EVENTFD

Section: Linux Programmer's Manual (2)
Updated: 2009-01-26
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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 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-blocking.

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-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 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), 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 call, if it is supported by the kernel.

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 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).)

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

#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");
    }
}

COLOPHON

This page is part of release 3.22 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/.

EXAMPLE

Program source

SEE ALSO

COLOPHON

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Time: 05:33:03 GMT, December 24, 2015