淺論epoll_wait

應用場景

相似libaio，屬於異步IO 模式，實現批量獲取完成的event.

使用方法

step1: 建立epolling fd

能夠把epolling fd 想象成一個容器或者代理，裏面裝須要偵聽的文件等描述符。建立polling 描述符對應的API及其接口說明以下：

EPOLL_CREATE(2)                                              Linux Programmer's Manual                                              EPOLL_CREATE(2)

NAME
       epoll_create - open an epoll file descriptor

SYNOPSIS
       #include <sys/epoll.h>

       int epoll_create(int size)

DESCRIPTION
       Open an epoll file descriptor by requesting the kernel allocate an event backing store dimensioned for size descriptors. The size is not the
       maximum size of the backing store but just a hint to the kernel about how to dimension internal structures.  The  returned  file  descriptor
       will  be  used  for all the subsequent calls to the epoll interface. The file descriptor returned by epoll_create(2) must be closed by using
       close(2).

RETURN VALUE
       When successful, epoll_create(2) returns a positive integer identifying the descriptor.  When an error occurs,  epoll_create(2)  returns  -1
       and errno is set appropriately.

ERRORS
       ENOMEM There was insufficient memory to create the kernel object.

epool_control

控制須要把那些文件描述符增長上面建立的容器裏，或者控制把那些文件描述符從容器裏去掉。

EPOLL_CTL(2)                                                 Linux Programmer's Manual                                                 EPOLL_CTL(2)

NAME
       epoll_ctl - control interface for an epoll descriptor

SYNOPSIS
       #include <sys/epoll.h>

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

DESCRIPTION
       Control  an  epoll descriptor, epfd, by requesting the operation op be performed on the target file descriptor, fd.  The event describes the
       object linked to the file descriptor fd.  The struct epoll_event is defined as :

            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 */
            };

       The events member is a bit set composed using the following available event types :

       EPOLLIN
              The associated file is available for read(2) operations.

       EPOLLOUT
              The associated file is available for write(2) operations.

       EPOLLPRI
              There is urgent data available for read(2) operations.

RETURN VALUE
       When successful, epoll_ctl(2) returns zero. When an error occurs, epoll_ctl(2) returns -1 and errno is set appropriately.

epoll_wait

它經過epfd 等待上面指定的event 是否出現：

NAME
       epoll_wait - wait for an I/O event on an epoll file descriptor

SYNOPSIS
       #include <sys/epoll.h>

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

DESCRIPTION
       Wait for events on the epoll file descriptor epfd for a maximum time of timeout milliseconds. The memory area pointed to by events will con-
       tain the events that will be available for the caller.  Up to maxevents are returned by epoll_wait(2).   The  maxevents  parameter  must  be
       greater  than  zero.  Specifying  a  timeout  of  -1  makes  epoll_wait(2) wait indefinitely, while specifying a timeout equal to zero makes
       epoll_wait(2) to return immediately even if no events are available ( return code equal to zero ).  The struct epoll_event is defined as :

            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 */
            };

       The data of each returned structure will contain the same data the user set with  a  epoll_ctl(2)  (EPOLL_CTL_ADD,EPOLL_CTL_MOD)  while  the
       events member will contain the returned event bit field.

RETURN VALUE
       When  successful,  epoll_wait(2)  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(2) returns -1 and errno is set appropriately.

ERRORS
       EBADF  epfd is not a valid file descriptor.

       EINVAL The supplied file descriptor, epfd, is not an epoll file descriptor, or the maxevents parameter is less than or equal to zero.

       EFAULT The memory area pointed to by events is not accessible with write permissions.

應用示例

_epfd = epoll_create(epoll_size);
  ...
    epoll_event evt = { EPOLLOUT, { NULL } };
        if (epoll_ctl(_epfd, EPOLL_CTL_ADD,
                      pipe[1], &evt) < 0) {
            COUT << "Fail to add pipe into epfd="
                        << _epfd;
            return -5;
        }

                ....
                epoll_event* e = new (std::nothrow) epoll_event[MAXEVENT];
                 int k = epoll_wait(_epfd, e, MAXEVENT, 0);
                ....

注意事項