eventfd_write


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 following values may be bitwise ORed in flags to change the  behav-
       iour 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  new open file
              description.  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.

       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)
              Each  successful  read(2)  returns an 8-byte integer.  A read(2)
              will fail with the error EINVAL if the size of the supplied buf-
              fer is less than 8 bytes.

              The  value  returned by read(2) is in host byte order, i.e., 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.

              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.

              *  If an overflow  of  the  counter  value  was  detected,  then
                 select(2)  indicates  the file descriptor as being both read-
                 able and writable, and poll(2) returns a POLLERR  event.   As
                 noted  above,  write(2) can never overflow the counter.  How-
                 ever 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), 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.


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.

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  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).)

   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
           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",

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-08-30                        EVENTFD(2)
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