Creates a new child_watch source.
The source will not initially be associated with any MainContext and must be added to one with attach before it will be executed.
Note that child watch sources can only be used in conjunction with `g_spawn...` when the g_spawn_do_not_reap_child flag is used.
Note that on platforms where Pid must be explicitly closed (see
pid must not be closed while the source is still
active. Typically, you will want to call close_pid in the callback function
for the source.
On POSIX platforms, the following restrictions apply to this API due to limitations in POSIX process interfaces:
pid must be a child of this process *
pid must be positive * the application must not call `waitpid` with a
non-positive first argument, for instance in another thread * the application must not wait for
pid to exit by any other
mechanism, including `waitpid(pid, ...)` or a second child-watch source for the same
pid * the application must not ignore
If any of those conditions are not met, this and related APIs will not work correctly. This can often be diagnosed via a GLib warning stating that `ECHILD` was received by `waitpid`.
Calling `waitpid` for specific processes other than
pid remains a valid thing to do.
process to watch. On POSIX the positive pid of a child process. On Windows a handle for a process (which doesn't have to be a child).
the newly-created child watch source