35.1. 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.

35.1.1. Large File Support

Several operating systems (including AIX, HP-UX, Irix 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 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" \

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


35.1.2. Notable Module Contents

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


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.


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.