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

       eventfd - create a file descriptor for event notification

       #include <sys/eventfd.h>

       int eventfd(unsigned int initval, int flags);

       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 un-
       signed 64-bit integer (uint64_t) counter that is maintained by the ker-
       nel.  This counter is initialized with the value specified in the argu-
       ment 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 behav-
       ior of eventfd():

       EFD_CLOEXEC (since Linux 2.6.27)
              Set the close-on-exec (FD_CLOEXEC) flag on the new file descrip-
              tor.   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  re-

       EFD_SEMAPHORE (since Linux 2.6.30)
              Provide semaphore-like semantics for reads from the new file de-
              scriptor.  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 re-
       turned 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  contain-
                 ing 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  be-
                 comes  nonzero  (at  which  time, the read(2) proceeds as de-
                 scribed above) or fails with the error EAGAIN if the file de-
                 scriptor 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 un-
              til a read(2) is performed on the file descriptor, or fails with
              the  error EAGAIN if the file descriptor has been made nonblock-

              A write(2) fails with the error EINVAL if the size of  the  sup-
              plied  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 argu-
                 ment; the poll(2) POLLIN flag) if the  counter  has  a  value
                 greater than 0.

              *  The file descriptor is writable (the select(2) writefds argu-
                 ment; 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 se-
                 lect(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 per-
                 formed 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.,

              The  eventfd  file  descriptor  also supports the other file-de-
              scriptor 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

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

       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

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There was insufficient memory to create a new eventfd  file  de-

       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.

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

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

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

       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, functionali-
       ties 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 moni-
       tored 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 re-
       cent eventfd2().  The former system call does not implement a flags ar-
       gument.   The  latter system call implements the flags values described
       above.  The glibc wrapper function will  use  eventfd2()  where  it  is

   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.

       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 ar-
       guments 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 <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, j;
           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 (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 %llu (0x%llx) from efd\n",
                       (unsigned long long) u, (unsigned long long) u);

           case -1:

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

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Linux                             2019-03-06                        EVENTFD(2)
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