Question

I’m having trouble with this code. When I execute “PrepareEncryption” (in example), the code returns a valid pointer, but when I pass it on to “EncryptBig” it isn’t valid no more (points to random numbers). My best bet is that the original struct is deleted. So how do I preserve it, if that’s the problem? And I know that there is a memory leak btw.

struct filecrypt
{
    FILE* bestand;
    FILE* nieuwbstnd;
    unsigned int positie;
    unsigned int size;
    unsigned int huidig;
    float procentum;
};

struct filecrypt *PrepareEncryption(char* locatie)
{
    struct stat file_status;
    struct filecrypt origineel, *mirror;
    int error;
    char* nieuw;

    if (stat(locatie, &file_status) != 0)
        return NULL;

    error = fopen_s(&origineel.bestand, locatie, "rb");
    if (error != 0)
        return NULL;

    error = strlen(locatie)+5;
    nieuw = (char*)malloc(error);
    if (nieuw == NULL)
        return NULL;
    strcpy_s(nieuw, error-3, locatie);
    strcat_s(nieuw, error, ".cpt");

    error = fopen_s(&origineel.nieuwbstnd, nieuw, "wb+");
    if (error != 0)
        return NULL;

    origineel.huidig = 0;
    origineel.positie = 0;
    origineel.procentum = 0.0f;
    origineel.size = file_status.st_size;
    mirror = &origineel;
    return mirror;
}

float EncryptBig(struct filecrypt *handle)
{
    int i, index = 0;
    float calc;
    char buf, *bytes = (char*)malloc(10485760); // 10 MB
    if (bytes == NULL)
    {
        handle = NULL;
        fcloseall();
        return -1.0f;
    }

    for (i = handle->huidig; i < (handle->huidig+10485760); i++)
    {
        if (i > handle->size)
            break;

        fseek(handle->bestand, i, SEEK_SET);
        fread_s(&buf, 1, 1, 1, handle->bestand);

        __asm
        {
            mov         eax, dword ptr [bytes]  
            add         eax, dword ptr [index]  
            mov         cl, byte ptr [buf]
            xor         cl, 18
            xor         cl, 75
            not         cl
            mov         byte ptr [eax], cl 
            mov         eax, dword ptr [index]  
            add         eax, 1  
            mov         dword ptr [index], eax
        }
    }

    fwrite(bytes, 1, i, handle->nieuwbstnd);
    fseek(handle->nieuwbstnd, i, SEEK_SET);

    handle->huidig += i;
    calc = (float)handle->huidig;
    calc /= (float)handle->size;
    calc *= 100.0f;

    if (calc == 100.0)
    {
        // GEHEUGEN LEK!
        // MOET NOG BIJGEWERKT WORDEN!
        fcloseall();
        handle = NULL;
    }
    return calc;
}

void example(char* path)
{
    float progress;
    struct filecrypt* handle;

    handle = PrepareEncryption(path);
    do
    {
        progress = EncryptBig(handle);
        printf_s("%f", progress);
    }
    while (handle != NULL);
}

Answer 1

It’s because you return a pointer to a local variable.

Local variables are stored on the stack, and when a function returns that area of the stack is reused by other functions, and you are left with a pointer that now points to unused memory or memory now occupied by something else. This is undefined behaviour, and might sometimes work, might sometimes give you “garbage” data, and may sometimes crash.