159 lines
5.7 KiB
TeX
159 lines
5.7 KiB
TeX
% Manual text and implementation by Jaap Vermeulen
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\section{Standard Module \sectcode{posixfile}}
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\bimodindex{posixfile}
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\indexii{posix}{file object}
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\emph{Note:} This module will become obsolete in a future release.
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The locking operation that it provides is done better and more
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portably by the \code{fcntl.lockf()} call.
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This module implements some additional functionality over the built-in
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file objects. In particular, it implements file locking, control over
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the file flags, and an easy interface to duplicate the file object.
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The module defines a new file object, the posixfile object. It
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has all the standard file object methods and adds the methods
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described below. This module only works for certain flavors of
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\UNIX{}, since it uses \code{fcntl()} for file locking.
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To instantiate a posixfile object, use the \code{open()} function in
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the posixfile module. The resulting object looks and feels roughly
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the same as a standard file object.
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The posixfile module defines the following constants:
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\renewcommand{\indexsubitem}{(in module posixfile)}
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\begin{datadesc}{SEEK_SET}
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offset is calculated from the start of the file
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\end{datadesc}
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\begin{datadesc}{SEEK_CUR}
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offset is calculated from the current position in the file
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\end{datadesc}
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\begin{datadesc}{SEEK_END}
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offset is calculated from the end of the file
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\end{datadesc}
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The posixfile module defines the following functions:
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\renewcommand{\indexsubitem}{(in module posixfile)}
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\begin{funcdesc}{open}{filename\optional{\, mode\optional{\, bufsize}}}
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Create a new posixfile object with the given filename and mode. The
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\var{filename}, \var{mode} and \var{bufsize} arguments are
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interpreted the same way as by the built-in \code{open()} function.
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\end{funcdesc}
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\begin{funcdesc}{fileopen}{fileobject}
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Create a new posixfile object with the given standard file object.
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The resulting object has the same filename and mode as the original
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file object.
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\end{funcdesc}
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The posixfile object defines the following additional methods:
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\renewcommand{\indexsubitem}{(posixfile method)}
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\begin{funcdesc}{lock}{fmt\, \optional{len\optional{\, start
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\optional{\, whence}}}}
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Lock the specified section of the file that the file object is
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referring to. The format is explained
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below in a table. The \var{len} argument specifies the length of the
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section that should be locked. The default is \code{0}. \var{start}
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specifies the starting offset of the section, where the default is
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\code{0}. The \var{whence} argument specifies where the offset is
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relative to. It accepts one of the constants \code{SEEK_SET},
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\code{SEEK_CUR} or \code{SEEK_END}. The default is \code{SEEK_SET}.
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For more information about the arguments refer to the fcntl
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manual page on your system.
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\end{funcdesc}
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\begin{funcdesc}{flags}{\optional{flags}}
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Set the specified flags for the file that the file object is referring
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to. The new flags are ORed with the old flags, unless specified
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otherwise. The format is explained below in a table. Without
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the \var{flags} argument
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a string indicating the current flags is returned (this is
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the same as the '?' modifier). For more information about the flags
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refer to the fcntl manual page on your system.
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\end{funcdesc}
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\begin{funcdesc}{dup}{}
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Duplicate the file object and the underlying file pointer and file
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descriptor. The resulting object behaves as if it were newly
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opened.
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\end{funcdesc}
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\begin{funcdesc}{dup2}{fd}
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Duplicate the file object and the underlying file pointer and file
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descriptor. The new object will have the given file descriptor.
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Otherwise the resulting object behaves as if it were newly opened.
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\end{funcdesc}
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\begin{funcdesc}{file}{}
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Return the standard file object that the posixfile object is based
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on. This is sometimes necessary for functions that insist on a
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standard file object.
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\end{funcdesc}
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All methods return \code{IOError} when the request fails.
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Format characters for the \code{lock()} method have the following meaning:
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\begin{tableiii}{|c|l|c|}{samp}{Format}{Meaning}{}
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\lineiii{u}{unlock the specified region}{}
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\lineiii{r}{request a read lock for the specified section}{}
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\lineiii{w}{request a write lock for the specified section}{}
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\end{tableiii}
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In addition the following modifiers can be added to the format:
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\begin{tableiii}{|c|l|c|}{samp}{Modifier}{Meaning}{Notes}
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\lineiii{|}{wait until the lock has been granted}{}
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\lineiii{?}{return the first lock conflicting with the requested lock, or
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\code{None} if there is no conflict.}{(1)}
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\end{tableiii}
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Note:
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(1) The lock returned is in the format \code{(mode, len, start,
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whence, pid)} where mode is a character representing the type of lock
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('r' or 'w'). This modifier prevents a request from being granted; it
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is for query purposes only.
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Format character for the \code{flags()} method have the following meaning:
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\begin{tableiii}{|c|l|c|}{samp}{Format}{Meaning}{}
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\lineiii{a}{append only flag}{}
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\lineiii{c}{close on exec flag}{}
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\lineiii{n}{no delay flag (also called non-blocking flag)}{}
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\lineiii{s}{synchronization flag}{}
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\end{tableiii}
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In addition the following modifiers can be added to the format:
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\begin{tableiii}{|c|l|c|}{samp}{Modifier}{Meaning}{Notes}
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\lineiii{!}{turn the specified flags 'off', instead of the default 'on'}{(1)}
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\lineiii{=}{replace the flags, instead of the default 'OR' operation}{(1)}
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\lineiii{?}{return a string in which the characters represent the flags that
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are set.}{(2)}
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\end{tableiii}
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Note:
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(1) The \code{!} and \code{=} modifiers are mutually exclusive.
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(2) This string represents the flags after they may have been altered
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by the same call.
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Examples:
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\bcode\begin{verbatim}
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from posixfile import *
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file = open('/tmp/test', 'w')
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file.lock('w|')
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...
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file.lock('u')
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file.close()
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\end{verbatim}\ecode
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