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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).

       ┌──────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────┼───────────────┼─────────┤
       │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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.