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EVENTFD(2) Linux Programmer's Manual EVENTFD(2)
eventfd - create a file descriptor for event notification
#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 userspace applications, and by the kernel to notify userspace
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.
The following values may be bitwise ORed in flags to change the behaviour 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 new open file description.
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.
In Linux up to version 2.6.26, the flags argument is unused, and must be
specified as zero.
As its return value, eventfd() returns a new file descriptor that can be used
to refer to the eventfd object. The following operations can be performed on
the file descriptor:
read(2)
Each successful read(2) returns an 8-byte integer. A read(2) will fail
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, i.e., 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) will fail 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), ppoll(2), and epoll(7).
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 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.
eventfd() is available on Linux since kernel 2.6.22. Working support is
provided in glibc since version 2.8. The eventfd2() system call (see NOTES)
is available on Linux since kernel 2.6.27. Since version 2.9, the glibc
eventfd() wrapper will employ the eventfd2() system call, if it is supported
by the kernel.
eventfd() and eventfd2() are Linux-specific.
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 kernel-
userspace bridge 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).)
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.
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.
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
#include <sys/eventfd.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h> /* Definition of uint64_t */
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
int efd, j;
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)
handle_error("eventfd");
switch (fork()) {
case 0:
for (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))
handle_error("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))
handle_error("read");
printf("Parent read %llu (0x%llx) from efd\n",
(unsigned long long) u, (unsigned long long) u);
exit(EXIT_SUCCESS);
case -1:
handle_error("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 release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-08-30 EVENTFD(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface