How can I convert a native (NT) pathname into a Win32 path name?

We do this in production code. As far as I know there is no API (public or private) that handles this. We just do some string comparisons with a few prefixes and it works for us.

Apparently there is a function named RtlNtPathNameToDosPathName() in ntdll.dll (introduced with XP?), but I have no idea what it does; I would guess it has more to do with stuff like \Device\Harddisk0, though.

I'm not sure there is really a need for such a function, though. Win32 passes paths (in the sense of CreateFile, etc) to NT; NT doesn't pass paths to Win32. So ntdll.dll doesn't really have a need to go from NT paths to Win32 paths. In the rare case where some NT query function returns a full path, any conversion function could be internal to the Win32 dll (e.g. not exported). I don't even know if they bother, as stuff like GetModuleFileName() will just return whatever path was used to load the image. I guess this is just a leaky abstraction.

Check this out for getting the canonical pathname in Win32. It may be helpful for you:

http://pdh11.blogspot.com/2009/05/pathcanonicalize-versus-what-it-says-on.html

This is a bit late, but I will still post my answer since even today this is a very good question!

I will share one of my functions tested and used for converting NT to DOS path. In my case, I also had to convert from ANSI to UNICODE so this is a small bonus for you to see and understand how this can be done.

All this code can be used in User Mode, so we need to first prepare some things.

Definitions & Structures:

typedef NTSTATUS(WINAPI* pRtlAnsiStringToUnicodeString)(PUNICODE_STRING, PANSI_STRING, BOOL);

typedef struct _RTL_BUFFER {
    PUCHAR    Buffer;
    PUCHAR    StaticBuffer;
    SIZE_T    Size;
    SIZE_T    StaticSize;
    SIZE_T    ReservedForAllocatedSize; // for future doubling
    PVOID     ReservedForIMalloc; // for future pluggable growth
} RTL_BUFFER, * PRTL_BUFFER;

typedef struct _RTL_UNICODE_STRING_BUFFER {
    UNICODE_STRING String;
    RTL_BUFFER     ByteBuffer;
    UCHAR          MinimumStaticBufferForTerminalNul[sizeof(WCHAR)];
} RTL_UNICODE_STRING_BUFFER, * PRTL_UNICODE_STRING_BUFFER;

#define RTL_NT_PATH_NAME_TO_DOS_PATH_NAME_AMBIGUOUS   (0x00000001)
#define RTL_NT_PATH_NAME_TO_DOS_PATH_NAME_UNC         (0x00000002)
#define RTL_NT_PATH_NAME_TO_DOS_PATH_NAME_DRIVE       (0x00000003)
#define RTL_NT_PATH_NAME_TO_DOS_PATH_NAME_ALREADY_DOS (0x00000004)

typedef NTSTATUS(WINAPI* pRtlNtPathNameToDosPathName)(__in ULONG Flags, __inout PRTL_UNICODE_STRING_BUFFER Path, __out_opt PULONG Disposition, __inout_opt PWSTR* FilePart);

#define RTL_DUPLICATE_UNICODE_STRING_NULL_TERMINATE (0x00000001)
#define RTL_DUPLICATE_UNICODE_STRING_ALLOCATE_NULL_STRING (0x00000002)
#define RTL_DUPSTR_ADD_NULL                          RTL_DUPLICATE_UNICODE_STRING_NULL_TERMINATE
#define RTL_DUPSTR_ALLOC_NULL                        RTL_DUPLICATE_UNICODE_STRING_ALLOCATE_NULL_STRING

typedef NTSTATUS(WINAPI* pRtlDuplicateUnicodeString)(_In_ ULONG Flags, _In_ PUNICODE_STRING StringIn, _Out_ PUNICODE_STRING StringOut);

Importing functions:

pRtlAnsiStringToUnicodeString MyRtlAnsiStringToUnicodeString;
pRtlNtPathNameToDosPathName MyRtlNtPathNameToDosPathName;
pRtlDuplicateUnicodeString MyRtlDuplicateUnicodeString;

MyRtlAnsiStringToUnicodeString = (pRtlAnsiStringToUnicodeString)GetProcAddress(GetModuleHandle("ntdll.dll"), "RtlAnsiStringToUnicodeString");
MyRtlNtPathNameToDosPathName = (pRtlNtPathNameToDosPathName)GetProcAddress(GetModuleHandle("ntdll.dll"), "RtlNtPathNameToDosPathName");
MyRtlDuplicateUnicodeString = (pRtlDuplicateUnicodeString)GetProcAddress(GetModuleHandle("ntdll.dll"), "RtlDuplicateUnicodeString");

Helper function:

NTSTATUS NtPathNameToDosPathName(PUNICODE_STRING DosPath, PUNICODE_STRING NtPath)
{
    NTSTATUS                    Status;
    ULONG_PTR                   BufferSize;
    PWSTR                       Buffer;
    RTL_UNICODE_STRING_BUFFER   UnicodeBuffer;

    BufferSize = NtPath->MaximumLength + MAX_PATH * sizeof(WCHAR);

    Buffer = (PWSTR)_alloca(BufferSize);

    ZeroMemory(&UnicodeBuffer, sizeof(UnicodeBuffer));

    UnicodeBuffer.String = *NtPath;
    UnicodeBuffer.String.Buffer = Buffer;
    UnicodeBuffer.String.MaximumLength = (USHORT)BufferSize;
    UnicodeBuffer.ByteBuffer.Buffer = (PUCHAR)Buffer;
    UnicodeBuffer.ByteBuffer.Size = BufferSize;

    CopyMemory(Buffer, NtPath->Buffer, NtPath->Length);

    MyRtlNtPathNameToDosPathName(0, &UnicodeBuffer, NULL, NULL);

    return MyRtlDuplicateUnicodeString(RTL_DUPSTR_ADD_NULL, &UnicodeBuffer.String, DosPath);
}

Function usage:

UNICODE_STRING us;
UNICODE_STRING DosPath;
ANSI_STRING as;

as.Buffer = (char*)malloc(strlen(NT_PATH_FILE_OR_DIR) + 1);
strcpy(as.Buffer, NT_PATH_FILE_OR_DIR);
as.Length = as.MaximumLength = us.MaximumLength = us.Length = strlen(NT_PATH_FILE_OR_DIR);

MyRtlAnsiStringToUnicodeString(&us, &as, TRUE);

NtPathNameToDosPathName(&DosPath, &us);

As mentioned, in my case I needed to convert from ANSI to UNICODE and this might not apply for your case, thus you can remove it.

Same as above can be used to create custom functions and convert paths as needed.

Here's something you could try. First use NtCreateFile to open the file, volume etc. for reading. Then use the returned HANDLE to get the full path as described here.