View source for Eventfd

[[Linux_APIs|Linux]] offers the eventfd abstraction for file descriptor-based userspace notification. It is backed by a single <tt>uint64_t</tt> in the kernel, unlike a pipe (which is usually backed by a much larger buffer). It is thus the most lightweight method of IPC available to userspace. The maximum value of the eventfd counter is 2^64-2 (0xfffffffffffffffe), and the minimum (and default) value is 0.

In the past, a full pipe was often used to interrupt a thread that was blocking in <tt>poll(2)</tt> (or <tt>select</tt>, or <tt>[[epoll]]</tt>, etc.). An eventfd can serve this same purpose while requiring less kernel memory.

<tt>write(2)</tt>s add the submitted eight-byte value to the value currently in the eventfd, unless they would cause the counter to exceed its maximum value, in which case they block. <tt>read(2)</tt>s return this eight-byte value, unless it is zero, in which case they block until the counter becomes non-zero. Following a successful <tt>read(2)</tt>, the counter is reset to zero (if <tt>EFD_SEMAPHORE</tt> was not specified) or decremented by one (if <tt>EFD_SEMAPHORE</tt> was specified).

<tt>POLLOUT</tt> is high whenever the value of the eventfd is less than its maximum, i.e. 0xfffffffffffffffd or less. It only guarantees that a value of 1 can be written without blocking.

==External links==
* [https://man7.org/linux/man-pages/man2/eventfd.2.html eventfd(2)] man page