# 8.1 Built-in Module `posix`

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 is `os.environ` is changed.

The descriptions below are very terse; refer to the corresponding Unix manual (or POSIX documentation) entry for more information. Arguments called path refer to a pathname given as a string.

Errors are reported as exceptions; the usual exceptions are given for type errors, while errors reported by the system calls raise `posix.error`, described below.

Module `posix` defines the following data items:

`environ`
  A dictionary representing the string environment at the time the interpreter was started. For example, `posix.environ['HOME']` 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()`.

  However: If you are using this module via the `os` module (as you should - see the introduction above), `environ` is a a mapping object that behaves almost like a dictionary but invokes `putenv()` automatically called whenever an item is changed.

`error`
  This exception is raised when a POSIX function returns a POSIX-related error (e.g., not for illegal argument types). Its string value is `'posix.error'`. The accompanying value is a pair containing the numeric error code from `errno` and the corresponding string, as would be printed by the C function `perror()`.

It defines the following functions and constants:

`chdir` (path)
  Change the current working directory to path.

`chmod` (path, mode)
  Change the mode of path to the numeric mode.

`chown` (path, uid, gid)
  Change the owner and group id of path to the numeric uid and gid. (Not on MS-DOS.)

`close` (fd)
  Close file descriptor fd.

  Note: this function is intended for low-level I/O and must be applied to a file descriptor as returned by `posix.open()` or `posix.pipe()`. To close a ``file object'' returned by the built-in function `open` or by `posix.popen` or `posix.fdopen`, use its `close()` method.

`dup` (fd)
  Return a duplicate of file descriptor fd.

`dup2` (fd, fd2)
  Duplicate file descriptor fd to fd2, closing the latter first if necessary. Return `None`.

`execv` (path, args)
  Execute the executable path with argument list args, replacing the current process (i.e., the Python interpreter). The argument list may be a tuple or list of strings. (Not on MS-DOS.)

`execve` (path, args, env)
  Execute the executable path with argument list args, and environment env, replacing the current process (i.e., the Python interpreter). The argument list may be a tuple or list of strings. The environment must be a dictionary mapping strings to strings. (Not on MS-DOS.)

`_exit` (n)
  Exit to the system with status n, without calling cleanup handlers, flushing stdio buffers, etc. (Not on MS-DOS.)

  Note: the standard way to exit is `sys.exit(n)`. `posix._exit()` should normally only be used in the child process after a `fork()`.

`fdopen` (fd[, mode[, bufsize]])
  Return an open file object connected to the file descriptor fd. The mode and bufsize arguments have the same meaning as the corresponding arguments to the built-in `open()` function.

`fork` ()
  Fork a child process. Return 0 in the child, the child's process id in the parent. (Not on MS-DOS.)

`fstat` (fd)
  Return status for file descriptor fd, like `stat()`.

`ftruncate` (fd, length)
  Truncate the file corresponding to file descriptor fd, so that it is at most length bytes in size.

`getcwd` ()
  Return a string representing the current working directory.

`getegid` ()
  Return the current process' effective group id. (Not on MS-DOS.)

`geteuid` ()
  Return the current process' effective user id. (Not on MS-DOS.)

`getgid` ()
  Return the current process' group id. (Not on MS-DOS.)

`getpgrp` ()
  Return the current process group id. (Not on MS-DOS.)

`getpid` ()
  Return the current process id. (Not on MS-DOS.)

`getppid` ()
  Return the parent's process id. (Not on MS-DOS.)

`getuid` ()
  Return the current process' user id. (Not on MS-DOS.)

`kill` (pid, sig)
  Kill the process pid with signal sig. (Not on MS-DOS.)

`link` (src, dst)
  Create a hard link pointing to src named dst. (Not on MS-DOS.)

`listdir` (path)
  Return a list containing the names of the entries in the directory. The list is in arbitrary order. It does not include the special entries `'.'` and `'..'` even if they are present in the directory.

`lseek` (fd, pos, how)
  Set the current position of file descriptor fd to position pos, modified by how: 0 to set the position relative to the beginning of the file; 1 to set it relative to the current position; 2 to set it relative to the end of the file.

`lstat` (path)
  Like `stat()`, but do not follow symbolic links. (On systems without symbolic links, this is identical to `posix.stat()`.)

`mkfifo` (path[, mode])
  Create a FIFO (a POSIX named pipe) named path with numeric mode mode. The default mode is 0666 (octal). The current umask value is first masked out from the mode. (Not on MS-DOS.)

  FIFOs are pipes that can be accessed like regular files. FIFOs exist until they are deleted (for example with `os.unlink`). Generally, FIFOs are used as rendez-vous between ``client'' and ``server'' type processes: the server opens the FIFO for reading, and the client opens it for writing. Note that `mkfifo()` doesn't open the FIFO - it just creates the rendez-vous point.

`mkdir` (path[, mode])
  Create a directory named path with numeric mode mode. The default mode is 0777 (octal). On some systems, mode is ignored. Where it is used, the current umask value is first masked out.

`nice` (increment)
  Add incr to the process' ``niceness''. Return the new niceness. (Not on MS-DOS.)

`open` (file, flags[, mode])
  Open the file file and set various flags according to flags and possibly its mode according to mode. The default mode is 0777 (octal), and the current umask value is first masked out. Return the file descriptor for the newly opened file.

  For a description of the flag and mode values, see the Unix or C run-time documentation; flag constants (like `O_RDONLY` and `O_WRONLY`) are defined in this module too (see below).

  Note: this function is intended for low-level I/O. For normal usage, use the built-in function `open`, which returns a ``file object'' with `read()` and `write()` methods (and many more).

`pipe` ()
  Create a pipe. Return a pair of file descriptors `(r, w)` usable for reading and writing, respectively. (Not on MS-DOS.)

`plock` (op)
  Lock program segments into memory. The value of op (defined in `<sys/lock.h>`) determines which segments are locked. (Not on MS-DOS.)

`popen` (command[, mode[, bufsize]])
  Open a pipe to or from command. The return value is an open file object connected to the pipe, which can be read or written depending on whether mode is `'r'` (default) or `'w'`. The bufsize argument has the same meaning as the corresponding argument to the built-in `open()` function. The exit status of the command (encoded in the format specified for `wait()`) is available as the return value of the `close()` method of the file object. (Not on MS-DOS.)

`putenv` (varname, value)
  Set the environment variable named varname to the string value. Such changes to the environment affect subprocesses started with `os.system()`, `os.popen()` or `os.fork()` and `os.execv()`. (Not on all systems.)

  When `putenv()` is supported, assignments to items in `os.environ` are automatically translated into corresponding calls to `os.putenv()`; however, calls to `os.putenv()` don't update `os.environ`, so it is actually preferable to assign to items of `os.environ`.

`strerror` (code)
  Return the error message corresponding to the error code in code.

`read` (fd, n)
  Read at most n bytes from file descriptor fd. Return a string containing the bytes read.

  Note: this function is intended for low-level I/O and must be applied to a file descriptor as returned by `posix.open()` or `posix.pipe()`. To read a ``file object'' returned by the built-in function `open` or by `posix.popen` or `posix.fdopen`, or `sys.stdin`, use its `read()` or `readline()` methods.

`readlink` (path)
  Return a string representing the path to which the symbolic link points. (On systems without symbolic links, this always raises `posix.error`.)

`remove` (path)
  Remove the file path. See `rmdir` below to remove a directory. This is identical to the `unlink` function documented below.

`rename` (src, dst)
  Rename the file or directory src to dst.

`rmdir` (path)
  Remove the directory path.

`setgid` (gid)
  Set the current process' group id. (Not on MS-DOS.)

`setpgrp` ()
  Calls the system call `setpgrp()` or `setpgrp(0, 0)` depending on which version is implemented (if any). See the Unix manual for the semantics. (Not on MS-DOS.)

`setpgid` (pid, pgrp)
  Calls the system call `setpgid()`. See the Unix manual for the semantics. (Not on MS-DOS.)

`setsid` ()
  Calls the system call `setsid()`. See the Unix manual for the semantics. (Not on MS-DOS.)

`setuid` (uid)
  Set the current process' user id. (Not on MS-DOS.)

`stat` (path)
  Perform a stat system call on the given path. The return value is a tuple of at least 10 integers giving the most important (and portable) members of the stat structure, in the order `st_mode`, `st_ino`, `st_dev`, `st_nlink`, `st_uid`, `st_gid`, `st_size`, `st_atime`, `st_mtime`, `st_ctime`. More items may be added at the end by some implementations. (On MS-DOS, some items are filled with dummy values.)

  Note: The standard module `stat` defines functions and constants that are useful for extracting information from a stat structure.

`symlink` (src, dst)
  Create a symbolic link pointing to src named dst. (On systems without symbolic links, this always raises `posix.error`.)

`system` (command)
  Execute the command (a string) in a subshell. This is implemented by calling the Standard C function `system()`, and has the same limitations. Changes to `posix.environ`, `sys.stdin` etc. are not reflected in the environment of the executed command. The return value is the exit status of the process encoded in the format specified for `wait()`.

`tcgetpgrp` (fd)
  Return the process group associated with the terminal given by fd (an open file descriptor as returned by `posix.open()`). (Not on MS-DOS.)

`tcsetpgrp` (fd, pg)
  Set the process group associated with the terminal given by fd (an open file descriptor as returned by `posix.open()`) to pg. (Not on MS-DOS.)

`times` ()
  Return a 5-tuple of floating point numbers indicating accumulated (CPU or other) times, in seconds. The items are: user time, system time, children's user time, children's system time, and elapsed real time since a fixed point in the past, in that order. See the Unix manual page times(2). (Not on MS-DOS.)

`umask` (mask)
  Set the current numeric umask and returns the previous umask. (Not on MS-DOS.)

`uname` ()
  Return a 5-tuple containing information identifying the current operating system. The tuple contains 5 strings: `(sysname, nodename, release, version, machine)`. Some systems truncate the nodename to 8 characters or to the leading component; a better way to get the hostname is `socket.gethostname()` or even `socket.gethostbyaddr(socket.gethostname())`. (Not on MS-DOS, nor on older Unix systems.)

`unlink` (path)
  Remove the file path. This is the same function as `remove`; the `unlink` name is its traditional Unix name.

`utime` (path, (atime, mtime))
  Set the access and modified time of the file to the given values. (The second argument is a tuple of two items.)

`wait` ()
  Wait for completion of a child process, and return a tuple containing its pid and exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced. (Not on MS-DOS.)

`waitpid` (pid, options)
  Wait for completion of a child process given by proces id, and return a tuple containing its pid and exit status indication (encoded as for `wait()`). The semantics of the call are affected by the value of the integer options, which should be 0 for normal operation. (If the system does not support `waitpid()`, this always raises `posix.error`. Not on MS-DOS.)

`write` (fd, str)
  Write the string str to file descriptor fd. Return the number of bytes actually written.

  Note: this function is intended for low-level I/O and must be applied to a file descriptor as returned by `posix.open()` or `posix.pipe()`. To write a ``file object'' returned by the built-in function `open` or by `posix.popen` or `posix.fdopen`, or `sys.stdout` or `sys.stderr`, use its `write()` method.

`WNOHANG`
  The option for `waitpid()` to avoid hanging if no child process status is available immediately.

`O_RDONLY`

`O_WRONLY`

`O_RDWR`

`O_NDELAY`

`O_NONBLOCK`

`O_APPEND`

`O_DSYNC`

`O_RSYNC`

`O_SYNC`

`O_NOCTTY`

`O_CREAT`

`O_EXCL`

`O_TRUNC`
  Options for the `flag` argument to the `open()` function. These can be bit-wise OR'd together.