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Python 3.6.5 Documentation > "fcntl" — The "fcntl" and "ioctl" system calls

"fcntl" — The "fcntl" and "ioctl" system calls
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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.

Changed in version 3.3: Operations in this module used to raise an
"IOError" where they now raise an "OSError".

The module defines the following functions:

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 the "fcntl" module, using the same names as used in
the relevant C header files. The argument *arg* can either be an
integer value, or a "bytes" object. With an integer value, the
return value of this function is the integer return value of the C
"fcntl()" call. When the argument is bytes it represents a binary
structure, e.g. created by "struct.pack()". 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 "bytes" 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, an "OSError" is raised.

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 (like "bytearray").

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 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 buffer.

If the "ioctl()" fails, an "OSError" 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, 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 using "fcntl()".)

If the "flock()" fails, an "OSError" exception is raised.

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 of the file to lock or unlock, and
*cmd* is one of the following values:

* "LOCK_UN" – unlock

* "LOCK_SH" – acquire a shared lock

* "LOCK_EX" – acquire an exclusive lock

When *cmd* is "LOCK_SH" or "LOCK_EX", it can also be bitwise ORed
with "LOCK_NB" to avoid blocking on lock acquisition. If "LOCK_NB"
is used and the lock cannot be acquired, an "OSError" will be
raised and the exception will have an *errno* attribute set to
"EACCES" or "EAGAIN" (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.

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.