:mod:`!posix` --- The most common POSIX system calls
====================================================

.. module:: posix
   :synopsis: The most common POSIX system calls (normally used via module os).

--------------

This module provides access to operating system functionality that is
standardized by the C Standard and the POSIX standard (a thinly disguised Unix
interface).

.. availability:: Unix.

.. index:: pair: module; os

**Do not import this module directly.**  Instead, import the module :mod:`os`,
which provides a *portable* version of this interface.  On Unix, the :mod:`os`
module provides a superset of the :mod:`!posix` interface.  On non-Unix operating
systems the :mod:`!posix` module is not available, but a subset is always
available through the :mod:`os` interface.  Once :mod:`os` is imported, there is
*no* performance penalty in using it instead of :mod:`!posix`.  In addition,
:mod:`os` provides some additional functionality, such as automatically calling
:func:`~os.putenv` when an entry in ``os.environ`` is changed.

Errors are reported as exceptions; the usual exceptions are given for type
errors, while errors reported by the system calls raise :exc:`OSError`.


.. _posix-large-files:

Large File Support
------------------

.. index::
   single: large files
   single: file; large files

Several operating systems (including AIX and Solaris) provide
support for files that are larger than 2 GiB from a C programming model where
:c:expr:`int` and :c:expr:`long` are 32-bit values. This is typically accomplished
by defining the relevant size and offset types as 64-bit values. Such files are
sometimes referred to as :dfn:`large files`.

Large file support is enabled in Python when the size of an :c:type:`off_t` is
larger than a :c:expr:`long` and the :c:expr:`long long` is at least as large
as an :c:type:`off_t`.
It may be necessary to configure and compile Python with certain compiler flags
to enable this mode. For example, with Solaris 2.6 and 2.7 you need to do
something like::

   CFLAGS="`getconf LFS_CFLAGS`" OPT="-g -O2 $CFLAGS" \
           ./configure

On large-file-capable Linux systems, this might work::

   CFLAGS='-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64' OPT="-g -O2 $CFLAGS" \
           ./configure


.. _posix-contents:

Notable Module Contents
-----------------------

In addition to many functions described in the :mod:`os` module documentation,
:mod:`!posix` defines the following data item:

.. data:: environ

   A dictionary representing the string environment at the time the interpreter
   was started. Keys and values are bytes on Unix and str on Windows. For
   example, ``environ[b'HOME']`` (``environ['HOME']`` on Windows) is the
   pathname of your home directory, equivalent to ``getenv("HOME")`` in C.

   Modifying this dictionary does not affect the string environment passed on by
   :func:`~os.execv`, :func:`~os.popen` or :func:`~os.system`; if you need to
   change the environment, pass ``environ`` to :func:`~os.execve` or add
   variable assignments and export statements to the command string for
   :func:`~os.system` or :func:`~os.popen`.

   .. versionchanged:: 3.2
      On Unix, keys and values are bytes.

   .. note::

      The :mod:`os` module provides an alternate implementation of ``environ``
      which updates the environment on modification. Note also that updating
      :data:`os.environ` will render this dictionary obsolete. Use of the
      :mod:`os` module version of this is recommended over direct access to the
      :mod:`!posix` module.