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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 userspace applications, and by the  kernel  to
       notify  userspace  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.

       Starting with Linux 2.6.27, the following values may be bitwise ORed in
       flags to change the behaviour of eventfd():

       EFD_NONBLOCK  Set  the O_NONBLOCK file status flag on the new open file
                     description.   Using  this  flag  saves  extra  calls  to
                     fcntl(2) to achieve the same result.

       EFD_CLOEXEC   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.

       In  Linux  up to version 2.6.26, the flags argument is unused, and must
       be specified as zero.

       As its return value, eventfd() returns a new file descriptor  that  can
       be  used  to refer to the eventfd object.  The following operations can
       be performed on the file descriptor:

       read(2)
              If the eventfd counter has a  non-zero  value,  then  a  read(2)
              returns  8  bytes containing that value, and the counter’s value
              is reset to zero.  (The returned value is in  host  byte  order,
              i.e., the native byte order for integers on the host machine.)

              If the counter is zero at the time of the read(2), then the call
              either blocks until the counter becomes non-zero, or fails  with
              the  error  EAGAIN  if  the  file  descriptor has been made non-
              blocking.

              A read(2) will fail with the error EINVAL if  the  size  of  the
              supplied buffer is less than 8 bytes.

       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 non-
              blocking.

              A write(2) will fail 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),   ppoll(2),   and
              epoll(7).

       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).

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 flags is invalid; or, in Linux 2.6.26 or earlier, flags is  non-
              zero.

       EMFILE The per-process limit on 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.

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
       kernel-userspace  bridge  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).)

   Underlying Linux system calls
       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.

EXAMPLE

       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 <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, j;
           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 (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 %llu (0x%llx) from efd\n",
                       (unsigned long long) u, (unsigned long long) 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 3.15 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.