10. 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 a file object, such as sys.stdin
itself, which
provides a fileno()
which returns a genuine file descriptor.
The module defines the following functions:
-
fcntl.
fcntl
(fd, op[, arg])¶ Perform the operation op on file descriptor fd (file objects providing a
fileno()
method are accepted as well). The values used for for op 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 is optional, and defaults to the integer value0
. 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 Cfcntl()
call. When the argument is a string 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 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.If the
fcntl()
fails, anIOError
is raised.
-
fcntl.
ioctl
(fd, op[, arg[, mutate_flag]])¶ This function is identical to the
fcntl()
function, except that the operations are typically defined in the library moduletermios
and the argument handling is even more complicated.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 integer
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
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 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 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 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.
If the
ioctl()
fails, anIOError
exception is raised.An example:
>>> 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])
-
fcntl.
flock
(fd, op)¶ Perform the lock operation op 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, anIOError
exception is raised.
-
fcntl.
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
– unlockLOCK_SH
– acquire a shared lockLOCK_EX
– acquire an exclusive lock
When operation 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, anIOError
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.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
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 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.