# `posix` — The most common POSIX system calls

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 and Solaris) provide support for files that are larger than 2 GiB 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 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, 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.