8、關于epoll的專家問答

    Q1    What happens if you add the same fd to an epoll_set twice?

    A1    You will probably get EEXIST. However, it is possible that two threads may add the same fd twice. This is a harmless condition. （同一個fd可以設定兩次）

    Q2    Can two epoll sets wait for the same fd? If so, are events reported to both epoll sets fds?

    A2    Yes. However, it is not recommended. Yes it would be reported to both. （同一個fd可以被兩個epoll 集等待）

    Q3    Is the epoll fd itself poll/epoll/selectable?

    A3    Yes. （epoll fd 同普通fd一樣，可設定，意為可用epoll_ctl）

    Q4    What happens if the epoll fd is put into its own fd set?

    A4    It will fail. However, you can add an epoll fd inside another epoll fd set.

    Q5    Can I send the epoll fd over a unix-socket to another process?

    A5    No. (隻能在本程序使用)

    Q6    Will the close of an fd cause it to be removed from all epoll sets automatically?

    A6    Yes. （智能删除）

    Q7    If more than one event comes in between epoll_wait(2) calls, are they combined or reported separately?

    A7    They will be combined. （這點要引起注意，也就是可讀和可寫同時有效）

    Q8    Does an operation on an fd affect the already collected but not yet reported events?

    A8    You can do two operations on an existing fd. Remove would be meaningless for this case. Modify will re-read available I/O.

    Q9    Do I need to continuously read/write an fd until EAGAIN when using the EPOLLET flag ( Edge Triggered behaviour ) ?

    A9    No you don't. Receiving an event from epoll_wait(2) should suggest to you that such file descriptor is ready for the requested I/O operation. You have simply to consider it ready until you will receive the next EAGAIN. When and how you will use such file descriptor is entirely up to you. Also, the condition that the read/write I/O space is exhausted（耗盡） can be detected by checking the amount of data read/write from/to the target file descriptor. For example, if you call read(2) by asking to read a certain amount of data and read(2) returns a lower number of bytes, you can be sure to have exhausted the read I/O space for such file descriptor. Same is valid when writing using the write(2) function.（也就是說，檢測到EAGAIN是一種保險的方法，另一種方法是，讀到的位元組數比可預期的少，那也說明讀緩存空了）

參考