Write access violation when trying to modify array

Question

The file from resources is passed to function Change which should XOR each byte value, but then I get write access violation error.

INT CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR lpCmdLine, INT nCmdShow)
{
    HRSRC hRes = FindResource(NULL, L"FILE", RT_RCDATA);

    if (hRes == NULL)
    {
        // print error
    }

    DWORD resSize = SizeofResource(NULL, hRes);
    HGLOBAL resData = LoadResource(NULL, hRes);
    byte *file = reinterpret_cast<byte*>(LockResource(resData));

    Change(file, resSize);

    return 0;
}

void Change(byte *data, int size)
{
    for (int i = 0; i < size; ++i)
    {
        data[i] ^= 2;
    }
}

Did you check the return value of LockResource to ensure that it wasn't null? — Xirema
– Xirema, Commented Apr 5, 2017 at 18:58
In memory window I can see that file variable has the bytes from my file. — Xor
– Xor, Commented Apr 5, 2017 at 19:11
Resources are in a read-only segment of the executable. You can't modify memory taken by the resource. — Igor Tandetnik
– Igor Tandetnik, Commented Apr 5, 2017 at 19:50

Remy Lebeau · Accepted Answer · 2017-04-05 20:32:48Z

2

Resources exist in read-only memory, you cannot write to them directly.

The only way to modify the contents of a resource is to use UpdateResource() (unless you WriteFile() directly to the executable file on disk), but you can't use either of those on a resource that belongs to an active running process, as the executable is locked by the OS.

So, the only way to do what you are attempting is to allocate a separate copy of the resource data in writable memory, and then you can do whatever you want with that copy, eg:

INT CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR lpCmdLine, INT nCmdShow)
{
    HRSRC hRes = FindResource(NULL, L"FILE", RT_RCDATA);
    if (hRes == NULL)
    {
        // print error
    }
    else
    {
        DWORD resSize = SizeofResource(NULL, hRes);
        HGLOBAL resData = LoadResource(NULL, hRes);
        LPVOID resDataPtr = LockResource(resData);

        byte *copy = new byte[resSize];
        memcpy(copy, resDataPtr, resSize);
        Change(copy, resSize);
        delete[] copy;
    }

    return 0;
}

answered Apr 5, 2017 at 20:32

Remy Lebeau

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1 Comment

Dan Over a year ago

Actually I found a way to alter resource that seemed locked, but I don't know if that would work here. See here: stackoverflow.com/questions/56302660/…

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