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_SEMAPHOREwas 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_SEMAPHOREwas 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)
readfdsargument; the poll(2)POLLINflag) if the counter has a value greater than 0.The file descriptor is writable (the select(2)
writefdsargument; the poll(2)POLLOUTflag) 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
POLLERRevent. 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.
An unsupported value was specified in flags.
The per-process limit on open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
Could not mount (internal) anonymous inode device.
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.
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 3.33 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at http://man7.org/linux/man-pages/.
|
Copyright (C) 2008 Michael Kerrisk <mtk.manpagesgmail.com> starting from a version by Davide Libenzi <davidelxmailserver.org> This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA 2008-10-10, mtk: describe eventfd2(), and EFD_NONBLOCK and EFD_CLOEXEC |