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119 lines
4.8 KiB
TeX
119 lines
4.8 KiB
TeX
\section{\module{fcntl} ---
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The \function{fcntl()} and \function{ioctl()} system calls}
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\declaremodule{builtin}{fcntl}
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\platform{Unix}
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\modulesynopsis{The \function{fcntl()} and \function{ioctl()} system calls.}
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\sectionauthor{Jaap Vermeulen}{}
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\indexii{UNIX@\UNIX{}}{file control}
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\indexii{UNIX@\UNIX{}}{I/O control}
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This module performs file control and I/O control on file descriptors.
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It is an interface to the \cfunction{fcntl()} and \cfunction{ioctl()}
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\UNIX{} routines. File descriptors can be obtained with the
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\method{fileno()} method of a file or socket object.
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The module defines the following functions:
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\begin{funcdesc}{fcntl}{fd, op\optional{, arg}}
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Perform the requested operation on file descriptor \var{fd}.
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The operation is defined by \var{op} and is operating system
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dependent. Typically these codes can be retrieved from the library
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module \module{FCNTL}\refstmodindex{FCNTL}. The argument \var{arg}
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is optional, and defaults to the integer value \code{0}. When
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present, it can either be an integer value, or a string. With
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the argument missing or an integer value, the return value of this
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function is the integer return value of the C \cfunction{fcntl()}
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call. When the argument is a string it represents a binary
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structure, e.g.\ created by \function{struct.pack()}. The binary
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data is copied to a buffer whose address is passed to the C
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\cfunction{fcntl()} call. The return value after a successful call
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is the contents of the buffer, converted to a string object. The length
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of the returned string will be the same as the length of the \var{arg}
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argument. This is limited to 1024 bytes. If the information returned
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in the buffer by the operating system is larger than 1024 bytes,
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this is most likely to result in a segmentation violation or a more
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subtle data corruption.
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If the \cfunction{fcntl()} fails, an \exception{IOError} is
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raised.
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\end{funcdesc}
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\begin{funcdesc}{ioctl}{fd, op, arg}
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This function is identical to the \function{fcntl()} function, except
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that the operations are typically defined in the library module
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\module{IOCTL}.
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\end{funcdesc}
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\begin{funcdesc}{flock}{fd, op}
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Perform the lock operation \var{op} on file descriptor \var{fd}.
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See the \UNIX{} manual \manpage{flock}{3} for details. (On some
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systems, this function is emulated using \cfunction{fcntl()}.)
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\end{funcdesc}
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\begin{funcdesc}{lockf}{fd, operation,
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\optional{len, \optional{start, \optional{whence}}}}
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This is essentially a wrapper around the \function{fcntl()} locking
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calls. \var{fd} is the file descriptor of the file to lock or unlock,
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and \var{operation} is one of the following values:
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\begin{itemize}
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\item \constant{LOCK_UN} -- unlock
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\item \constant{LOCK_SH} -- acquire a shared lock
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\item \constant{LOCK_EX} -- acquire an exclusive lock
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\end{itemize}
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When \var{operation} is \constant{LOCK_SH} or \constant{LOCK_EX}, it
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can also be bit-wise OR'd with \constant{LOCK_NB} to avoid blocking on
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lock acquisition. If \constant{LOCK_NB} is used and the lock cannot
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be acquired, an \exception{IOError} will be raised and the exception
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will have an \var{errno} attribute set to \constant{EACCES} or
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\constant{EAGAIN} (depending on the operating system; for portability,
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check for both values).
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\var{length} is the number of bytes to lock, \var{start} is the byte
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offset at which the lock starts, relative to \var{whence}, and
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\var{whence} is as with \function{fileobj.seek()}, specifically:
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\begin{itemize}
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\item \constant{0} -- relative to the start of the file
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(\constant{SEEK_SET})
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\item \constant{1} -- relative to the current buffer position
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(\constant{SEEK_CUR})
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\item \constant{2} -- relative to the end of the file
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(\constant{SEEK_END})
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\end{itemize}
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The default for \var{start} is 0, which means to start at the
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beginning of the file. The default for \var{length} is 0 which means
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to lock to the end of the file. The default for \var{whence} is also
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0.
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\end{funcdesc}
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If the library modules \module{FCNTL}\refstmodindex{FCNTL} or
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\module{IOCTL}\refstmodindex{IOCTL} are missing, you can find the
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opcodes in the C include files \code{<sys/fcntl.h>} and
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\code{<sys/ioctl.h>}. You can create the modules yourself with the
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\program{h2py} script, found in the \file{Tools/scripts/} directory.
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Examples (all on a SVR4 compliant system):
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\begin{verbatim}
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import struct, fcntl, FCNTL
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file = open(...)
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rv = fcntl(file.fileno(), FCNTL.O_NDELAY, 1)
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lockdata = struct.pack('hhllhh', FCNTL.F_WRLCK, 0, 0, 0, 0, 0)
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rv = fcntl.fcntl(file.fileno(), FCNTL.F_SETLKW, lockdata)
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\end{verbatim}
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Note that in the first example the return value variable \code{rv} will
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hold an integer value; in the second example it will hold a string
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value. The structure lay-out for the \var{lockdata} variable is
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system dependent --- therefore using the \function{flock()} call may be
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better.
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