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Eventfd: Difference between revisions
(Created page with "Linux offers the eventfd abstraction for file descriptor-based userspace notification. It is backed by merely 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 th...") |
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[[Linux_APIs|Linux]] offers the eventfd abstraction for file descriptor-based userspace notification. It is backed by | [[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. | 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. | ||
Revision as of 18:08, 22 May 2023
Linux offers the eventfd abstraction for file descriptor-based userspace notification. It is backed by a single uint64_t 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 poll(2) (or select, or epoll, etc.). An eventfd can serve this same purpose while requiring less kernel memory.
write(2)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. read(2)s return this eight-byte value, unless it is zero, in which case they block until the counter becomes non-zero. Following a successful read(2), the counter is reset to zero (if EFD_SEMAPHORE was not specified) or decremented by one (if EFD_SEMAPHORE was specified).
POLLOUT 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.
