#include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait(int *status);

All of these system calls are used to wait for state changes in a child
       of  the  calling  process, and obtain information about the child whose
       state has changed.  A state change  is  considered  to  be:  the  child
       terminated; the child was stopped by a signal; or the child was resumed
       by a signal.  In the case of a  terminated  child,  performing  a  wait

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