I'm not sure why my strncpy function breaks halfway thru it?


I'm running into the following error for my _asm code in Visual Studio: "First-chance exception at 0x00431BCF in HW7.exe: 0xC0000005: Access violation writing location 0x00000000."

#include <stdio.h>
#include <stddef.h>

// Function to implement strncpy() function
char* strncpy(char* destination, const char* source, size_t num)

    _asm {
            push    ebp
            mov ebp, esp
            sub esp, 208; 000000d0H
            push    ebx
            push    esi
            push    edi
            lea edi, DWORD PTR[ebp - 208]
            mov ecx, 52; 00000034H
            mov eax, -858993460; ccccccccH
            rep stosd
            cmp DWORD PTR[destination], 0
            jne label_3
            xor eax, eax
            jmp label_4
            mov eax, DWORD PTR[destination]
            mov DWORD PTR[ebp-08h], eax
            mov eax, DWORD PTR[source]
            movsx   ecx, BYTE PTR[eax]
            test    ecx, ecx
            je label_1
            mov eax, DWORD PTR[num]
            mov DWORD PTR[ebp-0D0h], eax
            mov ecx, DWORD PTR[num]
            sub ecx, 1
            mov DWORD PTR[num], ecx
            cmp DWORD PTR[ebp-0D0h], 0
            je label_1
            mov eax, DWORD PTR[destination]
            mov ecx, DWORD PTR[source]
            mov dl, BYTE PTR[ecx]
            mov BYTE PTR[eax], dl
            mov eax, DWORD PTR[destination]
            add eax, 1
            mov DWORD PTR[destination], eax
            mov eax, DWORD PTR[source]
            add eax, 1
            mov DWORD PTR[source], eax
            jmp label_2
            mov eax, DWORD PTR[destination]
            mov BYTE PTR[eax], 0
            mov eax, DWORD PTR[ebp-08h]
            pop edi
            pop esi
            pop ebx
            mov esp, ebp
            pop ebp
            ret 0

    // return if no memory is allocated to the destination
    if (destination == NULL)
    return NULL;
    // take a pointer pointing to the beginning of destination string
    char* ptr = destination;
    // copy first num characters of C-string pointed by source
    // into the array pointed by destination
    while (*source && num--)
    *destination = *source;
    // null terminate destination string
    *destination = '\0';
    // destination is returned by standard strncpy()
    return ptr;

// Implement strncpy() function in C
int main(void)
    char* source = "Crisp Rat";
    char destination[12];
    char* source = "Christians Later";
    char destination[12];
    char* source = "Huge Ackman";
    char destination[12];
    size_t num = 11;
    // Copies the first num characters of source to destination
    printf("%s\n", strncpy(destination, source, num));
    return 0;

I've stepped through the code and the error occurs at the instruction "mov byte ptr[eax],dl". My C code equivalent of the asm code is commented out just below it. I appreciate any help very much, thank you.

asked on Stack Overflow Apr 16, 2020 by duckyPluck • edited Apr 16, 2020 by duckyPluck

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