: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.