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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) will fail 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:

*

If EFD_SEMAPHORE was not specified and the eventfd counter has a nonzero value, then a read(2) returns 8 bytes containing that value, and the counter’s value is reset to zero.

*

If EFD_SEMAPHORE was specified and the eventfd counter has a nonzero value, then a read(2) returns 8 bytes containing the value 1, and the counter’s value is decremented by 1.

*

If the eventfd counter is zero at the time of the call to read(2) , then the call either blocks until the counter becomes nonzero (at which time, the read(2) proceeds as described above) or fails with the error EAGAIN if the file descriptor has been made nonblocking.

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) 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) 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 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

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) .)

C library/kernel ABI differences

Additional glibc features

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 14Child 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");
    }
}

See Also

Colophon

Table of Contents

• Name
• Synopsis
• Description
• Return Value
• Errors
• Versions
• Conforming to
• Notes
• C library/kernel ABI differences
• Additional glibc features
• Example
• Program source
• See Also
• Colophon