eventfd

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



NAME
       eventfd - create a file descriptor for event notification

SYNOPSIS
       #include <sys/eventfd.h>

       int eventfd(unsigned int initval, int flags);

DESCRIPTION
       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():

       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:

              *  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) 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).

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

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
       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
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                     Attribute     Value   │
       ├──────────────────────────────────────────────┼───────────────┼─────────┤
       │eventfd()                                     │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO
       eventfd() and eventfd2() are 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 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
       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 <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)

       int
       main(int argc, char *argv[])
       {
           int efd;
           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 (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))
                       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 %"PRIu64" (%#"PRIx64") from efd\n", u, 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)

COLOPHON
       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                         EVENTFD(2)