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.
eventfd() is available since Linux 2.6.22. Working support is provided
since glibc 2.8. The
eventfd2() system call (see NOTES) is available
since Linux 2.6.27. Since glibc 2.9, the
eventfd() wrapper will employ
the
eventfd2() system call, if it is supported by the kernel.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
eventfd () |
Thread safety |
MT-Safe |
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 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.
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 <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");
}
}
futex(2),
pipe(2),
poll(2),
read(2),
select(2),
signalfd(2),
timerfd_create(2),
write(2),
epoll(7),
sem_overview(7)