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120 lines
4.9 KiB
TeX
120 lines
4.9 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{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, 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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\refmodule{termios}.
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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{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). 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
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file = open(...)
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rv = fcntl(file, 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(file, 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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