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



NAME
       eventfd - create a file descriptor for event notification

SYNOPSIS
       #include <&lt;sys/eventfd.h>&gt;

       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-block-
              ing  (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 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.

              *  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 ker-
       nel-userspace bridge allowing, for example, functionalities  like  KAIO
       (kernel AIO) to signal to a file descriptor that some operation is com-
       plete.

       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 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 3.05 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/.



Linux                             2008-02-11                        EVENTFD(2)