Tcl_DetachPids(3) Tcl Library Procedures Tcl_DetachPids(3)
Tcl_DetachPids, Tcl_ReapDetachedProcs, Tcl_WaitPid - manage child processes in background
Tcl_WaitPid(pid, statPtr, options)
int numPids (in) Number of process ids contained in the array pointed to by
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
int options The options controlling the wait. WNOHANG specifies not to
wait when checking the process.
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 ker-
nel 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 cre-
ates children and doesn't wait on them, the system's process table will eventually over-
flow, 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 pre-
sumably 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 run-
ning. It is used by Tcl_ReapDetachedProcs and the channel system to portably access the
background, child, detach, process, wait