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Notify the io_service of a fork-related event.
        
void notify_fork( asio::io_service::fork_event event);
          This function is used to inform the io_service that the process is
          about to fork, or has just forked. This allows the io_service, and the services it
          contains, to perform any necessary housekeeping to ensure correct operation
          following a fork.
        
          This function must not be called while any other io_service function, or any function
          on an I/O object associated with the io_service, is being called in
          another thread. It is, however, safe to call this function from within
          a completion handler, provided no other thread is accessing the io_service.
        
A fork-related event.
                Thrown on failure. If the notification fails the io_service object should
                no longer be used and should be destroyed.
              
          The following code illustrates how to incorporate the notify_fork() function:
        
my_io_service.notify_fork(asio::io_service::fork_prepare); if (fork() == 0) { // This is the child process. my_io_service.notify_fork(asio::io_service::fork_child); } else { // This is the parent process. my_io_service.notify_fork(asio::io_service::fork_parent); }
          For each service object svc
          in the io_service
          set, performs svc->fork_service();.
          When processing the fork_prepare event, services are visited in reverse
          order of the beginning of service object lifetime. Otherwise, services
          are visited in order of the beginning of service object lifetime.