8.10 Built-in Module fcntl

       

This module performs file control and I/O control on file descriptors. It is an interface to the fcntl() and ioctl() Unix routines. File descriptors can be obtained with the fileno() method of a file or socket object.

The module defines the following functions:

fcntl (fd, op[, arg])

Perform the requested operation on file descriptor fd. The operation is defined by op and is operating system dependent. Typically these codes can be retrieved from the library module FCNTL. The argument arg is optional, and defaults to the integer value 0. When it is 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 real fcntl() call. When the argument is a string 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 real fcntl() call. The return value after a successful call is the contents of the buffer, converted to a string object. In case the fcntl() fails, an IOError will be raised.

ioctl (fd, op, arg)

This function is identical to the fcntl() function, except that the operations are typically defined in the library module IOCTL.

flock (fd, op)

Perform the lock operation op on file descriptor fd. See the Unix manual for details. (On some systems, this function is emulated using fcntl().)

lockf (fd, code, [len, [start, [whence]]])

This is a wrapper around the F_SETLK and F_SETLKW fcntl() calls. See the Unix manual for details.

If the library modules FCNTL or IOCTL are missing, you can find the opcodes in the C include files `sys/fcntl.h' and `sys/ioctl.h'. You can create the modules yourself with the h2py script, found in the `Tools/scripts' directory.    

Examples (all on a SVR4 compliant system):

import struct, FCNTL

file = open(...)
rv = fcntl(file.fileno(), FCNTL.O_NDELAY, 1)

lockdata = struct.pack('hhllhh', FCNTL.F_WRLCK, 0, 0, 0, 0, 0)
rv = fcntl(file.fileno(), 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 lockadata variable is system dependent - therefore using the flock() call may be better.