In unix in most cases child processes don't completely disappear
when they exit. After exit, they can deliver status information
to their parent. The Unix wait(2) and
waitpid(2) functions are the mechanism
by which the parent process obtains this status.
Obtaining child status is also sometimes called "reaping".
In most cases, child processes created by Tcl's exec or open commands are reaped by the interpreter's event handling subsystem. Subprocesse started by other means are not automatically reaped and this wait command can be used to perform that task.
The command returns a two element list containing the process id of the process that satisfied the wait and the status code returned by the process. If no matching process exited, the list contains a pair of zeroes (list 0 0). Any errors returned from the underlying wait system service result in a POSIX like Tcl error.
Note that all options are optional. By default, the command blocks until the next
-pid pidSpecifies what to wait for.
A value of -1 waits for the
next exiting child process. This the
default when no -pid option
is supplied.
A value greater than zero waits for only the process with the specified PID to exit.
A value of 0 waits for any process in the process group of which we are a member to exit.
A value less than zero waits for any process
in the process group that is the absolute value
of pid.
-poll
If this option is provided, the command returns
immediately. Otherwise it blocks until a process matching
the -pid specification exits. This
is equivalent to the
WNOHANG flag described in the
wait(2) manpage.