epoll

Its function is to monitor multiple file descriptors to see if I/O is possible on any of them.

epoll API

epoll_create

int epoll_create();

Creates an epoll object and returns its file descriptor.

epoll_ctl

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

This system call performs control operations on the epoll() instance referred to by the file descriptor epfd. It requests that the operation op be performed for the target file descriptor, fd.

When successful, epoll_ctl() returns zero. When an error occurs, epoll_ctl() returns -1 and errno is set appropriately.

• op

  • EPOLL_CTL_ADD
    Register the target file descriptor fd on the epoll instance
    referred to by the file descriptor epfd and associate the
    event event with the internal file linked to fd.

  • EPOLL_CTL_MOD
    Change the event event associated with the target file
    descriptor fd.

  • EPOLL_CTL_DEL
    Remove (deregister) the target file descriptor fd from the
    epoll instance referred to by epfd. The event is ignored and
    can be NULL .

• struct epoll_event

   typedef union epoll_data {
               void        *ptr;
               int          fd;
               uint32_t     u32;
               uint64_t     u64;
           } epoll_data_t;

           struct epoll_event {
               uint32_t     events;      /* Epoll events */
               epoll_data_t data;        /* User data variable */
           };

• evnets

  • EPOLLIN
    The associated file is available for read() operations.

  • EPOLLOUT
    The associated file is available for write() operations.

  • EPOLLET
    Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for epoll is Level Triggered.

epoll_wait

int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);

The epoll_wait() system call waits for events on the epoll() instance referred to by the file descriptor epfd. The memory area pointed to by events will contain the events that will be available for the caller. Up to maxevents are returned by epoll_wait(). The timeout argument specifies the number of milliseconds that
epoll_wait() will block. Specifying a timeout of -1 causes epoll_wait() to block indefinitely, while specifying a timeout equal to zero cause epoll_wait() to return immediately, even if no events are available.

When successful, epoll_wait() returns the number of file descriptors ready for the requested I/O, or zero if no file descriptor became ready during the requested timeout milliseconds. When an error occurs, epoll_wait() returns -1 and errno is set appropriately.

Level-triggered and edge-triggered

• LT(level triggered)

  • 无境界 同学说，LT 是缺省的工作方式，并且同时支持block和no-block socket.在这种做法中，内核告诉你一个文件描述符是否就绪了，然后你可以对这个就绪的fd进行IO操作。如果你不作任何操作，内核还是会继续通知你的。

  • LEO 同学说，水平触发

    1. 对于读操作 只要缓冲内容不为空，LT模式返回读就绪。
    2. 对于写操作 只要缓冲区还不满，LT模式会返回写就绪。

• ET(edge-triggered)

  • 无境界 同学说，ET 是高速工作方式，只支持no-block socket。在这种模式下，当描述符从未就绪变为就绪时，内核通过epoll告诉你。然后它会假设你知道文件描述符已经就绪，并且不会再为那个文件描述符发送更多的就绪通知，直到你做了某些操作导致那个文件描述符不再为就绪状态了。但是请注意，如果一直不对这个fd作IO操作(从而导致它再次变成未就绪)，内核不会发送更多的通知(only once).

  • LEO 同学说，边缘触发

    1. 对于读操作
       （1）当缓冲区由不可读变为可读的时候，即缓冲区由空变为不空的时候。
       （2）当有新数据到达时，即缓冲区中的待读数据变多的时候。
       （3）当缓冲区有数据可读，且应用进程对相应的描述符进行EPOLL_CTL_MOD 修改EPOLLIN事件时。

    2. 对于写操作
       （1）当缓冲区由不可写变为可写时。
       （2）当有旧数据被发送走，即缓冲区中的内容变少的时候。
       （3）当缓冲区有空间可写，且应用进程对相应的描述符进行EPOLL_CTL_MOD 修改EPOLLOUT事件时。

• 阻塞IO
  dontknow 同学说，当你去读一个阻塞的文件描述符时，如果在该文件描述符上没有数据可读，那么它会一直阻塞(通俗一点就是一直卡在调用函数那里)，直到有数据可读。当你去写一个阻塞的文件描述符时，如果在该文件描述符上没有空间(通常是缓冲区)可写，那么它会一直阻塞，直到有空间可写。

• 非阻塞IO
  dontknow 同学说, 当你去读写一个非阻塞的文件描述符时，不管可不可以读写，它都会立即返回，返回成功说明读写操作完成了，返回失败会设置相应errno状态码，根据这个errno可以进一步执行其他处理。它不会像阻塞IO那样，卡在那里不动！！！

Python: select模块

• select.epoll(sizehint=-1, flags=0)
  Return an edge polling object, which can be used as Edge or Level Triggered interface for I/O events.

• eventmask
  EPOLLIN: Available for read
  EPOLLOUT: Available for write
  EPOLLET: Set Edge Trigger behavior, the default is Level Trigger behavior

• epoll.close()
  Close the control file descriptor of the epoll object.

• epoll.register(fd[, eventmask])
  Register a fd descriptor with the epoll object.

• epoll.modify(fd, eventmask)
  Modify a registered file descriptor.

• epoll.unregister(fd)
  Remove a registered file descriptor from the epoll object.

• epoll.poll(timeout=-1, maxevents=-1)
  Wait for events.

参考

• epoll 水平触发与边缘触发
• 实例浅析epoll的水平触发和边缘触发，以及边缘触发为什么要使用非阻塞IO
• epoll精髓
• Linux manual: epoll
• Wiki: epoll
• Python: select

read more

• 彻底学会使用epoll（二）——ET和LT的触发方式
• How to use epoll? A complete example in C
• 预备知识：我读过的对epoll最好的讲解
• select、poll、epoll之间的区别总结[整理]