Argument of type "BYTE*" is incompatible with parameter of type "LPCWSTR" on Windows

Question

I'm trying to make a randomly named file on the desktop by getting the desktop path and then combining it with the buffer of BCryptGetRandom but the function PathCombineW doesn't accept the buffer

#include <Windows.h>
#include <Shlobj.h>

int main()
{
    BYTE Buffer[15];
    DWORD size = sizeof(Buffer);
    BCryptGenRandom(NULL, Buffer, size, BCRYPT_USE_SYSTEM_PREFERRED_RNG);

    LPWSTR desktop;
    SHGetKnownFolderPath(FOLDERID_Desktop, 0, NULL, &desktop);

    WCHAR filePath[MAX_PATH];
    PathCombineW(filePath, desktop, Buffer); //the error is here
    CreateFileW(filePath, GENERIC_ALL, FILE_SHARE_WRITE, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
}

Answer 1

WCHAR on Windows is UTF-16LE and you therefore should not use random bytes directly because not everything is going to be valid UTF-16.

You can use a WCHAR Buffer[15] = {}; and fill 0-13 with numbers in a known valid range, for example from 0x20 to 0xD7FF (excluding things that are invalid in filenames ).

If you want to interoperate with non-Unicode programs you might want to limit yourself to ASCII...

Answer 2

A byte array is not a valid null-terminated UTF16 string, which is why PathCombineW() won't accept it.

Also, random bytes are not guaranteed to represent valid UTF16 characters.

What you can do is hex-encode the generated bytes into an ASCII string in UTF16 encoding, eg:

#include <Windows.h>
#include <Shlobj.h>

int main()
{
    BYTE Buffer[15];
    BCryptGenRandom(NULL, Buffer, sizeof(Buffer), BCRYPT_USE_SYSTEM_PREFERRED_RNG);

    LPWSTR desktop = NULL;
    SHGetKnownFolderPath(FOLDERID_Desktop, 0, NULL, &desktop);

    WCHAR fileName[(sizeof(Buffer)*2)+1];
    for(int i = 0, j = 0; i < sizeof(Buffer); ++i, j += 2) {
        wsprintfW(&fileName[j], L"02X", Buffer[i]);
    }

    WCHAR filePath[MAX_PATH];
    PathCombineW(filePath, desktop, fileName);

    HANDLE hFile = CreateFileW(filePath, GENERIC_ALL, FILE_SHARE_WRITE, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    CloseHandle(hFile);

    CoTaskMemFree(desktop);
}