, 2 min read
Working with Windows Junctions in Python
Original post is here eklausmeier.goip.de/blog/2015/10-27-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.