6.1.5 Process Management

These functions may be used to create and manage processes.

The various exec*() functions take a list of arguments for the new program loaded into the process. In each case, the first of these arguments is passed to the new program as its own name rather than as an argument a user may have typed on a command line. For the C programmer, this is the argv[0] passed to a program's main(). For example, "os.execv('/bin/echo', ['foo', 'bar'])" will only print "bar" on standard output; "foo"will seem to be ignored.

abort(): Generate a SIGABRT signal to the current process. On Unix, the default behavior is to produce a core dump; on Windows, the process immediately returns an exit code of 3. Be aware that programs which use signal.signal() to register a handler for SIGABRT will behave differently. Availability: Unix, Windows.

execl(path, arg0, arg1, ...): This is equivalent to "execv(path, (arg0, arg1, ...))". Availability: Unix, Windows.

execle(path, arg0, arg1, ..., env): This is equivalent to "execve(path, (arg0, arg1, ...), env)". Availability: Unix, Windows.

execlp(path, arg0, arg1, ...): This is equivalent to "execvp(path, (arg0, arg1, ...))". Availability: Unix, Windows.

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. Availability: Unix, Windows.

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. Availability: Unix, Windows.

execvp(path, args): This is like "execv(path, args)" but duplicates the shell's actions in searching for an executable file in a list of directories. The directory list is obtained from environ['PATH']. Availability: Unix, Windows.

execvpe(path, args, env): This is a cross between execve() and execvp(). The directory list is obtained from env['PATH']. Availability: Unix, Windows.

_exit(n): Exit to the system with status n, without calling cleanup handlers, flushing stdio buffers, etc. Availability: Unix, Windows.
Note: the standard way to exit is sys.exit(n). _exit() should normally only be used in the child process after a fork().

fork(): Fork a child process. Return 0 in the child, the child's process id in the parent. Availability: Unix.

forkpty(): Fork a child process, using a new pseudo-terminal as the child's controlling terminal. Return a pair of (pid, fd), where pid is 0 in the child, the new child's process id in the parent, and fd is the file descriptor of the master end of the pseudo-terminal. For a more portable approach, use the pty module. Availability: Some flavors of Unix

kill(pid, sig): Kill the process pid with signal sig. Availability: Unix.

nice(increment): Add increment to the process's ``niceness''. Return the new niceness. Availability: Unix.

plock(op): Lock program segments into memory. The value of op (defined in <sys/lock.h>) determines which segments are locked. Availability: Unix.

popen(...)
popen2(...)
popen3(...)
popen4(...): Run child processes, returning opened pipes for communications. These functions are described in section 6.1.2.

spawnv(mode, path, args): Execute the program path in a new process, passing the arguments specified in args as command-line parameters. args may be a list or a tuple. mode is a magic operational constant. See the Visual C++ Runtime Library documentation for further information; the constants are exposed to the Python programmer as listed below. Availability: Unix, Windows. New in version 1.6.

spawnve(mode, path, args, env): Execute the program path in a new process, passing the arguments specified in args as command-line parameters and the contents of the mapping env as the environment. args may be a list or a tuple. mode is a magic operational constant. See the Visual C++ Runtime Library documentation for further information; the constants are exposed to the Python programmer as listed below. Availability: Unix, Windows. New in version 1.6.

P_WAIT
P_NOWAIT
P_NOWAITO: Possible values for the mode parameter to spawnv() and spawnve(). Availability: Unix, Windows. New in version 1.6.

P_OVERLAY
P_DETACH: Possible values for the mode parameter to spawnv() and spawnve(). These are less portable than those listed above. Availability: Windows. New in version 1.6.

startfile(path): Start a file with its associated application. This acts like double-clicking the file in Windows Explorer, or giving the file name as an argument to the DOS start command: the file is opened with whatever application (if any) its extension is associated.
startfile() returns as soon as the associated application is launched. There is no option to wait for the application to close, and no way to retrieve the application's exit status. The path parameter is relative to the current directory. If you want to use an absolute path, make sure the first character is not a slash ("/"); the underlying Win32 ShellExecute() function doesn't work it is. Use the os.path.normpath() function to ensure that the path is properly encoded for Win32. Availability: Windows. New in version 2.0.

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(), except on Windows 95 and 98, where it is always 0. Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent. Availability: Unix, Windows.

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) or the corresponding Windows Platform API documentation. Availability: Unix, Windows.

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.

waitpid(pid, options): Wait for completion of a child process given by process id pid, and return a tuple containing its process id 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. Availability: Unix.
If pid is greater than 0, waitpid() requests status information for that specific process. If pid is 0, the request is for the status of any child in the process group of the current process. If pid is -1, the request pertains to any child of the current process. If pid is less than -1, status is requested for any process in the process group -pid (the absolute value of pid).

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

The following functions take a process status code as returned by system(), wait(), or waitpid() as a parameter. They may be used to determine the disposition of a process.

WIFSTOPPED(status): Return true if the process has been stopped. Availability: Unix.

WIFSIGNALED(status): Return true if the process exited due to a signal. Availability: Unix.

WIFEXITED(status): Return true if the process exited using the exit(2) system call. Availability: Unix.

WEXITSTATUS(status): If WIFEXITED(status) is true, return the integer parameter to the exit(2) system call. Otherwise, the return value is meaningless. Availability: Unix.

WSTOPSIG(status): Return the signal which caused the process to stop. Availability: Unix.

WTERMSIG(status): Return the signal which caused the process to exit. Availability: Unix.

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