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--------------
9
10This module provides access to operating system functionality that is
11standardized by the C Standard and the POSIX standard (a thinly disguised Unix
12interface).
13
14.. index:: module: os
15
16**Do not import this module directly.**  Instead, import the module :mod:`os`,
17which provides a *portable* version of this interface.  On Unix, the :mod:`os`
18module provides a superset of the :mod:`posix` interface.  On non-Unix operating
19systems the :mod:`posix` module is not available, but a subset is always
20available through the :mod:`os` interface.  Once :mod:`os` is imported, there is
21*no* performance penalty in using it instead of :mod:`posix`.  In addition,
22:mod:`os` provides some additional functionality, such as automatically calling
23:func:`~os.putenv` when an entry in ``os.environ`` is changed.
24
25Errors are reported as exceptions; the usual exceptions are given for type
26errors, while errors reported by the system calls raise :exc:`OSError`.
27
28
29.. _posix-large-files:
30
31Large File Support
32------------------
33
34.. index::
35   single: large files
36   single: file; large files
37
38.. sectionauthor:: Steve Clift <clift@mail.anacapa.net>
39
40Several operating systems (including AIX, HP-UX, Irix and Solaris) provide
41support for files that are larger than 2 GiB from a C programming model where
42:c:type:`int` and :c:type:`long` are 32-bit values. This is typically accomplished
43by defining the relevant size and offset types as 64-bit values. Such files are
44sometimes referred to as :dfn:`large files`.
45
46Large file support is enabled in Python when the size of an :c:type:`off_t` is
47larger than a :c:type:`long` and the :c:type:`long long` is at least as large
48as an :c:type:`off_t`.
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. Keys and values are bytes on Unix and str on Windows. For
74   example, ``environ[b'HOME']`` (``environ['HOME']`` on Windows) is the
75   pathname of your home directory, equivalent to ``getenv("HOME")`` in C.
76
77   Modifying this dictionary does not affect the string environment passed on by
78   :func:`~os.execv`, :func:`~os.popen` or :func:`~os.system`; if you need to
79   change the environment, pass ``environ`` to :func:`~os.execve` or add
80   variable assignments and export statements to the command string for
81   :func:`~os.system` or :func:`~os.popen`.
82
83   .. versionchanged:: 3.2
84      On Unix, keys and values are bytes.
85
86   .. note::
87
88      The :mod:`os` module provides an alternate implementation of ``environ``
89      which updates the environment on modification. Note also that updating
90      :data:`os.environ` will render this dictionary obsolete. Use of the
91      :mod:`os` module version of this is recommended over direct access to the
92      :mod:`posix` module.
93