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就绪 fd 如何回传用户态

• select
  • copy fd_set to user context
  • return fd count
  • user context dont know which fd is ready, have to traverse
• poll
  • copy fd array to user context
  • return fd count
  • user context dont know which fd is ready, have to traverse
• epoll
  • check whether the ready fd list is empty
  • memory share
    • between user and kernel
    • avoid copying

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