fcntl — The fcntl and ioctl system calls (original) (raw)

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.

All functions in this module take a file descriptor fd as their first argument. This can be an integer file descriptor, such as returned bysys.stdin.fileno(), or an io.IOBase object, such as sys.stdinitself, which provides a fileno() that returns a genuine file descriptor.

Changed in version 3.3: Operations in this module used to raise an IOError where they now raise an OSError.

Changed in version 3.8: The fcntl module now contains F_ADD_SEALS, F_GET_SEALS, andF_SEAL_* constants for sealing of os.memfd_create() file descriptors.

Changed in version 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.

Changed in version 3.10: On Linux >= 2.6.11, the fcntl module exposes the F_GETPIPE_SZ andF_SETPIPE_SZ constants, which allow to check and modify a pipe’s size respectively.

Changed in version 3.11: On FreeBSD, the fcntl module exposes the F_DUP2FD andF_DUP2FD_CLOEXEC constants, which allow to duplicate a file descriptor, the latter setting FD_CLOEXEC flag in addition.

Changed in version 3.12: On Linux >= 4.5, the fcntl module exposes the FICLONE andFICLONERANGE 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”.

Changed in version 3.13: On Linux >= 2.6.32, the fcntl module exposes theF_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 theF_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 theF_SEAL_FUTURE_WRITE constant for use with F_ADD_SEALS andF_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_RDAHEADconstant. On NetBSD and AIX, the fcntl module exposes the F_CLOSEMconstant. On NetBSD, the fcntl module exposes the F_MAXFD constant. On macOS and NetBSD, the fcntl module exposes the F_GETNOSIGPIPEand F_SETNOSIGPIPE constant.

The module defines the following functions:

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, abytes 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.

Note

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.

Raises an auditing event fcntl.fcntl with arguments fd, cmd, arg.

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

This function is identical to the fcntl() function, except that the argument handling is even more complicated.

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.

Note

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.

An example:

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])

Raises an auditing event fcntl.ioctl with arguments fd, request, arg.

fcntl.flock(fd, operation, /)¶

Perform the lock operation operation on file descriptor fd (file objects providing a fileno() method are accepted as well). See the Unix manual_flock(2)_ for details. (On some systems, this function is emulated using fcntl().)

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

Raises an auditing event fcntl.flock with arguments fd, operation.

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

This is essentially a wrapper around the fcntl() locking calls.fd is the file descriptor (file objects providing a fileno()method are accepted as well) of the file to lock or unlock, and _cmd_is one of the following values:

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, anOSError 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 is the number of bytes to lock, start is the byte offset at which the lock starts, relative to whence, and whence is as withio.IOBase.seek(), specifically:

0 – relative to the start of the file (os.SEEK_SET)
1 – relative to the current buffer position (os.SEEK_CUR)
2 – relative to the end of the file (os.SEEK_END)

The default for start is 0, which means to start at the beginning of the file. The default for len is 0 which means to lock to the end of the file. The default for whence is also 0.

Raises an auditing event fcntl.lockf with arguments fd, cmd, len, start, whence.

Examples (all on a SVR4 compliant system):

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)

Note that in the first example the return value variable rv will hold an integer value; in the second example it will hold a bytes object. The structure lay-out for the lockdata variable is system dependent — therefore using the flock() call may be better.