eventfd_write
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 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.
Starting with Linux 2.6.27, the following values may be bitwise ORed in
flags to change the behaviour of eventfd():
EFD_NONBLOCK 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_CLOEXEC 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.
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)
If the eventfd counter has a non-zero value, then a read(2)
returns 8 bytes containing that value, and the counter's value
is reset to zero. (The returned value is in host byte order,
i.e., the native byte order for integers on the host machine.)
If the counter is zero at the time of the read(2), then the call
either blocks until the counter becomes non-zero, or fails with
the error EAGAIN if the file descriptor has been made non-block-
ing.
A read(2) will fail with the error EINVAL if the size of the
supplied buffer is less than 8 bytes.
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 non-
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
select(2) indicates the file descriptor as being both read-
able and writable, and poll(2) returns a POLLERR event. As
noted above, write(2) can never overflow the counter. How-
ever 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).
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 flags is invalid; or, in Linux 2.6.26 or earlier, flags is non-
zero.
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.
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
When used in the kernel, an eventfd file descriptor can provide a ker-
nel-userspace bridge allowing, for example, functionalities like KAIO
(kernel AIO) to signal to a file descriptor that some operation is com-
plete.
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).)
Underlying Linux system calls
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.
EXAMPLE
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
{
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");
}
}
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
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