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