) {
    char words[50];
    char newwords[50];
    int num;

    for (;;) {
        printf("Type a word, or type 'quit' to quit: ");
        if (scanf("%49s", words) != 1) {
            printf("Invalid input!\n");
            return 0; 
        }
        if (!strcmp(words, "quit")) {
            printf("Good bye!\n");
            return 0; 
        }
        printf("Type the # of chars to copy: ");
         if (scanf("%d", &num) != 1) {
            printf("Invalid input!\n");
            return 0; 
        }
        copywords(newwords, words, num);
        printf("The word was %s\n", words);
        printf("and the copied word is %s\n", newwords);
    }
}

char *copywords(char *dest, const char *source, size_t n) {
    size_t i;
    for (i = 0; i < n && source[i] != '\0'; i++) {
        dest[i] = source[i];
    }
    dest[i] = '\0';
    return dest;
}

Answer 1

Some debug code to make sure things are working as intended for your strings:

// Place this at the top of your file, after your `#include` statements 
// and use it in place of the 50s in your array defines.
#define MAX_ARRAY_SIZE 50

char *copywords(char *dest, const char *source, size_t n) {
    size_t i;
    for (i = 0; i < n && source[i] != '\0'; i++) {
        if(i >= MAX_ARRAY_SIZE)
        {
            printf("I just blew up memory at %i\n", %i);
        }
        dest[i] = source[i];
    }
    dest[i] = '\0';
    return dest;
}

Note: you can check the array size each iteration instead within the for loop to be sure that you haven't overrun your buffers.

Not a direct answer, but something to try and easier than using comments. If the printf is triggered, you know bad has happened...

Even better, because you pass in the size as n, you can check array size before the loop every starts and handle bad data immediately.

char *copywords(char *dest, const char *source, size_t n) {
    if(n >= MAX_ARRAY_SIZE) 
    {
        printf("Array will overflow by %d bytes- truncating\n", n - MAX_ARRAY_SIZE);
        n = MAX_ARRAY_SIZE;
    }