
``posix`` --- The most common POSIX system calls
************************************************

*Platforms: *Unix

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

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


Large File Support
==================

Several operating systems (including AIX, HP-UX, Irix and Solaris)
provide support for files that are larger than 2 GB from a C
programming model where ``int`` and ``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 *large
files*.

Large file support is enabled in Python when the size of an ``off_t``
is larger than a ``long`` and the ``long long`` type is available and
is at least as large as an ``off_t``. It may be necessary to configure
and compile Python with certain compiler flags to enable this mode.
For example, it is enabled by default with recent versions of Irix,
but 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


Notable Module Contents
=======================

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

posix.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 ``execv()``, ``popen()`` or ``system()``; if you need
   to change the environment, pass ``environ`` to ``execve()`` or add
   variable assignments and export statements to the command string
   for ``system()`` or ``popen()``.

   Changed in version 3.2: On Unix, keys and values are bytes.

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