# 8.12 `fcntl` -- The `fcntl()` and `ioctl()` system calls

Availability: Unix.

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 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. 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).

  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.

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, FCNTL

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

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