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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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eventfd(2) System Calls Manual eventfd(2)
eventfd - create a file descriptor for event notification
Standard C library (libc, -lc)
#include <sys/eventfd.h>
int eventfd(unsigned int initval, int flags);
eventfd() creates an "eventfd object" that can be used as an event
wait/notify mechanism by user-space applications, and by the
kernel to notify user-space applications of events. The object
contains an unsigned 64-bit integer (uint64_t) counter that is
maintained by the kernel. This counter is initialized with the
value specified in the argument initval.
As its return value, eventfd() returns a new file descriptor that
can be used to refer to the eventfd object.
The following values may be bitwise ORed in flags to change the
behavior of eventfd():
EFD_CLOEXEC (since Linux 2.6.27)
Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
EFD_NONBLOCK (since Linux 2.6.27)
Set the O_NONBLOCK file status flag on the open file
description (see open(2)) referred to by the new file
descriptor. Using this flag saves extra calls to fcntl(2)
to achieve the same result.
EFD_SEMAPHORE (since Linux 2.6.30)
Provide semaphore-like semantics for reads from the new
file descriptor. See below.
Up to Linux 2.6.26, the flags argument is unused, and must be
specified as zero.
The following operations can be performed on the file descriptor
returned by eventfd():
read(2)
Each successful read(2) returns an 8-byte integer. A
read(2) fails with the error EINVAL if the size of the
supplied buffer is less than 8 bytes.
The value returned by read(2) is in host byte order—that
is, the native byte order for integers on the host machine.
The semantics of read(2) depend on whether the eventfd
counter currently has a nonzero value and whether the
EFD_SEMAPHORE flag was specified when creating the eventfd
file descriptor:
• If EFD_SEMAPHORE was not specified and the eventfd
counter has a nonzero value, then a read(2) returns 8
bytes containing that value, and the counter's value is
reset to zero.
• If EFD_SEMAPHORE was specified and the eventfd counter
has a nonzero value, then a read(2) returns 8 bytes
containing the value 1, and the counter's value is
decremented by 1.
• If the eventfd counter is zero at the time of the call
to read(2), then the call either blocks until the
counter becomes nonzero (at which time, the read(2)
proceeds as described above) or fails with the error
EAGAIN if the file descriptor has been made nonblocking.
write(2)
A write(2) call adds the 8-byte integer value supplied in
its buffer to the counter. The maximum value that may be
stored in the counter is the largest unsigned 64-bit value
minus 1 (i.e., 0xfffffffffffffffe). If the addition would
cause the counter's value to exceed the maximum, then the
write(2) either blocks until a read(2) is performed on the
file descriptor, or fails with the error EAGAIN if the file
descriptor has been made nonblocking.
A write(2) fails with the error EINVAL if the size of the
supplied buffer is less than 8 bytes, or if an attempt is
made to write the value 0xffffffffffffffff.
poll(2)
select(2)
(and similar)
The returned file descriptor supports poll(2) (and
analogously epoll(7)) and select(2), as follows:
• The file descriptor is readable (the select(2) readfds
argument; the poll(2) POLLIN flag) if the counter has a
value greater than 0.
• The file descriptor is writable (the select(2) writefds
argument; the poll(2) POLLOUT flag) if it is possible to
write a value of at least "1" without blocking.
• If an overflow of the counter value was detected, then
select(2) indicates the file descriptor as being both
readable and writable, and poll(2) returns a POLLERR
event. As noted above, write(2) can never overflow the
counter. However an overflow can occur if 2^64 eventfd
"signal posts" were performed by the KAIO subsystem
(theoretically possible, but practically unlikely). If
an overflow has occurred, then read(2) will return that
maximum uint64_t value (i.e., 0xffffffffffffffff).
The eventfd file descriptor also supports the other file-
descriptor multiplexing APIs: pselect(2) and ppoll(2).
close(2)
When the file descriptor is no longer required it should be
closed. When all file descriptors associated with the same
eventfd object have been closed, the resources for object
are freed by the kernel.
A copy of the file descriptor created by eventfd() is inherited by
the child produced by fork(2). The duplicate file descriptor is
associated with the same eventfd object. File descriptors created
by eventfd() are preserved across execve(2), unless the close-on-
exec flag has been set.
On success, eventfd() returns a new eventfd file descriptor. On
error, -1 is returned and errno is set to indicate the error.
EINVAL An unsupported value was specified in flags.
EMFILE The per-process limit on the number of open file
descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has
been reached.
ENODEV Could not mount (internal) anonymous inode device.
ENOMEM There was insufficient memory to create a new eventfd file
descriptor.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ eventfd() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C library/kernel differences
There are two underlying Linux system calls: eventfd() and the
more recent eventfd2(). The former system call does not implement
a flags argument. The latter system call implements the flags
values described above. The glibc wrapper function will use
eventfd2() where it is available.
Additional glibc features
The GNU C library defines an additional type, and two functions
that attempt to abstract some of the details of reading and
writing on an eventfd file descriptor:
typedef uint64_t eventfd_t;
int eventfd_read(int fd, eventfd_t *value);
int eventfd_write(int fd, eventfd_t value);
The functions perform the read and write operations on an eventfd
file descriptor, returning 0 if the correct number of bytes was
transferred, or -1 otherwise.
Linux, GNU.
eventfd()
Linux 2.6.22, glibc 2.8.
eventfd2()
Linux 2.6.27 (see VERSIONS). Since glibc 2.9, the
eventfd() wrapper will employ the eventfd2() system call,
if it is supported by the kernel.
Applications can use an eventfd file descriptor instead of a pipe
(see pipe(2)) in all cases where a pipe is used simply to signal
events. The kernel overhead of an eventfd file descriptor is much
lower than that of a pipe, and only one file descriptor is
required (versus the two required for a pipe).
When used in the kernel, an eventfd file descriptor can provide a
bridge from kernel to user space, allowing, for example,
functionalities like KAIO (kernel AIO) to signal to a file
descriptor that some operation is complete.
A key point about an eventfd file descriptor is that it can be
monitored just like any other file descriptor using select(2),
poll(2), or epoll(7). This means that an application can
simultaneously monitor the readiness of "traditional" files and
the readiness of other kernel mechanisms that support the eventfd
interface. (Without the eventfd() interface, these mechanisms
could not be multiplexed via select(2), poll(2), or epoll(7).)
The current value of an eventfd counter can be viewed via the
entry for the corresponding file descriptor in the process's
/proc/pid/fdinfo directory. See proc(5) for further details.
The following program creates an eventfd file descriptor and then
forks to create a child process. While the parent briefly sleeps,
the child writes each of the integers supplied in the program's
command-line arguments to the eventfd file descriptor. When the
parent has finished sleeping, it reads from the eventfd file
descriptor.
The following shell session shows a sample run of the program:
$ ./a.out 1 2 4 7 14
Child writing 1 to efd
Child writing 2 to efd
Child writing 4 to efd
Child writing 7 to efd
Child writing 14 to efd
Child completed write loop
Parent about to read
Parent read 28 (0x1c) from efd
Program source
#include <err.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/eventfd.h>
#include <sys/types.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int efd;
uint64_t u;
ssize_t s;
if (argc < 2) {
fprintf(stderr, "Usage: %s <num>...\n", argv[0]);
exit(EXIT_FAILURE);
}
efd = eventfd(0, 0);
if (efd == -1)
err(EXIT_FAILURE, "eventfd");
switch (fork()) {
case 0:
for (size_t j = 1; j < argc; j++) {
printf("Child writing %s to efd\n", argv[j]);
u = strtoull(argv[j], NULL, 0);
/* strtoull() allows various bases */
s = write(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
err(EXIT_FAILURE, "write");
}
printf("Child completed write loop\n");
exit(EXIT_SUCCESS);
default:
sleep(2);
printf("Parent about to read\n");
s = read(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
err(EXIT_FAILURE, "read");
printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u);
exit(EXIT_SUCCESS);
case -1:
err(EXIT_FAILURE, "fork");
}
}
futex(2), pipe(2), poll(2), read(2), select(2), signalfd(2),
timerfd_create(2), write(2), epoll(7), sem_overview(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 eventfd(2)
Pages that refer to this page: io_uring_register(2), signalfd(2), syscalls(2), timerfd_create(2), io_uring_register_eventfd(3), io_uring_register_eventfd_async(3), io_uring_unregister_eventfd(3), proc_pid_fd(5), proc_pid_fdinfo(5), systemd.exec(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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