NAME

       Tcl_DetachPids, Tcl_ReapDetachedProcs, Tcl_WaitPid - manage child processes in background

SYNOPSIS

       #include <tcl.h>

       Tcl_DetachPids(numPids, pidPtr)

       Tcl_ReapDetachedProcs()

       Tcl_Pid
       Tcl_WaitPid(pid, statPtr, options)

ARGUMENTS

       int   numPids      (in)      Number  of  process  ids contained in the array pointed to by
                                    pidPtr.

       int   *pidPtr      (in)      Address of array containing numPids process ids.

       Tcl_Pid            pid(in)   The id of the process (pipe) to wait for.

       int*  statPtr      (out)     The result of waiting  on  a  process  (pipe).  Either  0  or
                                    ECHILD.

       int   options                The  options  controlling  the wait. WNOHANG specifies not to
                                    wait when checking the process.
_________________________________________________________________

DESCRIPTION

       Tcl_DetachPids and Tcl_ReapDetachedProcs 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 waitpid 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.

       Tcl_DetachPids may be called to ask Tcl to take responsibility for one or  more  processes
       whose  process  ids  are  contained  in  the  pidPtr array passed as argument.  The caller
       presumably has started these processes running in background and doesn't want to  have  to
       deal with them again.

       Tcl_ReapDetachedProcs  invokes the waitpid kernel call on each of the background processes
       so that its state can be cleaned up if it has exited.  If the process hasn't  exited  yet,
       Tcl_ReapDetachedProcs  doesn't  wait for it to exit;  it will check again the next time it
       is invoked.  Tcl automatically calls Tcl_ReapDetachedProcs each time the exec  command  is
       executed,  so  in  most  cases  it  isn't  necessary for any code outside of Tcl to invoke
       Tcl_ReapDetachedProcs.  However, if you call Tcl_DetachPids in situations where  the  exec
       command  may  never  get executed, you may wish to call Tcl_ReapDetachedProcs from time to
       time so that background processes can be cleaned up.

       Tcl_WaitPid 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 Tcl_ReapDetachedProcs and the channel system to portably access the
       operating system.

KEYWORDS

       background, child, detach, process, wait