1:mod:`fcntl` --- The ``fcntl`` and ``ioctl`` system calls 2========================================================= 3 4.. module:: fcntl 5 :platform: Unix 6 :synopsis: The fcntl() and ioctl() system calls. 7 8.. sectionauthor:: Jaap Vermeulen 9 10.. index:: 11 pair: UNIX; file control 12 pair: UNIX; I/O control 13 14---------------- 15 16This module performs file control and I/O control on file descriptors. It is an 17interface to the :c:func:`fcntl` and :c:func:`ioctl` Unix routines. For a 18complete description of these calls, see :manpage:`fcntl(2)` and 19:manpage:`ioctl(2)` Unix manual pages. 20 21All functions in this module take a file descriptor *fd* as their first 22argument. This can be an integer file descriptor, such as returned by 23``sys.stdin.fileno()``, or an :class:`io.IOBase` object, such as ``sys.stdin`` 24itself, which provides a :meth:`~io.IOBase.fileno` that returns a genuine file 25descriptor. 26 27.. versionchanged:: 3.3 28 Operations in this module used to raise an :exc:`IOError` where they now 29 raise an :exc:`OSError`. 30 31.. versionchanged:: 3.8 32 The fcntl module now contains ``F_ADD_SEALS``, ``F_GET_SEALS``, and 33 ``F_SEAL_*`` constants for sealing of :func:`os.memfd_create` file 34 descriptors. 35 36.. versionchanged:: 3.9 37 On macOS, the fcntl module exposes the ``F_GETPATH`` constant, which obtains 38 the path of a file from a file descriptor. 39 On Linux(>=3.15), the fcntl module exposes the ``F_OFD_GETLK``, ``F_OFD_SETLK`` 40 and ``F_OFD_SETLKW`` constants, which working with open file description locks. 41 42The module defines the following functions: 43 44 45.. function:: fcntl(fd, cmd, arg=0) 46 47 Perform the operation *cmd* on file descriptor *fd* (file objects providing 48 a :meth:`~io.IOBase.fileno` method are accepted as well). The values used 49 for *cmd* are operating system dependent, and are available as constants 50 in the :mod:`fcntl` module, using the same names as used in the relevant C 51 header files. The argument *arg* can either be an integer value, or a 52 :class:`bytes` object. With an integer value, the return value of this 53 function is the integer return value of the C :c:func:`fcntl` call. When 54 the argument is bytes it represents a binary structure, e.g. created by 55 :func:`struct.pack`. The binary data is copied to a buffer whose address is 56 passed to the C :c:func:`fcntl` call. The return value after a successful 57 call is the contents of the buffer, converted to a :class:`bytes` object. 58 The length of the returned object will be the same as the length of the 59 *arg* argument. This is limited to 1024 bytes. If the information returned 60 in the buffer by the operating system is larger than 1024 bytes, this is 61 most likely to result in a segmentation violation or a more subtle data 62 corruption. 63 64 If the :c:func:`fcntl` fails, an :exc:`OSError` is raised. 65 66 .. audit-event:: fcntl.fcntl fd,cmd,arg fcntl.fcntl 67 68 69.. function:: ioctl(fd, request, arg=0, mutate_flag=True) 70 71 This function is identical to the :func:`~fcntl.fcntl` function, except 72 that the argument handling is even more complicated. 73 74 The *request* parameter is limited to values that can fit in 32-bits. 75 Additional constants of interest for use as the *request* argument can be 76 found in the :mod:`termios` module, under the same names as used in 77 the relevant C header files. 78 79 The parameter *arg* can be one of an integer, an object supporting the 80 read-only buffer interface (like :class:`bytes`) or an object supporting 81 the read-write buffer interface (like :class:`bytearray`). 82 83 In all but the last case, behaviour is as for the :func:`~fcntl.fcntl` 84 function. 85 86 If a mutable buffer is passed, then the behaviour is determined by the value of 87 the *mutate_flag* parameter. 88 89 If it is false, the buffer's mutability is ignored and behaviour is as for a 90 read-only buffer, except that the 1024 byte limit mentioned above is avoided -- 91 so long as the buffer you pass is at least as long as what the operating system 92 wants to put there, things should work. 93 94 If *mutate_flag* is true (the default), then the buffer is (in effect) passed 95 to the underlying :func:`ioctl` system call, the latter's return code is 96 passed back to the calling Python, and the buffer's new contents reflect the 97 action of the :func:`ioctl`. This is a slight simplification, because if the 98 supplied buffer is less than 1024 bytes long it is first copied into a static 99 buffer 1024 bytes long which is then passed to :func:`ioctl` and copied back 100 into the supplied buffer. 101 102 If the :c:func:`ioctl` fails, an :exc:`OSError` exception is raised. 103 104 An example:: 105 106 >>> import array, fcntl, struct, termios, os 107 >>> os.getpgrp() 108 13341 109 >>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, " "))[0] 110 13341 111 >>> buf = array.array('h', [0]) 112 >>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1) 113 0 114 >>> buf 115 array('h', [13341]) 116 117 .. audit-event:: fcntl.ioctl fd,request,arg fcntl.ioctl 118 119 120.. function:: flock(fd, operation) 121 122 Perform the lock operation *operation* on file descriptor *fd* (file objects providing 123 a :meth:`~io.IOBase.fileno` method are accepted as well). See the Unix manual 124 :manpage:`flock(2)` for details. (On some systems, this function is emulated 125 using :c:func:`fcntl`.) 126 127 If the :c:func:`flock` fails, an :exc:`OSError` exception is raised. 128 129 .. audit-event:: fcntl.flock fd,operation fcntl.flock 130 131 132.. function:: lockf(fd, cmd, len=0, start=0, whence=0) 133 134 This is essentially a wrapper around the :func:`~fcntl.fcntl` locking calls. 135 *fd* is the file descriptor (file objects providing a :meth:`~io.IOBase.fileno` 136 method are accepted as well) of the file to lock or unlock, and *cmd* 137 is one of the following values: 138 139 * :const:`LOCK_UN` -- unlock 140 * :const:`LOCK_SH` -- acquire a shared lock 141 * :const:`LOCK_EX` -- acquire an exclusive lock 142 143 When *cmd* is :const:`LOCK_SH` or :const:`LOCK_EX`, it can also be 144 bitwise ORed with :const:`LOCK_NB` to avoid blocking on lock acquisition. 145 If :const:`LOCK_NB` is used and the lock cannot be acquired, an 146 :exc:`OSError` will be raised and the exception will have an *errno* 147 attribute set to :const:`EACCES` or :const:`EAGAIN` (depending on the 148 operating system; for portability, check for both values). On at least some 149 systems, :const:`LOCK_EX` can only be used if the file descriptor refers to a 150 file opened for writing. 151 152 *len* is the number of bytes to lock, *start* is the byte offset at 153 which the lock starts, relative to *whence*, and *whence* is as with 154 :func:`io.IOBase.seek`, specifically: 155 156 * :const:`0` -- relative to the start of the file (:data:`os.SEEK_SET`) 157 * :const:`1` -- relative to the current buffer position (:data:`os.SEEK_CUR`) 158 * :const:`2` -- relative to the end of the file (:data:`os.SEEK_END`) 159 160 The default for *start* is 0, which means to start at the beginning of the file. 161 The default for *len* is 0 which means to lock to the end of the file. The 162 default for *whence* is also 0. 163 164 .. audit-event:: fcntl.lockf fd,cmd,len,start,whence fcntl.lockf 165 166Examples (all on a SVR4 compliant system):: 167 168 import struct, fcntl, os 169 170 f = open(...) 171 rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY) 172 173 lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0) 174 rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata) 175 176Note that in the first example the return value variable *rv* will hold an 177integer value; in the second example it will hold a :class:`bytes` object. The 178structure lay-out for the *lockdata* variable is system dependent --- therefore 179using the :func:`flock` call may be better. 180 181 182.. seealso:: 183 184 Module :mod:`os` 185 If the locking flags :data:`~os.O_SHLOCK` and :data:`~os.O_EXLOCK` are 186 present in the :mod:`os` module (on BSD only), the :func:`os.open` 187 function provides an alternative to the :func:`lockf` and :func:`flock` 188 functions. 189