source: downloads/tcl8.5.2/doc/DetachPids.3 @ 43

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'\" RCS: @(#) $Id: DetachPids.3,v 1.7 2007/12/13 15:22:31 dgp Exp $
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.so man.macros
.TH Tcl_DetachPids 3 "" Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_DetachPids, Tcl_ReapDetachedProcs, Tcl_WaitPid \- manage child processes in background
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_DetachPids\fR(\fInumPids, pidPtr\fR)
.sp
\fBTcl_ReapDetachedProcs\fR()
.sp
Tcl_Pid
\fBTcl_WaitPid\fR(\fIpid, statusPtr, options\fR)
.SH ARGUMENTS
.AS Tcl_Pid *statusPtr out
.AP int numPids in
Number of process ids contained in the array pointed to by \fIpidPtr\fR.
.AP int *pidPtr in
Address of array containing \fInumPids\fR process ids.
.AP Tcl_Pid pid in
The id of the process (pipe) to wait for.
.AP int *statusPtr out
The result of waiting on a process (pipe). Either 0 or ECHILD.
.AP int options in
The options controlling the wait. WNOHANG specifies not to wait when
checking the process.
.BE
.SH DESCRIPTION
.PP
\fBTcl_DetachPids\fR and \fBTcl_ReapDetachedProcs\fR provide a
mechanism for managing subprocesses that are running in background.
These procedures are needed because the parent of a process must
eventually invoke the \fBwaitpid\fR kernel call (or one of a few other
similar kernel calls) to wait for the child to exit.  Until the
parent waits for the child, the child's state cannot be completely
reclaimed by the system.  If a parent continually creates children
and doesn't wait on them, the system's process table will eventually
overflow, even if all the children have exited.
.PP
\fBTcl_DetachPids\fR may be called to ask Tcl to take responsibility
for one or more processes whose process ids are contained in the
\fIpidPtr\fR array passed as argument.  The caller presumably
has started these processes running in background and does not
want to have to deal with them again.
.PP
\fBTcl_ReapDetachedProcs\fR invokes the \fBwaitpid\fR kernel call
on each of the background processes so that its state can be cleaned
up if it has exited.  If the process has not exited yet,
\fBTcl_ReapDetachedProcs\fR does not wait for it to exit;  it will check again
the next time it is invoked.
Tcl automatically calls \fBTcl_ReapDetachedProcs\fR each time the
\fBexec\fR command is executed, so in most cases it is not necessary
for any code outside of Tcl to invoke \fBTcl_ReapDetachedProcs\fR.
However, if you call \fBTcl_DetachPids\fR in situations where the
\fBexec\fR command may never get executed, you may wish to call
\fBTcl_ReapDetachedProcs\fR from time to time so that background
processes can be cleaned up.
.PP
\fBTcl_WaitPid\fR is a thin wrapper around the facilities provided by
the operating system to wait on the end of a spawned process and to
check a whether spawned process is still running. It is used by
\fBTcl_ReapDetachedProcs\fR and the channel system to portably access
the operating system.

.SH KEYWORDS
background, child, detach, process, wait