fcntl
—
fcntl
and
ioctl
系统调用
¶
This module performs file control and I/O control on file descriptors. It is an interface to the
fcntl()
and
ioctl()
Unix routines. For a complete description of these calls, see
fcntl(2)
and
ioctl(2)
Unix manual pages.
All functions in this module take a file descriptor
fd
as their first argument. This can be an integer file descriptor, such as returned by
sys.stdin.fileno()
, or an
io.IOBase
object, such as
sys.stdin
itself, which provides a
fileno()
that returns a genuine file descriptor.
模块定义了下列函数：
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, or a
bytes
object. With an integer value, the return value of this function is the integer return value of the C
fcntl()
call. When the argument is bytes it represents a binary structure, e.g. created by
struct.pack()
. 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 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.
若
fcntl()
fails, an
OSError
被引发。
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.
request
parameter is limited to values that can fit in 32-bits. 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 one of an integer, an object supporting the read-only buffer interface (like
bytes
) or an object supporting the read-write buffer interface (like
bytearray
).
In all but the last case, behaviour is as for the
fcntl()
函数。
If a mutable buffer is passed, then the behaviour is determined by the value of the mutate_flag 参数。
If it is false, the buffer’s mutability is ignored and behaviour is as for a read-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 system wants to put there, things should work.
若
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.
若
ioctl()
fails, an
OSError
异常被引发。
范例：
>>> 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 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()
.)
若
flock()
fails, an
OSError
异常被引发。
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 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
当
cmd
is
LOCK_SH
or
LOCK_EX
, it can also be bitwise ORed with
LOCK_NB
to avoid blocking on lock acquisition. If
LOCK_NB
is used and the lock cannot be acquired, an
OSError
will be raised and the exception will have an
errno
属性设置为
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
，和
whence
is as with
io.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
)
默认为 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.
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.