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On UNIX systems we kill a tree of processes by performing a DFS walk of the tree. We send SIGSTOP to each process encountered, recursively handle its children, and then send SIGKILL. We once used the above approach in the past, but it was removed by the commit "Do not send both SIGSTOP and SIGKILL when killing a process". The commit was meant to work-around an OS X 10.3 bug in which the child would not always honor SIGKILL after SIGSTOP. At the time we wrongly assumed that the process tree remains intact after SIGKILL and before the child is reaped. In fact the grandchildren may be re-parented to ppid=1 even before the child is reaped, which causes the DFS walk to miss them.
KWSys provides a platform-independent API to many common system features that are implemented differently on every platform. This library is intended to be shared among many projects, so it has a configurable namespace. Each project should configure KWSys to use a namespace unique to itself. See comments in CMakeLists.txt for details. You are probably reading this file in the source tree of a surrounding project. In that case, see "../README.kwsys" for details of using KWSys in your project.