fcntl —- The fcntl and ioctl system calls

This module performs file control and I/O control on file descriptors. It is aninterface to the fcntl() and ioctl() Unix routines. For acomplete description of these calls, see fcntl(2)) andioctl(2)) Unix manual pages.

All functions in this module take a file descriptor fd as their firstargument. 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 filedescriptor.

在 3.3 版更改: Operations in this module used to raise an IOError where they nowraise an OSError.

在 3.8 版更改: The fcntl module now contains FADD_SEALS, F_GET_SEALS, andF_SEAL* constants for sealing of os.memfd_create() filedescriptors.

这个模块定义了以下函数：

• fcntl.fcntl(fd, cmd, arg=0)
• Perform the operation cmd on file descriptor fd (file objects providinga fileno() method are accepted as well). The values usedfor cmd are operating system dependent, and are available as constantsin the fcntl module, using the same names as used in the relevant Cheader files. The argument arg can either be an integer value, or abytes object. With an integer value, the return value of thisfunction is the integer return value of the C fcntl() call. Whenthe argument is bytes it represents a binary structure, e.g. created bystruct.pack(). The binary data is copied to a buffer whose address ispassed to the C fcntl() call. The return value after a successfulcall 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 thearg argument. This is limited to 1024 bytes. If the information returnedin the buffer by the operating system is larger than 1024 bytes, this ismost likely to result in a segmentation violation or a more subtle datacorruption.

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

• fcntl.ioctl(fd, request, arg=0, mutate_flag=True)
• This function is identical to the fcntl() function, exceptthat the argument handling is even more complicated.

The request parameter is limited to values that can fit in 32-bits.Additional constants of interest for use as the request argument can befound in the termios module, under the same names as used inthe relevant C header files.

The parameter arg can be one of an integer, an object supporting theread-only buffer interface (like bytes) or an object supportingthe read-write buffer interface (like bytearray).

In all but the last case, behaviour is as for the fcntl()function.

If a mutable buffer is passed, then the behaviour is determined by the value ofthe mutate_flag parameter.

If it is false, the buffer's mutability is ignored and behaviour is as for aread-only buffer, except that the 1024 byte limit mentioned above is avoided —so long as the buffer you pass is at least as long as what the operating systemwants to put there, things should work.

If mutate_flag is true (the default), then the buffer is (in effect) passedto the underlying ioctl() system call, the latter's return code ispassed back to the calling Python, and the buffer's new contents reflect theaction of the ioctl(). This is a slight simplification, because if thesupplied buffer is less than 1024 bytes long it is first copied into a staticbuffer 1024 bytes long which is then passed to ioctl() and copied backinto the supplied buffer.

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

举个例子：

>>> 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])

• fcntl.flock(fd, operation)
• Perform the lock operation operation on file descriptor fd (file objects providinga fileno() method are accepted as well). See the Unix manualflock(2)) for details. (On some systems, this function is emulatedusing fcntl().)

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

• 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:

  • LOCK_UN — unlock

  • LOCK_SH — acquire a shared lock

  • LOCK_EX — acquire an exclusive lock

When cmd is LOCK_SH or LOCK_EX, it can also bebitwise ORed with LOCK_NB to avoid blocking on lock acquisition.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 theoperating system; for portability, check for both values). On at least somesystems, LOCK_EX can only be used if the file descriptor refers to afile opened for writing.

len is the number of bytes to lock, start is the byte offset atwhich 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. Thedefault for whence is also 0.

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 aninteger value; in the second example it will hold a bytes object. Thestructure lay-out for the lockdata variable is system dependent —- thereforeusing the flock() call may be better.

参见

• 模块 os
• If the locking flags O_SHLOCK and O_EXLOCK arepresent in the os module (on BSD only), the os.open()function provides an alternative to the lockf() and flock()functions.