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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 % Manual text by Jaap Vermeulen
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2 \section{Built-in Module \sectcode{fcntl}}
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3 \bimodindex{fcntl}
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4 \indexii{\UNIX{}}{file control}
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5 \indexii{\UNIX{}}{I/O control}
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6
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7 This module performs file control and I/O control on file descriptors.
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8 It is an interface to the \dfn{fcntl()} and \dfn{ioctl()} \UNIX{} routines.
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9 File descriptors can be obtained with the \dfn{fileno()} method of a
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10 file or socket object.
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11
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12 The module defines the following functions:
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13
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14 \renewcommand{\indexsubitem}{(in module struct)}
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15
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16 \begin{funcdesc}{fcntl}{fd\, op\optional{\, arg}}
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17 Perform the requested operation on file descriptor \code{\var{fd}}.
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18 The operation is defined by \code{\var{op}} and is operating system
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19 dependent. Typically these codes can be retrieved from the library
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20 module \code{FCNTL}. The argument \code{\var{arg}} is optional, and
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21 defaults to the integer value \code{0}. When
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22 it is present, it can either be an integer value, or a string. With
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23 the argument missing or an integer value, the return value of this
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24 function is the integer return value of the real \code{fcntl()}
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25 call. When the argument is a string it represents a binary
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26 structure, e.g.\ created by \code{struct.pack()}. The binary data is
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27 copied to a buffer whose address is passed to the real \code{fcntl()}
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28 call. The return value after a successful call is the contents of
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29 the buffer, converted to a string object. In case the
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30 \code{fcntl()} fails, an \code{IOError} will be raised.
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31 \end{funcdesc}
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32
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33 \begin{funcdesc}{ioctl}{fd\, op\, arg}
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34 This function is identical to the \code{fcntl()} function, except
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35 that the operations are typically defined in the library module
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36 \code{IOCTL}.
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37 \end{funcdesc}
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38
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39 \begin{funcdesc}{flock}{fd\, op}
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40 Perform the lock operation \var{op} on file descriptor \var{fd}.
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41 See the Unix manual for details. (On some systems, this function is
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42 emulated using \code{fcntl}.)
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43 \end{funcdesc}
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45 \begin{funcdesc}{lockf}{fd\, code\, \optional{len\, \optional{start\, \optional{whence}}}}
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46 This is a wrapper around the \code{F_SETLK} and \code{F_SETLKW}
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47 \code{fcntl()} calls. See the Unix manual for details.
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48 \end{funcdesc}
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49
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50 If the library modules \code{FCNTL} or \code{IOCTL} are missing, you
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51 can find the opcodes in the C include files \code{sys/fcntl} and
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52 \code{sys/ioctl}. You can create the modules yourself with the h2py
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53 script, found in the \code{Tools/scripts} directory.
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54 \stmodindex{FCNTL}
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55 \stmodindex{IOCTL}
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57 Examples (all on a SVR4 compliant system):
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58
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59 \bcode\begin{verbatim}
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60 import struct, FCNTL
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61
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62 file = open(...)
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63 rv = fcntl(file.fileno(), FCNTL.O_NDELAY, 1)
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64
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65 lockdata = struct.pack('hhllhh', FCNTL.F_WRLCK, 0, 0, 0, 0, 0)
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66 rv = fcntl(file.fileno(), FCNTL.F_SETLKW, lockdata)
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67 \end{verbatim}\ecode
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68
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69 Note that in the first example the return value variable \code{rv} will
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70 hold an integer value; in the second example it will hold a string
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71 value. The structure lay-out for the \var{lockadata} variable is
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72 system dependent -- therefore using the \code{flock()} call may be
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73 better.