fcntl
--- The fcntl
and ioctl
system calls¶
This module performs file control and I/O control on file descriptors. It is an
interface to the fcntl()
and ioctl()
Unix routines. For a
complete description of these calls, see fcntl(2) and
ioctl(2) Unix manual pages.
All functions in this module take a file descriptor fd as their first
argument. This can be an integer file descriptor, such as returned by
sys.stdin.fileno()
, or an io.IOBase
object, such as sys.stdin
itself, which provides a fileno()
that returns a genuine file
descriptor.
Modifié dans la version 3.3: Operations in this module used to raise an IOError
where they now
raise an OSError
.
Modifié dans la version 3.8: The fcntl module now contains F_ADD_SEALS
, F_GET_SEALS
, and
F_SEAL_*
constants for sealing of os.memfd_create()
file
descriptors.
Modifié dans la version 3.9: On macOS, the fcntl module exposes the F_GETPATH
constant, which obtains
the path of a file from a file descriptor.
On Linux(>=3.15), the fcntl module exposes the F_OFD_GETLK
, F_OFD_SETLK
and F_OFD_SETLKW
constants, which are used when working with open file
description locks.
Le module définit les fonctions suivantes :
-
fcntl.
fcntl
(fd, cmd, arg=0)¶ Perform the operation cmd on file descriptor fd (file objects providing a
fileno()
method are accepted as well). The values used for cmd are operating system dependent, and are available as constants in thefcntl
module, using the same names as used in the relevant C header files. The argument arg can either be an integer value, or abytes
object. With an integer value, the return value of this function is the integer return value of the Cfcntl()
call. When the argument is bytes it represents a binary structure, e.g. created bystruct.pack()
. The binary data is copied to a buffer whose address is passed to the Cfcntl()
call. The return value after a successful call is the contents of the buffer, converted to abytes
object. The length of the returned object will be the same as the length of the arg argument. This is limited to 1024 bytes. If the information returned in the buffer by the operating system is larger than 1024 bytes, this is most likely to result in a segmentation violation or a more subtle data corruption.If the
fcntl()
fails, anOSError
is raised.Raises an auditing event
fcntl.fcntl
with argumentsfd
,cmd
,arg
.
-
fcntl.
ioctl
(fd, request, arg=0, mutate_flag=True)¶ This function is identical to the
fcntl()
function, except that the argument handling is even more complicated.The request parameter is limited to values that can fit in 32-bits. Additional constants of interest for use as the request argument can be found in the
termios
module, under the same names as used in the relevant C header files.The parameter arg can be one of an integer, an object supporting the read-only buffer interface (like
bytes
) or an object supporting the read-write buffer interface (likebytearray
).In all but the last case, behaviour is as for the
fcntl()
function.If a mutable buffer is passed, then the behaviour is determined by the value of the mutate_flag parameter.
If it is false, the buffer's mutability is ignored and behaviour is as for a read-only buffer, except that the 1024 byte limit mentioned above is avoided -- so long as the buffer you pass is at least as long as what the operating system wants to put there, things should work.
If mutate_flag is true (the default), then the buffer is (in effect) passed to the underlying
ioctl()
system call, the latter's return code is passed back to the calling Python, and the buffer's new contents reflect the action of theioctl()
. This is a slight simplification, because if the supplied buffer is less than 1024 bytes long it is first copied into a static buffer 1024 bytes long which is then passed toioctl()
and copied back into the supplied buffer.If the
ioctl()
fails, anOSError
exception is raised.Un exemple :
>>> import array, fcntl, struct, termios, os >>> os.getpgrp() 13341 >>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, " "))[0] 13341 >>> buf = array.array('h', [0]) >>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1) 0 >>> buf array('h', [13341])
Raises an auditing event
fcntl.ioctl
with argumentsfd
,request
,arg
.
-
fcntl.
flock
(fd, operation)¶ Perform the lock operation operation on file descriptor fd (file objects providing a
fileno()
method are accepted as well). See the Unix manual flock(2) for details. (On some systems, this function is emulated usingfcntl()
.)If the
flock()
fails, anOSError
exception is raised.Raises an auditing event
fcntl.flock
with argumentsfd
,operation
.
-
fcntl.
lockf
(fd, cmd, len=0, start=0, whence=0)¶ This is essentially a wrapper around the
fcntl()
locking calls. fd is the file descriptor (file objects providing afileno()
method are accepted as well) of the file to lock or unlock, and cmd is one of the following values:LOCK_UN
-- unlockLOCK_SH
-- acquire a shared lockLOCK_EX
-- acquire an exclusive lock
When cmd is
LOCK_SH
orLOCK_EX
, it can also be bitwise ORed withLOCK_NB
to avoid blocking on lock acquisition. IfLOCK_NB
is used and the lock cannot be acquired, anOSError
will be raised and the exception will have an errno attribute set toEACCES
orEAGAIN
(depending on the operating system; for portability, check for both values). On at least some systems,LOCK_EX
can only be used if the file descriptor refers to a file opened for writing.len is the number of bytes to lock, start is the byte offset at which the lock starts, relative to whence, and whence is as with
io.IOBase.seek()
, specifically:0
-- relative to the start of the file (os.SEEK_SET
)1
-- relative to the current buffer position (os.SEEK_CUR
)2
-- relative to the end of the file (os.SEEK_END
)
The default for start is 0, which means to start at the beginning of the file. The default for len is 0 which means to lock to the end of the file. The default for whence is also 0.
Raises an auditing event
fcntl.lockf
with argumentsfd
,cmd
,len
,start
,whence
.
Examples (all on a SVR4 compliant system):
import struct, fcntl, os
f = open(...)
rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)
lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
Note that in the first example the return value variable rv will hold an
integer value; in the second example it will hold a bytes
object. The
structure lay-out for the lockdata variable is system dependent --- therefore
using the flock()
call may be better.