The Event Poll Interface

Recognizing the limitations of both poll() and select() , the 2.6 Linux kernel* intro duced the event poll (epoll) facility. While more complex than the two earlier interfaces, epoll solves the fundamental performance problem shared by both of them, and adds several new features.

Both poll() and select() (discussed in Chapter 2) require the full list of file descriptors to watch on each invocation. The kernel must then walk the list of each file descriptor to be monitored. When this list grows large—it may contain hundreds or even thousands of file descriptors—walking the list on each invocation becomes a scalability bottleneck.

Epoll circumvents this problem by decoupling the monitor registration from the actual monitoring. One system call initializes an epoll context, another adds monitored file descriptors to or removes them from the context, and a third performs the actual event wait.

Creating a New Epoll Instance

An epoll context is created via epoll_create() :

  #include <sys/epoll.h>

  int epoll_create (int size)

A successful call to epoll_create() instantiates a new epoll instance, and returns a file descriptor associated with the instance. This file descriptor has no relationship to a real file; it is just a handle to be used with subsequent calls using the epoll facility. The size parameter is a hint to the kernel about the number of file descriptors that are going to be monitored; it is not the maximum number. Passing in a good approximation will result in better performance, but the exact number is not required. On error, the call returns -1 , and sets errno to one of the following:

EINVAL
   The size parameter is not a positive number.

ENFILE
   The system has reached the limit on the total number
   of open files.

ENOMEM
   Insufficient memory was available to complete the
   operation.

A typical call is:

  int epfd;

  epfd = epoll_create (100); /* plan to watch ~100 fds * /
  if (epfd < 0)
          perror ("epoll_create");

The file descriptor returned from epoll_create() should be destroyed via a call to close() after polling is finished.