/Doc/library/posix.rst

 1:mod:`posix` --- The most common POSIX system calls
 2===================================================
 3
 4.. module:: posix
 5   :platform: Unix
 6   :synopsis: The most common POSIX system calls (normally used via module os).
 7
 8
 9This module provides access to operating system functionality that is
10standardized by the C Standard and the POSIX standard (a thinly disguised Unix
11interface).
12
13.. index:: module: os
14
15**Do not import this module directly.**  Instead, import the module :mod:`os`,
16which provides a *portable* version of this interface.  On Unix, the :mod:`os`
17module provides a superset of the :mod:`posix` interface.  On non-Unix operating
18systems the :mod:`posix` module is not available, but a subset is always
19available through the :mod:`os` interface.  Once :mod:`os` is imported, there is
20*no* performance penalty in using it instead of :mod:`posix`.  In addition,
21:mod:`os` provides some additional functionality, such as automatically calling
22:func:`putenv` when an entry in ``os.environ`` is changed.
23
24Errors are reported as exceptions; the usual exceptions are given for type
25errors, while errors reported by the system calls raise :exc:`OSError`.
26
27
28.. _posix-large-files:
29
30Large File Support
31------------------
32
33.. index::
34   single: large files
35   single: file; large files
36
37.. sectionauthor:: Steve Clift <clift@mail.anacapa.net>
38
39Several operating systems (including AIX, HP-UX, Irix and Solaris) provide
40support for files that are larger than 2 GB from a C programming model where
41:ctype:`int` and :ctype:`long` are 32-bit values. This is typically accomplished
42by defining the relevant size and offset types as 64-bit values. Such files are
43sometimes referred to as :dfn:`large files`.
44
45Large file support is enabled in Python when the size of an :ctype:`off_t` is
46larger than a :ctype:`long` and the :ctype:`long long` type is available and is
47at least as large as an :ctype:`off_t`. Python longs are then used to represent
48file sizes, offsets and other values that can exceed the range of a Python int.
49It may be necessary to configure and compile Python with certain compiler flags
50to enable this mode. For example, it is enabled by default with recent versions
51of Irix, but with Solaris 2.6 and 2.7 you need to do something like::
52
53   CFLAGS="`getconf LFS_CFLAGS`" OPT="-g -O2 $CFLAGS" \
54           ./configure
55
56On large-file-capable Linux systems, this might work::
57
58   CFLAGS='-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64' OPT="-g -O2 $CFLAGS" \
59           ./configure
60
61
62.. _posix-contents:
63
64Notable Module Contents
65-----------------------
66
67In addition to many functions described in the :mod:`os` module documentation,
68:mod:`posix` defines the following data item:
69
70.. data:: environ
71
72   A dictionary representing the string environment at the time the interpreter
73   was started.  For example, ``environ['HOME']`` is the pathname of your home
74   directory, equivalent to ``getenv("HOME")`` in C.
75
76   Modifying this dictionary does not affect the string environment passed on by
77   :func:`execv`, :func:`popen` or :func:`system`; if you need to change the
78   environment, pass ``environ`` to :func:`execve` or add variable assignments and
79   export statements to the command string for :func:`system` or :func:`popen`.
80
81   .. note::
82
83      The :mod:`os` module provides an alternate implementation of ``environ`` which
84      updates the environment on modification.  Note also that updating ``os.environ``
85      will render this dictionary obsolete.  Use of the :mod:`os` module version of
86      this is recommended over direct access to the :mod:`posix` module.