eventfd_read(3) — Linux manual page


EVENTFD(2)                Linux Programmer's Manual               EVENTFD(2)

NAME         top

       eventfd - create a file descriptor for event notification

SYNOPSIS         top

       #include <sys/eventfd.h>

       int eventfd(unsigned int initval, int flags);

DESCRIPTION         top

       eventfd() creates an "eventfd object" that can be used as an event
       wait/notify mechanism by user-space applications, and by the kernel
       to notify user-space 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.

       As its return value, eventfd() returns a new file descriptor that can
       be used to refer to the eventfd object.

       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 open file
              description (see open(2)) referred to by the new file
              descriptor.  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.

       The following operations can be performed on the file descriptor
       returned by eventfd():

              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:

              *  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

              *  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.

              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 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).

              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         top

       On success, eventfd() returns a new eventfd file descriptor.  On
       error, -1 is returned and errno is set to indicate the error.

ERRORS         top

       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

VERSIONS         top

       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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value   │
       │eventfd() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       eventfd() and eventfd2() are Linux-specific.

NOTES         top

       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
       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.

   C library/kernel differences
       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.

EXAMPLES         top

       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

       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 <inttypes.h>           /* Definition of PRIu64 & PRIx64 */
       #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)

       main(int argc, char *argv[])
           int efd;
           uint64_t u;
           ssize_t s;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <num>...\n", argv[0]);

           efd = eventfd(0, 0);
           if (efd == -1)

           switch (fork()) {
           case 0:
               for (int 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))
               printf("Child completed write loop\n");



               printf("Parent about to read\n");
               s = read(efd, &u, sizeof(uint64_t));
               if (s != sizeof(uint64_t))
               printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u);

           case -1:

SEE ALSO         top

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

COLOPHON         top

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       latest version of this page, can be found at

Linux                            2020-11-01                       EVENTFD(2)