Doc/lib/libos.tex

\section{\module{os} ---
         Miscellaneous OS interfaces}

\declaremodule{standard}{os}
\modulesynopsis{Miscellaneous OS interfaces.}


This module provides a more portable way of using operating system
(OS) dependent functionality than importing an OS dependent built-in
module like \refmodule{posix} or \module{nt}.

This module searches for an OS dependent built-in module like
\module{mac} or \refmodule{posix} and exports the same functions and data
as found there.  The design of all Python's built-in OS dependent
modules is such that as long as the same functionality is available,
it uses the same interface; e.g., the function
\code{os.stat(\var{path})} returns stat information about \var{path} in
the same format (which happens to have originated with the
\POSIX{} interface).

Extensions peculiar to a particular OS are also available through the
\module{os} module, but using them is of course a threat to
portability!

Note that after the first time \module{os} is imported, there is
\emph{no} performance penalty in using functions from \module{os}
instead of directly from the OS dependent built-in module, so there
should be \emph{no} reason not to use \module{os}!


% Frank Stajano <fstajano@uk.research.att.com> complained that it
% wasn't clear that the entries described in the subsections were all
% available at the module level (most uses of subsections are
% different); I think this is only a problem for the HTML version,
% where the relationship may not be as clear.
%
\ifhtml
The \module{os} module contains many functions and data values.
The items below and in the following sub-sections are all available
directly from the \module{os} module.
\fi


\begin{excdesc}{error}
This exception is raised when a function returns a
system-related error (e.g., not for illegal argument types).  This is
also known as the built-in exception \exception{OSError}.  The
accompanying value is a pair containing the numeric error code from
\cdata{errno} and the corresponding string, as would be printed by the
C function \cfunction{perror()}.  See the module
\refmodule{errno}\refbimodindex{errno}, which contains names for the
error codes defined by the underlying operating system.

When exceptions are classes, this exception carries two attributes,
\member{errno} and \member{strerror}.  The first holds the value of
the C \cdata{errno} variable, and the latter holds the corresponding
error message from \cfunction{strerror()}.  For exceptions that
involve a file system path (e.g. \function{chdir()} or
\function{unlink()}), the exception instance will contain a third
attribute, \member{filename}, which is the file name passed to the
function.

When exceptions are strings, the string for the exception is
\code{'OSError'}.
\end{excdesc}

\begin{datadesc}{name}
The name of the OS dependent module imported.  The following names
have currently been registered: \code{'posix'}, \code{'nt'},
\code{'dos'}, \code{'mac'}, \code{'os2'}, \code{'ce'}.
\end{datadesc}

\begin{datadesc}{path}
The corresponding OS dependent standard module for pathname
operations, e.g., \module{posixpath} or \module{macpath}.  Thus, given
the proper imports, \code{os.path.split(\var{file})} is equivalent to but
more portable than \code{posixpath.split(\var{file})}.  Note that this 
is also a valid module: it may be imported directly as
\refmodule{os.path}.
\end{datadesc}



\subsection{Process Parameters \label{os-procinfo}}

These functions and data items provide information and operate on the
current process and user.

\begin{datadesc}{environ}
A mapping object representing the string environment. For example,
\code{environ['HOME']} is the pathname of your home directory (on some
platforms), and is equivalent to \code{getenv("HOME")} in C.

If the platform supports the \function{putenv()} function, this
mapping may be used to modify the environment as well as query the
environment.  \function{putenv()} will be called automatically when
the mapping is modified.

If \function{putenv()} is not provided, this mapping may be passed to
the appropriate process-creation functions to cause child processes to 
use a modified environment.
\end{datadesc}

\begin{funcdescni}{chdir}{path}
\funclineni{getcwd}{}
These functions are described in ``Files and Directories'' (section
\ref{os-file-dir}).
\end{funcdescni}

\begin{funcdesc}{getegid}{}
Return the current process' effective group id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{geteuid}{}
\index{user!effective id}
Return the current process' effective user id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{getgid}{}
\index{process!group}
Return the current process' group id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{getpgrp}{}
\index{process!group}
Return the current process group id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{getpid}{}
\index{process!id}
Return the current process id.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{getppid}{}
\index{process!id of parent}
Return the parent's process id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{getuid}{}
\index{user!id}
Return the current process' user id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{putenv}{varname, value}
\index{environment variables!setting}
Set the environment variable named \var{varname} to the string
\var{value}.  Such changes to the environment affect subprocesses
started with \function{os.system()}, \function{popen()} or
\function{fork()} and \function{execv()}.
Availability: most flavors of \UNIX{}, Windows.

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

\begin{funcdesc}{setgid}{gid}
Set the current process' group id.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{setpgrp}{}
Calls the system call \cfunction{setpgrp()} or \cfunction{setpgrp(0,
0)} depending on which version is implemented (if any).  See the
\UNIX{} manual for the semantics.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{setpgid}{pid, pgrp}
Calls the system call \cfunction{setpgid()}.  See the \UNIX{} manual
for the semantics.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{setsid}{}
Calls the system call \cfunction{setsid()}.  See the \UNIX{} manual
for the semantics.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{setuid}{uid}
\index{user!id, setting}
Set the current process' user id.
Availability: \UNIX{}.
\end{funcdesc}

% placed in this section since it relates to errno.... a little weak ;-(
\begin{funcdesc}{strerror}{code}
Return the error message corresponding to the error code in
\var{code}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{umask}{mask}
Set the current numeric umask and returns the previous umask.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{uname}{}
Return a 5-tuple containing information identifying the current
operating system.  The tuple contains 5 strings:
\code{(\var{sysname}, \var{nodename}, \var{release}, \var{version},
\var{machine})}.  Some systems truncate the nodename to 8
characters or to the leading component; a better way to get the
hostname is \function{socket.gethostname()}
\withsubitem{(in module socket)}{\ttindex{gethostname()}}
or even
\withsubitem{(in module socket)}{\ttindex{gethostbyaddr()}}
\code{socket.gethostbyaddr(socket.gethostname())}.
Availability: recent flavors of \UNIX{}.
\end{funcdesc}



\subsection{File Object Creation \label{os-newstreams}}

These functions create new file objects.


\begin{funcdesc}{fdopen}{fd\optional{, mode\optional{, bufsize}}}
Return an open file object connected to the file descriptor \var{fd}.
\index{I/O control!buffering}
The \var{mode} and \var{bufsize} arguments have the same meaning as
the corresponding arguments to the built-in \function{open()}
function.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{popen}{command\optional{, mode\optional{, bufsize}}}
Open a pipe to or from \var{command}.  The return value is an open
file object connected to the pipe, which can be read or written
depending on whether \var{mode} is \code{'r'} (default) or \code{'w'}.
The \var{bufsize} argument has the same meaning as the corresponding
argument to the built-in \function{open()} function.  The exit status of
the command (encoded in the format specified for \function{wait()}) is
available as the return value of the \method{close()} method of the file
object, except that when the exit status is zero (termination without
errors), \code{None} is returned.
Availability: \UNIX{}, Windows.
\end{funcdesc}



\subsection{File Descriptor Operations \label{os-fd-ops}}

These functions operate on I/O streams referred to
using file descriptors.


\begin{funcdesc}{close}{fd}
Close file descriptor \var{fd}.
Availability: Macintosh, \UNIX{}, Windows.

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

\begin{funcdesc}{dup}{fd}
Return a duplicate of file descriptor \var{fd}.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{dup2}{fd, fd2}
Duplicate file descriptor \var{fd} to \var{fd2}, closing the latter
first if necessary.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{fstat}{fd}
Return status for file descriptor \var{fd}, like \function{stat()}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{fstatvfs}{fd}
Return information about the filesystem containing the file associated
with file descriptor \var{fd}, like \function{statvfs()}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{ftruncate}{fd, length}
Truncate the file corresponding to file descriptor \var{fd}, 
so that it is at most \var{length} bytes in size.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{lseek}{fd, pos, how}
Set the current position of file descriptor \var{fd} to position
\var{pos}, modified by \var{how}: \code{0} to set the position
relative to the beginning of the file; \code{1} to set it relative to
the current position; \code{2} to set it relative to the end of the
file.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{open}{file, flags\optional{, mode}}
Open the file \var{file} and set various flags according to
\var{flags} and possibly its mode according to \var{mode}.
The default \var{mode} is \code{0777} (octal), and the current umask
value is first masked out.  Return the file descriptor for the newly
opened file.
Availability: Macintosh, \UNIX{}, Windows.

For a description of the flag and mode values, see the C run-time
documentation; flag constants (like \constant{O_RDONLY} and
\constant{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 \function{open()}, which returns a ``file
object'' with \method{read()} and \method{write()} methods (and many
more).
\end{funcdesc}

\begin{funcdesc}{pipe}{}
Create a pipe.  Return a pair of file descriptors \code{(\var{r},
\var{w})} usable for reading and writing, respectively.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{read}{fd, n}
Read at most \var{n} bytes from file descriptor \var{fd}.
Return a string containing the bytes read.
Availability: Macintosh, \UNIX{}, Windows.

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

\begin{funcdesc}{tcgetpgrp}{fd}
Return the process group associated with the terminal given by
\var{fd} (an open file descriptor as returned by \function{open()}).
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{tcsetpgrp}{fd, pg}
Set the process group associated with the terminal given by
\var{fd} (an open file descriptor as returned by \function{open()})
to \var{pg}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{ttyname}{fd}
Return a string which specifies the terminal device associated with
file-descriptor \var{fd}.  If \var{fd} is not associated with a terminal
device, an exception is raised.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{write}{fd, str}
Write the string \var{str} to file descriptor \var{fd}.
Return the number of bytes actually written.
Availability: Macintosh, \UNIX{}, Windows.

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


The following data items are available for use in constructing the
\var{flags} parameter to the \function{open()} function.

\begin{datadesc}{O_RDONLY}
\dataline{O_WRONLY}
\dataline{O_RDWR}
\dataline{O_NDELAY}
\dataline{O_NONBLOCK}
\dataline{O_APPEND}
\dataline{O_DSYNC}
\dataline{O_RSYNC}
\dataline{O_SYNC}
\dataline{O_NOCTTY}
\dataline{O_CREAT}
\dataline{O_EXCL}
\dataline{O_TRUNC}
Options for the \var{flag} argument to the \function{open()} function.
These can be bit-wise OR'd together.
Availability: Macintosh, \UNIX{}, Windows.
\end{datadesc}


\subsection{Files and Directories \label{os-file-dir}}

\begin{funcdesc}{access}{path, mode}
Check read/write/execute permissions for this process or extance of file
\var{path}.  Return \code{1} if access is granted, \code{0} if not.
See the \UNIX{} manual for the semantics.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{chdir}{path}
\index{directory!changing}
Change the current working directory to \var{path}.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{getcwd}{}
Return a string representing the current working directory.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{chmod}{path, mode}
Change the mode of \var{path} to the numeric \var{mode}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{chown}{path, uid, gid}
Change the owner and group id of \var{path} to the numeric \var{uid}
and \var{gid}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{link}{src, dst}
Create a hard link pointing to \var{src} named \var{dst}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{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 \code{'.'} and \code{'..'} even if they are present in the
directory.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{lstat}{path}
Like \function{stat()}, but do not follow symbolic links.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{mkfifo}{path\optional{, mode}}
Create a FIFO (a named pipe) named \var{path} with numeric mode
\var{mode}.  The default \var{mode} is \code{0666} (octal).  The current
umask value is first masked out from the mode.
Availability: \UNIX{}.

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

\begin{funcdesc}{mkdir}{path\optional{, mode}}
Create a directory named \var{path} with numeric mode \var{mode}.
The default \var{mode} is \code{0777} (octal).  On some systems,
\var{mode} is ignored.  Where it is used, the current umask value is
first masked out.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{makedirs}{path\optional{, mode}}
\index{directory!creating}
Recursive directory creation function.  Like \function{mkdir()},
but makes all intermediate-level directories needed to contain the
leaf directory.  Throws an \exception{error} exception if the leaf
directory already exists or cannot be created.  The default \var{mode}
is \code{0777} (octal).
\versionadded{1.5.2}
\end{funcdesc}

\begin{funcdesc}{readlink}{path}
Return a string representing the path to which the symbolic link
points.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{remove}{path}
Remove the file \var{path}.  See \function{rmdir()} below to remove a
directory.  This is identical to the \function{unlink()} function
documented below.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{removedirs}{path}
\index{directory!deleting}
Recursive directory removal function.  Works like
\function{rmdir()} except that, if the leaf directory is
successfully removed, directories corresponding to rightmost path
segments will be pruned way until either the whole path is consumed or
an error is raised (which is ignored, because it generally means that
a parent directory is not empty).  Throws an \exception{error}
exception if the leaf directory could not be successfully removed.
\versionadded{1.5.2}
\end{funcdesc}

\begin{funcdesc}{rename}{src, dst}
Rename the file or directory \var{src} to \var{dst}.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{renames}{old, new}
Recursive directory or file renaming function.
Works like \function{rename()}, except creation of any intermediate
directories needed to make the new pathname good is attempted first.
After the rename, directories corresponding to rightmost path segments
of the old name will be pruned away using \function{removedirs()}.

Note: this function can fail with the new directory structure made if
you lack permissions needed to remove the leaf directory or file.
\versionadded{1.5.2}
\end{funcdesc}

\begin{funcdesc}{rmdir}{path}
Remove the directory \var{path}.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{stat}{path}
Perform a \cfunction{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 \emph{stat} structure, in the
order
\code{st_mode},
\code{st_ino},
\code{st_dev},
\code{st_nlink},
\code{st_uid},
\code{st_gid},
\code{st_size},
\code{st_atime},
\code{st_mtime},
\code{st_ctime}.
More items may be added at the end by some implementations.
(On MS Windows, some items are filled with dummy values.)
Availability: Macintosh, \UNIX{}, Windows.

Note: The standard module \refmodule{stat}\refstmodindex{stat} defines
functions and constants that are useful for extracting information
from a \ctype{stat} structure.
\end{funcdesc}

\begin{funcdesc}{statvfs}{path}
Perform a \cfunction{statvfs()} system call on the given path.  The
return value is a tuple of 10 integers giving the most common
members of the \ctype{statvfs} structure, in the order
\code{f_bsize},
\code{f_frsize},
\code{f_blocks},
\code{f_bfree},
\code{f_bavail},
\code{f_files},
\code{f_ffree},
\code{f_favail},
\code{f_flag},
\code{f_namemax}.
Availability: \UNIX{}.

Note: The standard module \module{statvfs}\refstmodindex{statvfs}
defines constants that are useful for extracting information
from a \ctype{statvfs} structure.
\end{funcdesc}

\begin{funcdesc}{symlink}{src, dst}
Create a symbolic link pointing to \var{src} named \var{dst}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{unlink}{path}
Remove the file \var{path}.  This is the same function as
\function{remove()}; the \function{unlink()} name is its traditional
\UNIX{} name.
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{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.)
Availability: Macintosh, \UNIX{}, Windows.
\end{funcdesc}


\subsection{Process Management \label{os-process}}

These functions may be used to create and manage additional
processes.


\begin{funcdesc}{execl}{path, arg0, arg1, ...}
This is equivalent to
\samp{execv(\var{path}, (\var{arg0}, \var{arg1}, ...))}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execle}{path, arg0, arg1, ..., env}
This is equivalent to
\samp{execve(\var{path}, (\var{arg0}, \var{arg1}, ...), \var{env})}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execlp}{path, arg0, arg1, ...}
This is equivalent to
\samp{execvp(\var{path}, (\var{arg0}, \var{arg1}, ...))}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execv}{path, args}
Execute the executable \var{path} with argument list \var{args},
replacing the current process (i.e., the Python interpreter).
The argument list may be a tuple or list of strings.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execve}{path, args, env}
Execute the executable \var{path} with argument list \var{args},
and environment \var{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.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execvp}{path, args}
This is like \samp{execv(\var{path}, \var{args})} but duplicates
the shell's actions in searching for an executable file in a list of
directories.  The directory list is obtained from
\code{environ['PATH']}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{execvpe}{path, args, env}
This is a cross between \function{execve()} and \function{execvp()}.
The directory list is obtained from \code{\var{env}['PATH']}.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{_exit}{n}
Exit to the system with status \var{n}, without calling cleanup
handlers, flushing stdio buffers, etc.
Availability: \UNIX{}, Windows.

Note: the standard way to exit is \code{sys.exit(\var{n})}.
\function{_exit()} should normally only be used in the child process
after a \function{fork()}.
\end{funcdesc}

\begin{funcdesc}{fork}{}
Fork a child process.  Return \code{0} in the child, the child's
process id in the parent.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{kill}{pid, sig}
\index{process!killing}
\index{process!signalling}
Kill the process \var{pid} with signal \var{sig}.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{nice}{increment}
Add \var{increment} to the process's ``niceness''.  Return the new
niceness.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{plock}{op}
Lock program segments into memory.  The value of \var{op}
(defined in \code{<sys/lock.h>}) determines which segments are locked.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{spawnv}{mode, path, args}
Execute the program \var{path} in a new process, passing the arguments 
specified in \var{args} as command-line parameters.  \var{args} may be 
a list or a tuple.  \var{mode} is a magic operational constant.  See
the Visual \Cpp{} Runtime Library documentation for further
information; the constants are exposed to the Python programmer as
listed below.
Availability: Windows.
\versionadded{1.5.2}
\end{funcdesc}

\begin{funcdesc}{spawnve}{mode, path, args, env}
Execute the program \var{path} in a new process, passing the arguments 
specified in \var{args} as command-line parameters and the contents of 
the mapping \var{env} as the environment.  \var{args} may be a list or
a tuple.  \var{mode} is a magic operational constant.  See the Visual
\Cpp{} Runtime Library documentation for further information; the
constants are exposed to the Python programmer as listed below.
Availability: Windows.
\versionadded{1.5.2}
\end{funcdesc}

\begin{datadesc}{P_WAIT}
\dataline{P_NOWAIT}
\dataline{P_NOWAITO}
\dataline{P_OVERLAY}
\dataline{P_DETACH}
Possible values for the \var{mode} parameter to \function{spawnv()}
and \function{spawnve()}.
Availability: Windows.
\versionadded{1.5.2}
\end{datadesc}

\begin{funcdesc}{system}{command}
Execute the command (a string) in a subshell.  This is implemented by
calling the Standard C function \cfunction{system()}, and has the
same limitations.  Changes to \code{posix.environ}, \code{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 \function{wait()}, except on Windows 95 and 98,
where it is always \code{0}.  Note that \POSIX{} does not specify the
meaning of the return value of the C \cfunction{system()} function,
so the return value of the Python function is system-dependent.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{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
\manpage{times}{2} or the corresponding Windows Platform API
documentation.
Availability: \UNIX{}, Windows.
\end{funcdesc}

\begin{funcdesc}{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.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{waitpid}{pid, options}
Wait for completion of a child process given by process id, and return
a tuple containing its process id and exit status indication (encoded
as for \function{wait()}).  The semantics of the call are affected by
the value of the integer \var{options}, which should be \code{0} for
normal operation.
Availability: \UNIX{}.
\end{funcdesc}

\begin{datadesc}{WNOHANG}
The option for \function{waitpid()} to avoid hanging if no child
process status is available immediately.
Availability: \UNIX{}.
\end{datadesc}

The following functions take a process stats code as returned by
\function{waitpid()} as a parameter.  They may be used to determine
the disposition of a process.

\begin{funcdesc}{WIFSTOPPED}{status}
Return true if the process has been stopped.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{WIFSIGNALED}{status}
Return true if the process exited due to a signal.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{WIFEXITED}{status}
Return true if the process exited using the \manpage{exit}{2} system
call.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{WEXITSTATUS}{status}
If \code{WIFEXITED(\var{status})} is true, return the integer
parameter to the \manpage{exit}{2} system call.  Otherwise, the return 
value is meaningless.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{WSTOPSIG}{status}
Return the signal which caused the process to stop.
Availability: \UNIX{}.
\end{funcdesc}

\begin{funcdesc}{WTERMSIG}{status}
Return the signal which caused the process to exit.
Availability: \UNIX{}.
\end{funcdesc}


\subsection{Miscellanenous System Data \label{os-path}}

The follow data values are used to support path manipulation
operations.  These are defined for all platforms.

Higher-level operations on pathnames are defined in the
\refmodule{os.path} module.


\begin{datadesc}{curdir}
The constant string used by the OS to refer to the current directory,
e.g.\ \code{'.'} for \POSIX{} or \code{':'} for the Macintosh.
\end{datadesc}

\begin{datadesc}{pardir}
The constant string used by the OS to refer to the parent directory,
e.g.\ \code{'..'} for \POSIX{} or \code{'::'} for the Macintosh.
\end{datadesc}

\begin{datadesc}{sep}
The character used by the OS to separate pathname components,
e.g.\ \character{/} for \POSIX{} or \character{:} for the Macintosh.
Note that knowing this is not sufficient to be able to parse or
concatenate pathnames --- use \function{os.path.split()} and
\function{os.path.join()} --- but it is occasionally useful.
\end{datadesc}

\begin{datadesc}{altsep}
An alternative character used by the OS to separate pathname components,
or \code{None} if only one separator character exists.  This is set to
\character{/} on DOS and Windows systems where \code{sep} is a backslash.
\end{datadesc}

\begin{datadesc}{pathsep}
The character conventionally used by the OS to separate search patch
components (as in \envvar{PATH}), e.g.\ \character{:} for \POSIX{} or
\character{;} for DOS and Windows.
\end{datadesc}

\begin{datadesc}{defpath}
The default search path used by \function{exec*p*()} if the environment
doesn't have a \code{'PATH'} key.
\end{datadesc}

\begin{datadesc}{linesep}
The string used to separate (or, rather, terminate) lines on the
current platform.  This may be a single character,
e.g.\ \code{'\e n'} for \POSIX{} or \code{'\e r'} for MacOS, or multiple
characters, e.g.\ \code{'\e r\e n'} for MS-DOS and MS Windows.
\end{datadesc}