EVENTFD
Section: Linux Programmer's Manual (2)
Updated: 2021-03-22
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NAME
eventfd - create a file descriptor for event notification
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 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.
In Linux up to version 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.
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 descriptor.
VERSIONS
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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface | Attribute | Value
|
eventfd()
| Thread safety | MT-Safe
|
CONFORMING TO
eventfd()
and
eventfd2()
are Linux-specific.
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, 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.
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.
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 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 <sys/eventfd.h>
#include <unistd.h>
#include <inttypes.h> /* Definition of PRIu64 & PRIx64 */
#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;
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 (int 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 %"PRIu64" (%#"PRIx64") from efd\n", u, u);
exit(EXIT_SUCCESS);
case -1:
handle_error("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)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- Additional glibc features
-
- EXAMPLES
-
- Program source
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:44 GMT, May 09, 2021