# `fcntl` — The `fcntl` and `ioctl` system calls

This module performs file and I/O control on file descriptors. It is an interface to the `fcntl()` and `ioctl()` Unix routines. See the fcntl(2) and ioctl(2) Unix manual pages for full details.

Availability: Unix, not WASI.

Todas las funciones de este módulo toman un descriptor de fichero fd como su primer argumento. Puede ser un descriptor de fichero entero, como el retornado por `sys.stdin.fileno()`, o un objeto `io.IOBase`, como `sys.stdin`, que proporciona un `fileno()` que retorna un descriptor de fichero original.

Distinto en la versión 3.3: Las operaciones en este módulo solían lanzar un `IOError` donde ahora lanzan un `OSError`.

Distinto en la versión 3.8: The `fcntl` module now contains `F_ADD_SEALS`, `F_GET_SEALS`, and `F_SEAL_*` constants for sealing of `os.memfd_create()` file descriptors.

Distinto en la versión 3.9: On macOS, the `fcntl` module exposes the `F_GETPATH` constant, which obtains the path of a file from a file descriptor. On Linux(>=3.15), the `fcntl` module exposes the `F_OFD_GETLK`, `F_OFD_SETLK` and `F_OFD_SETLKW` constants, which are used when working with open file description locks.

Distinto en la versión 3.10: On Linux >= 2.6.11, the `fcntl` module exposes the `F_GETPIPE_SZ` and `F_SETPIPE_SZ` constants, which allow to check and modify a pipe’s size respectively.

Distinto en la versión 3.11: On FreeBSD, the `fcntl` module exposes the `F_DUP2FD` and `F_DUP2FD_CLOEXEC` constants, which allow to duplicate a file descriptor, the latter setting `FD_CLOEXEC` flag in addition.

Distinto en la versión 3.12: On Linux >= 4.5, the `fcntl` module exposes the `FICLONE` and `FICLONERANGE` constants, which allow to share some data of one file with another file by reflinking on some filesystems (e.g., btrfs, OCFS2, and XFS). This behavior is commonly referred to as «copy-on-write».

Distinto en la versión 3.13: On Linux >= 2.6.32, the `fcntl` module exposes the `F_GETOWN_EX`, `F_SETOWN_EX`, `F_OWNER_TID`, `F_OWNER_PID`, `F_OWNER_PGRP` constants, which allow to direct I/O availability signals to a specific thread, process, or process group. On Linux >= 4.13, the `fcntl` module exposes the `F_GET_RW_HINT`, `F_SET_RW_HINT`, `F_GET_FILE_RW_HINT`, `F_SET_FILE_RW_HINT`, and `RWH_WRITE_LIFE_*` constants, which allow to inform the kernel about the relative expected lifetime of writes on a given inode or via a particular open file description. On Linux >= 5.1 and NetBSD, the `fcntl` module exposes the `F_SEAL_FUTURE_WRITE` constant for use with `F_ADD_SEALS` and `F_GET_SEALS` operations. On FreeBSD, the `fcntl` module exposes the `F_READAHEAD`, `F_ISUNIONSTACK`, and `F_KINFO` constants. On macOS and FreeBSD, the `fcntl` module exposes the `F_RDAHEAD` constant. On NetBSD and AIX, the `fcntl` module exposes the `F_CLOSEM` constant. On NetBSD, the `fcntl` module exposes the `F_MAXFD` constant. On macOS and NetBSD, the `fcntl` module exposes the `F_GETNOSIGPIPE` and `F_SETNOSIGPIPE` constant.

Distinto en la versión 3.14: On Linux >= 6.1, the `fcntl` module exposes the `F_DUPFD_QUERY` to query a file descriptor pointing to the same file.

El módulo define las siguientes funciones:

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, a bytes-like object, or a string. The type and size of arg must match the type and size of the argument of the operation as specified in the relevant C documentation.

  When arg is an integer, the function returns the integer return value of the C `fcntl()` call.

  When the argument is bytes-like object, it represents a binary structure, for example, created by `struct.pack()`. A string value is encoded to binary using the UTF-8 encoding. 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.

  If the `fcntl()` call fails, an `OSError` is raised.

  Nota

  If the type or size of arg does not match the type or size of the operation’s argument (for example, if an integer is passed when a pointer is expected, or the information returned in the buffer by the operating system is larger than the size of arg), this is most likely to result in a segmentation violation or a more subtle data corruption.

  Lanza un evento de auditoria `fcntl.fcntl` con argumentos `fd`, `cmd`, `arg`.

  Distinto en la versión 3.14: Add support of arbitrary bytes-like objects, not only `bytes`.

  Distinto en la versión 3.15: The size of bytes-like objects is no longer limited to 1024 bytes.

fcntl.ioctl(fd, request, arg=0, mutate_flag=True, /)

  Esta función es idéntica a la función `fcntl()`, excepto por el manejo de los argumentos que es aún más complicado.

  The request parameter is limited to values that can fit in 32-bits or 64-bits, depending on the platform. 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 an integer, a bytes-like object, or a string. The type and size of arg must match the type and size of the argument of the operation as specified in the relevant C documentation.

  If arg does not support the read-write buffer interface or the mutate_flag is false, behavior is as for the `fcntl()` function.

  If arg supports the read-write buffer interface (like `bytearray`) and 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()` call fails, an `OSError` exception is raised.

  Nota

  If the type or size of arg does not match the type or size of the operation’s argument (for example, if an integer is passed when a pointer is expected, or the information returned in the buffer by the operating system is larger than the size of arg), this is most likely to result in a segmentation violation or a more subtle data corruption.

  Un ejemplo:

      >>> 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])

  Lanza un evento de auditoria `fcntl.ioctl` con argumentos `fd`, `request`, `arg`.

  Distinto en la versión 3.14: The GIL is always released during a system call. System calls failing with EINTR are automatically retried.

  Distinto en la versión 3.15: The size of not mutated bytes-like objects is no longer limited to 1024 bytes.

fcntl.flock(fd, operation, /)

  Realiza la operación de bloqueo operation sobre el descriptor de fichero fd (también se aceptan objetos de fichero que proporcionen un método `fileno()`). Ver el manual de Unix flock(2) para más detalles. (En algunos sistemas, esta función es emulada usando `fcntl()`.)

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

  Lanza un evento de auditoria `fcntl.flock` con argumentos `fd`, `operation`.

fcntl.lockf(fd, cmd, len=0, start=0, whence=0, /)

  Esto es esencialmente un «wrapper» de las llamadas de bloqueo `fcntl()`. fd es el descriptor de fichero (también se aceptan objetos de fichero que proporcionen un método `fileno()`) del archivo para bloquear o desbloquear, y cmd es uno de los siguientes valores:

  fcntl.LOCK_UN

    Release an existing lock.

  fcntl.LOCK_SH

    Acquire a shared lock.

  fcntl.LOCK_EX

    Acquire an exclusive lock.

  fcntl.LOCK_NB

    Bitwise OR with any of the other three `LOCK_*` constants to make the request non-blocking.

  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 es el número de bytes a bloquear, start es el desplazamiento de bytes en el que comienza el bloqueo, relativo a whence, y whence es como con `io.IOBase.seek()`, específicamente:

  - `0` – relativo al inicio del archivo (`os.SEEK_SET`)

  - `1` – relativo a la posición actual del buffer (`os.SEEK_CUR`)

  - `2` – relativo al final del archivo (`os.SEEK_END`)

  El valor por defecto de*start* es 0, lo que significa que comienza al inicio del archivo. El valor por defecto para len es 0 lo que significa bloquear hasta el final del archivo. El valor por defecto para whence también es 0.

  Lanza un evento de auditoria `fcntl.lockf` con argumentos `fd`, `cmd`, `len`, `start`, `whence`.

Ejemplos (todos en un sistema compatible con SVR4):

    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)

Tenga en cuenta que en el primer ejemplo, el valor de la variable retornada rv contendrá un valor entero; en el segundo ejemplo contendrá un objeto `bytes`. La estructura para la variable lockdata depende del sistema — por lo tanto, usar la llamada `flock()` puede ser mejor.

Ver también

Módulo `os`

  Si los indicadores de bloqueo `O_SHLOCK` y `O_EXLOCK` están presentes en el módulo `os` (sólo en BSD), la función `os.open()` proporciona una alternativa a las funciones `lockf()` y `flock()`.