EVENTFD(2) Linux Programmers Manual EVENTFD(2)
NAME
eventfd - create a file descriptor for event notification
SYNOPSIS
#include
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.
The flags argument is currently unused, and must be specified as zero.
In the future, it may be used to request additional functionality.
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 counters 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 (via the use of the fcntl(2) F_SETFL operation to set the
O_NONBLOCK flag).
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 counters
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-
blocking.
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 argu
ment; the poll(2) POLLIN flag) if the counter has a value
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.
* The file descriptor indicates an exceptional condition (the
select(2) exceptfds argument; the poll(2) POLLERR flag) if an
overflow of the counter value was detected. 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).
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 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.
CONFORMING TO
eventfd() is 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 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).)
The flags argument is a glibc addition to the underlying system call,
which takes only the initval argument.
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 programs 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
#include
#include
#include
#include /* 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 ...\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
This page is part of release 3.05 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 2008-02-11 EVENTFD(2)
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