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:

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

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 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).)

The flags argument is a glibc addition to the underlying system call, which takes only the initval argument.

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:

#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");
    }
}

SEE ALSO

futex(2), pipe(2), poll(2), read(2), select(2), signalfd(2), timerfd_create(2), write(2), epoll(7), sem_overview(7)