wait - check child process status
wait_nohang looks for zombies (child processes that have exited). If it sees a zombie, it eliminates the zombie, puts the zombie’s exit status into wstat, and returns the zombie’s process ID. If there are several zombies, wait_nohang picks one. If there are children but no zombies, wait_nohang returns 0. If there are no children, wait_nohang returns -1, setting errno appropriately.
wait_stopnohang is similar to wait_nohang, but it also looks for children that have stopped.
wait_stop is similar to wait_stopnohang, but if there are children it will pause waiting for one of them to stop or exit.
wait_pid waits for child process pid to exit. It eliminates any zombie that shows up in the meantime, discarding the exit status.
wait_stop and wait_pid retry upon error_intr.
If the child stopped, wait_stopped is nonzero; wait_stopsig is the signal that caused the child to stop.
If the child exited by crashing, wait_stopped is zero; wait_crashed is nonzero.
If the child exited normally, wait_stopped is zero; wait_crashed is zero; and wait_exitcode is the child’s exit code.