NAME¶

SYNOPSIS¶

int eventfd(unsigned int initval, int flags);

DESCRIPTION¶

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 new open file description. 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.

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)

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:

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 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.

RETURN VALUE¶

ERRORS¶

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.

VERSIONS¶

ATTRIBUTES¶

CONFORMING TO¶

NOTES¶

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.

typedef uint64_t eventfd_t;
int eventfd_read(int fd, eventfd_t *value);
int eventfd_write(int fd, eventfd_t value);

EXAMPLE¶

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 <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¶

COLOPHON¶