eventfd(2) — Linux manual page


eventfd(2)                 System Calls Manual                eventfd(2)

NAME         top

       eventfd - create a file descriptor for event notification

LIBRARY         top

       Standard C library (libc, -lc)

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.

       Up to Linux 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

              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

              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.

       (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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ eventfd()                           │ Thread safety │ MT-Safe │

VERSIONS         top

   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.

STANDARDS         top

       Linux, GNU.

HISTORY         top

              Linux 2.6.22, glibc 2.8.

              Linux 2.6.27 (see VERSIONS).  Since glibc 2.9, the
              eventfd() wrapper will employ the eventfd2() system call,
              if it is supported by the kernel.

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.

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 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 <err.h>
       #include <inttypes.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/eventfd.h>
       #include <sys/types.h>
       #include <unistd.h>

       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)
               err(EXIT_FAILURE, "eventfd");

           switch (fork()) {
           case 0:
               for (size_t 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))
                       err(EXIT_FAILURE, "write");
               printf("Child completed write loop\n");



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

           case -1:
               err(EXIT_FAILURE, "fork");

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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Linux man-pages 6.9.1          2024-06-15                     eventfd(2)

Pages that refer to this page: signalfd(2)syscalls(2)timerfd_create(2)io_uring_register_eventfd(3)io_uring_register_eventfd_async(3)io_uring_unregister_eventfd(3)proc_pid_fd(5)proc_pid_fdinfo(5)systemd.exec(5)