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175 lines
7.0 KiB
TeX
175 lines
7.0 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.
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All functions in this module take a file descriptor \var{fd} as their
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first argument. This can be an integer file descriptor, such as
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returned by \code{sys.stdin.fileno()}, or a file object, such as
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\code{sys.stdin} itself, which provides a \method{fileno()} which
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returns a genuine file descriptor.
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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} (file
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objects providing a \method{fileno()} method are accepted as well).
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The operation is defined by \var{op} and is operating system
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dependent. These codes are also found in the \module{fcntl}
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module. The argument \var{arg} is optional, and defaults to the
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integer value \code{0}. When present, it can either be an integer
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value, or a string. With the argument missing or an integer value,
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the return value of this function is the integer return value of the
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C \cfunction{fcntl()} call. When the argument is a string it
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represents a binary structure, e.g.\ created by
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\function{\refmodule{struct}.pack()}. The binary data is copied to a buffer
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whose address is passed to the C \cfunction{fcntl()} call. The
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return value after a successful call is the contents of the buffer,
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converted to a string object. The length of the returned string
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will be the same as the length of the \var{arg} argument. This is
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limited to 1024 bytes. If the information returned in the buffer by
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the operating system is larger than 1024 bytes, this is most likely
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to result in a segmentation violation or a more subtle data
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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\optional{, arg\optional{, mutate_flag}}}
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This function is identical to the \function{fcntl()} function,
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except that the operations are typically defined in the library
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module \refmodule{termios} and the argument handling is even more
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complicated.
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The parameter \var{arg} can be one of an integer, absent (treated
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identically to the integer \code{0}), an object supporting the
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read-only buffer interface (most likely a plain Python string) or an
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object supporting the read-write buffer interface.
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In all but the last case, behaviour is as for the \function{fcntl()}
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function.
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If a mutable buffer is passed, then the behaviour is determined by
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the value of the \var{mutate_flag} parameter.
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If it is false, the buffer's mutability is ignored and behaviour is
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as for a read-only buffer, except that the 1024 byte limit mentioned
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above is avoided -- so long as the buffer you pass is as least as
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long as what the operating system wants to put there, things should
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work.
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If \var{mutate_flag} is true, then the buffer is (in effect) passed
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to the underlying \function{ioctl()} system call, the latter's
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return code is passed back to the calling Python, and the buffer's
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new contents reflect the action of the \function{ioctl()}. This is a
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slight simplification, because if the supplied buffer is less than
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1024 bytes long it is first copied into a static buffer 1024 bytes
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long which is then passed to \function{ioctl()} and copied back into
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the supplied buffer.
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If \var{mutate_flag} is not supplied, then from Python 2.5 it
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defaults to true, which is a change from versions 2.3 and 2.4.
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Supply the argument explicitly if version portability is a priority.
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An example:
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\begin{verbatim}
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>>> import array, fcntl, struct, termios, os
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>>> os.getpgrp()
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13341
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>>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, " "))[0]
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13341
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>>> buf = array.array('h', [0])
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>>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
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0
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>>> buf
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array('h', [13341])
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\end{verbatim}
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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} (file
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objects providing a \method{fileno()} method are accepted as well).
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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{length, \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). On at least some systems, \constant{LOCK_EX}
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can only be used if the file descriptor refers to a file opened for
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writing.
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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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Examples (all on a SVR4 compliant system):
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\begin{verbatim}
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import struct, fcntl, os
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f = open(...)
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rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)
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lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
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rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
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\end{verbatim}
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Note that in the first example the return value variable \var{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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\begin{seealso}
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\seemodule{os}{If the locking flags \constant{O_SHLOCK} and
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\constant{O_EXLOCK} are present in the \module{os} module,
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the \function{os.open()} function provides a more
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platform-independent alternative to the \function{lockf()}
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and \function{flock()} functions.}
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\end{seealso}
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