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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.

       The flags argument is currently unused, and must be specified as  zero.
       In the future, it may be used to request additional functionality.

       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  (via  the use of the fcntl(2) F_SETFL operation to set
              the O_NONBLOCK flag).

              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.

              *  The  file  descriptor indicates an exceptional condition (the
                 select(2) exceptfds argument; the poll(2) POLLERR flag) if an
                 overflow  of the counter value was detected.  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 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.

CONFORMING TO

       eventfd() is 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).)

       The  flags  argument is a glibc addition to the underlying system call,
       which takes only the initval argument.

   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

       #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 3.01 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/.