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The `fcntl()' and `ioctl()' system calls
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The `fcntl()' and `ioctl()' system calls.

This manual section was written by Jaap Vermeulen <>.
This module performs file control and I/O control on file descriptors.
It is an interface to the `fcntl()' and `ioctl()' UNIX routines.

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(fd, op[, arg])'
     Perform the requested operation on file descriptor FD (file
     objects providing a `fileno()' method are accepted as well).  The
     operation is defined by OP and is operating system dependent.
     These codes are also found in the `fcntl' module. The argument ARG
     is optional, and defaults to the integer value `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 `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 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, an `IOError' is 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 (file objects
     providing a `fileno()' method are accepted as well).  See the UNIX
     manual `flock(3)' for details.  (On some systems, this function is
     emulated using `fcntl()'.)

`lockf(fd, operation, [len, [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_SH' or `LOCK_EX', it can also be bit-wise
     OR'd with `LOCK_NB' to avoid blocking on lock acquisition.  If
     `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 `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.

     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
     `fileobj.seek()', specifically:

        * `0' - relative to the start of the file (`SEEK_SET')

        * `1' - relative to the current buffer position (`SEEK_CUR')

        * `2' - relative to the end of the file (`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
     
     file = open(...)
     rv = fcntl(file, fcntl.F_SETFL, os.O_NDELAY)
     
     lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
     rv = fcntl.fcntl(file, 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.