eventfd(2) System Calls Manual eventfd(2)
NAME
eventfd - create a file descriptor for event notification
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/eventfd.h>
int eventfd(unsigned int initval, int flags);
DESCRIPTION
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 un-
signed 64-bit integer (uint64_t) counter that is maintained by the ker-
nel. This counter is initialized with the value specified in the argu-
ment 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 behav-
ior of eventfd():
EFD_CLOEXEC (since Linux 2.6.27)
Set the close-on-exec (FD_CLOEXEC) flag on the new file descrip-
tor. 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 re-
sult.
EFD_SEMAPHORE (since Linux 2.6.30)
Provide semaphore-like semantics for reads from the new file de-
scriptor. 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 re-
turned 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 contain-
ing 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 be-
comes nonzero (at which time, the read(2) proceeds as de-
scribed above) or fails with the error EAGAIN if the file de-
scriptor 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 un-
til a read(2) is performed on the file descriptor, or fails with
the error EAGAIN if the file descriptor has been made nonblock-
ing.
A write(2) fails with the error EINVAL if the size of the sup-
plied 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 argu-
ment; the poll(2) POLLIN flag) if the counter has a value
greater than 0.
• The file descriptor is writable (the select(2) writefds argu-
ment; 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 se-
lect(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 per-
formed 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-de-
scriptor 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.
RETURN VALUE
On success, eventfd() returns a new eventfd file descriptor. On error,
-1 is returned and errno is set to indicate the error.
ERRORS
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 de-
scriptor.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ eventfd() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
C library/kernel differences
There are two underlying Linux system calls: eventfd() and the more re-
cent eventfd2(). The former system call does not implement a flags ar-
gument. 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.
STANDARDS
Linux, GNU.
HISTORY
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 sup-
ported by the kernel.
NOTES
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, functionali-
ties 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 moni-
tored 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 di-
rectory. See proc(5) for further details.
EXAMPLES
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 ar-
guments 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 <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");
}
}
SEE ALSO
futex(2), pipe(2), poll(2), read(2), select(2), signalfd(2),
timerfd_create(2), write(2), epoll(7), sem_overview(7)
Linux man-pages 6.7 2023-10-31 eventfd(2)
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