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The epoll API

Edge-triggered notification

• Specify (EPOLLET) in ev.events when calling epoll_ctl().

ev.data.fd = fd;              
ev.events = EPOLLIN | EPOLLET;
epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, ev);

• "Has file descriptor had an I/O event occurred since we last monitored it?"
  
  
• Somewhat similar to signal-driven I/O.
  
  
• Each call to epoll_wait() removes items ready list.
  
  
• File descriptor is only re-added to the ready list if an I/O event has occurred since last epoll_wait() call.
  
  
• If monitoring (say) a socket for input, behavior of second epoll_wait() call will vary depending on whether notification is level-triggered or edge-triggered.
  
  
  1. Input arrives on the socket.
     
     
  2. We call epoll_wait(); regardless of the model, the call says that the socket is ready.
     
     
  3. We call epoll_wait().
     
     
• Caller must mark file descriptors non-blocking (O_NONBLOCK), because:
  
  
  • No further notification will occur until next I/O event.
    
    
  • Therefore we must perform as much I/O is possible...

    read() until EAGAIN

    
    
    

(C) 2006, Michael Kerrisk