This module performs file control and I/O control on file descriptors. It is an
interface to the
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 a file object, such as
sys.stdin itself, which
fileno() which returns a genuine file descriptor.
The module defines the following functions:
fcntl(fd, op[, arg])
Perform the operation op on file descriptor fd (file objects providing
fileno() method are accepted as well). The values used
for for op are operating system dependent, and are available as constants
fcntl module, using the same names as used in the relevant C
header files. The argument arg is optional, and defaults to the integer
0. When present, it can either be an integer value, or a string.
With the argument missing or an integer value, the return value of this function
is the integer return value of the C
fcntl() call. When the argument is
a string it represents a binary structure, e.g. created by
The binary data is copied to a buffer whose address is passed to the C
fcntl() call. The return value after a successful call is the contents
of the buffer, converted to a string object. The length of the returned string
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.
fcntl() fails, an
IOError is raised.
ioctl(fd, op[, arg[, mutate_flag]])
The op parameter is limited to values that can fit in 32-bits.
Additional constants of interest for use as the op 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, absent (treated identically to the
0), an object supporting the read-only buffer interface (most likely
a plain Python string) or an object supporting the read-write buffer interface.
In all but the last case, behaviour is as for the
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 as least as long as what the operating system wants to put there, things should work.
If mutate_flag is true, then the buffer is (in effect) passed to the
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 the
ioctl(). 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 to
ioctl() and copied back into the supplied
If mutate_flag is not supplied, then from Python 2.5 it defaults to true, which is a change from versions 2.3 and 2.4. Supply the argument explicitly if version portability is a priority.
ioctl() fails, an
IOError exception is raised.
>>> import array, fcntl, struct, termios, os >>> os.getpgrp() 13341 >>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, " ")) 13341 >>> buf = array.array('h', ) >>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1) 0 >>> buf array('h', )
Perform the lock operation op on file descriptor fd (file objects providing
fileno() method are accepted as well). See the Unix manual
flock(2) for details. (On some systems, this function is emulated
flock() fails, an
IOError exception is raised.
lockf(fd, operation[, length[, start[, whence]]])
This is essentially a wrapper around the
fcntl() locking calls.
fd is the file descriptor of the file to lock or unlock, and operation
is one of the following values:
LOCK_UN – unlock
LOCK_SH – acquire a shared lock
LOCK_EX – acquire an exclusive lock
When operation is
LOCK_EX, it can also be
bitwise ORed with
LOCK_NB to avoid blocking on lock acquisition.
LOCK_NB is used and the lock cannot be acquired, an
IOError will be raised and the exception will have an errno
attribute set to
EAGAIN (depending on the
operating system; for portability, check for both values). On at least some
LOCK_EX can only be used if the file descriptor refers to a
file opened for writing.
length 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
0 – relative to the start of the file (
1 – relative to the current buffer position (
2 – relative to the end of the file (
The default for start is 0, which means to start at the beginning of the file. The default for length is 0 which means to lock to the end of the file. The default for whence is also 0.
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 string value. The structure
lay-out for the lockdata variable is system dependent — therefore using the
flock() call may be better.