27th October 2015, 2 min read

Working with Windows Junctions in Python

I had to detect Windows junctions (similar but not identical to symbolic links) in Python. On stackoverflow.com I read the following Python code given by a user named eryksun for handling them, as os.islink() does not work for junctions.

The routines access Windows functions CreateFileW(), DeviceIoControl(), CloseHandle() in kernel32.

from ctypes import *
from ctypes.wintypes import *

kernel32 = WinDLL('kernel32')
LPDWORD = POINTER(DWORD)
UCHAR = c_ubyte

GetFileAttributesW = kernel32.GetFileAttributesW
GetFileAttributesW.restype = DWORD
GetFileAttributesW.argtypes = (LPCWSTR,) #lpFileName In

INVALID_FILE_ATTRIBUTES = 0xFFFFFFFF
FILE_ATTRIBUTE_REPARSE_POINT = 0x00400

CreateFileW = kernel32.CreateFileW
CreateFileW.restype = HANDLE
CreateFileW.argtypes = (LPCWSTR, #lpFileName In
                        DWORD,   #dwDesiredAccess In
                        DWORD,   #dwShareMode In
                        LPVOID,  #lpSecurityAttributes In_opt
                        DWORD,   #dwCreationDisposition In  
                        DWORD,   #dwFlagsAndAttributes In
                        HANDLE)  #hTemplateFile In_opt

CloseHandle = kernel32.CloseHandle
CloseHandle.restype = BOOL
CloseHandle.argtypes = (HANDLE,) #hObject In

INVALID_HANDLE_VALUE = HANDLE(-1).value
OPEN_EXISTING = 3
FILE_FLAG_BACKUP_SEMANTICS = 0x02000000
FILE_FLAG_OPEN_REPARSE_POINT = 0x00200000

DeviceIoControl = kernel32.DeviceIoControl
DeviceIoControl.restype = BOOL
DeviceIoControl.argtypes = (HANDLE,  #hDevice In
                            DWORD,   #dwIoControlCode In
                            LPVOID,  #lpInBuffer In_opt
                            DWORD,   #nInBufferSize In
                            LPVOID,  #lpOutBuffer Out_opt
                            DWORD,   #nOutBufferSize In
                            LPDWORD, #lpBytesReturned Out_opt
                            LPVOID)  #lpOverlapped Inout_opt

FSCTL_GET_REPARSE_POINT = 0x000900A8
IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
IO_REPARSE_TAG_SYMLINK = 0xA000000C
MAXIMUM_REPARSE_DATA_BUFFER_SIZE = 0x4000

class GENERIC_REPARSE_BUFFER(Structure):
    _fields_ = (('DataBuffer', UCHAR * 1),)


class SYMBOLIC_LINK_REPARSE_BUFFER(Structure):
    _fields_ = (('SubstituteNameOffset', USHORT),
                ('SubstituteNameLength', USHORT),
                ('PrintNameOffset', USHORT),
                ('PrintNameLength', USHORT),
                ('Flags', ULONG),
                ('PathBuffer', WCHAR * 1))
    @property
    def PrintName(self):
        arrayt = WCHAR * (self.PrintNameLength // 2)
        offset = type(self).PathBuffer.offset + self.PrintNameOffset
        return arrayt.from_address(addressof(self) + offset).value


class MOUNT_POINT_REPARSE_BUFFER(Structure):
    _fields_ = (('SubstituteNameOffset', USHORT),
                ('SubstituteNameLength', USHORT),
                ('PrintNameOffset', USHORT),
                ('PrintNameLength', USHORT),
                ('PathBuffer', WCHAR * 1))
    @property
    def PrintName(self):
        arrayt = WCHAR * (self.PrintNameLength // 2)
        offset = type(self).PathBuffer.offset + self.PrintNameOffset
        return arrayt.from_address(addressof(self) + offset).value


class REPARSE_DATA_BUFFER(Structure):
    class REPARSE_BUFFER(Union):
        _fields_ = (('SymbolicLinkReparseBuffer', 
                        SYMBOLIC_LINK_REPARSE_BUFFER),
                    ('MountPointReparseBuffer', 
                        MOUNT_POINT_REPARSE_BUFFER),
                    ('GenericReparseBuffer', 
                        GENERIC_REPARSE_BUFFER))
    _fields_ = (('ReparseTag', ULONG),
                ('ReparseDataLength', USHORT),
                ('Reserved', USHORT),
                ('ReparseBuffer', REPARSE_BUFFER))
    _anonymous_ = ('ReparseBuffer',)


def islink(path):
    result = GetFileAttributesW(path)
    if result == INVALID_FILE_ATTRIBUTES:
        raise WinError()
    return bool(result & FILE_ATTRIBUTE_REPARSE_POINT)

def readlink(path):
    reparse_point_handle = CreateFileW(path, 
                                       0,
                                       0,
                                       None,
                                       OPEN_EXISTING,
                                       FILE_FLAG_OPEN_REPARSE_POINT |
                                       FILE_FLAG_BACKUP_SEMANTICS,
                                       None)
    if reparse_point_handle == INVALID_HANDLE_VALUE:
        raise WinError()
    target_buffer = c_buffer(MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
    n_bytes_returned = DWORD()
    io_result = DeviceIoControl(reparse_point_handle, 
                                FSCTL_GET_REPARSE_POINT, 
                                None, 0,
                                target_buffer, len(target_buffer),
                                byref(n_bytes_returned),
                                None)
    CloseHandle(reparse_point_handle)
    if not io_result:
        raise WinError()
    rdb = REPARSE_DATA_BUFFER.from_buffer(target_buffer)        
    if rdb.ReparseTag == IO_REPARSE_TAG_SYMLINK:
        return rdb.SymbolicLinkReparseBuffer.PrintName
    elif rdb.ReparseTag == IO_REPARSE_TAG_MOUNT_POINT:
        return rdb.MountPointReparseBuffer.PrintName
    raise ValueError("not a link")

The user calls islink() to detect whether the supplied path-name is a junction or not. readlink() gives you the path to which the arguments points to. So if X points to Y, then readlink(X) gives Y.

One comment in stackoverflow.com says: This works out of the box. Indeed.