Tcl_DetachPids man page

Tcl_DetachPids, Tcl_ReapDetachedProcs, Tcl_WaitPid — manage child processes in background


#include <tcl.h>

Tcl_DetachPids(numPids, pidPtr)


Tcl_WaitPid(pid, statusPtr, options)


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 *statusPtr (out)
The result of waiting on a process (pipe). Either 0 or ECHILD.
int options (in)
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 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 does not 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 has not exited yet, Tcl_ReapDetachedProcs does not 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 is not 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.


background, child, detach, process, wait

Referenced By

Tcl_ReapDetachedProcs(3) and Tcl_WaitPid(3) are aliases of Tcl_DetachPids(3).

Tcl Tcl Library Procedures