epoll_wait 在“epoll_event”中存储了一个不同的值

Question

I'll try keep this simple. I have the code below:

epoll_event event;

event.events = EPOLLIN | EPOLLET;
event.data.fd = clientSock; // this is equal to "7"
event.data.ptr = myPtr;
epoll_ctl(epoll, EPOLL_CTL_ADD, client, &event);


//Another thread

 epoll_wait(epoll, &event2, MAX_EVENTS, EPOLL_TIMEOUT);

 // This is the strange part... 
 cout << event2.data.fd; //output is different from "7"

But, if I dont add a ptr to event.data.ptr (which I did before I called epoll_wait ), the value of event2.data.fd is correct (7). What's causing this?

Answer 1

The type of the data member of struct epoll_event is a union . As such, only one of its members contains a value at any given time, so when you assign to event.data.ptr you replace the value previously written to event.data.fd . The subsequent epoll_ctl call therefore probably does not express interest in the events you think it does, but in any case, you should expect to read back only the ptr member from the resulting event data, not the fd member.