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: En Linux >= 4.5, el módulo fcntl expone las constantes FICLONE y FICLONERANGE, que permiten compartir algunos datos de un fichero con otro fichero mediante el reflinking en algunos sistemas de ficheros (por ejemplo, btrfs, OCFS2 y XFS). Este comportamiento se conoce comúnmente como «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.

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 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, 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. This is limited to 1024 bytes.

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

Nota

If the type or the size of arg does not match the type or size of the argument of the operation (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 1024 bytes), 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.

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 1024 bytes, or the size of the mutable bytes-like object is too small), 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.

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:

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().