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       eventfd - create a file descriptor for event notification


       #include <sys/eventfd.h>

       int eventfd(unsigned int initval, int flags);


       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.

       The following values may  be  bitwise  ORed  in  flags  to  change  the
       behaviour 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  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:

              Each  successful  read(2)  returns an 8-byte integer.  A read(2)
              will fail 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

              *  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.

              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

              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.,

              The  eventfd  file  descriptor  also  supports  the  other file-
              descriptor multiplexing APIs: pselect(2) and ppoll(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


       On success, eventfd() returns a new eventfd file descriptor.  On error,
       -1 is returned and errno is set to indicate the error.


       EINVAL An unsupported value was specified in flags.

       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

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There  was  insufficient  memory  to  create  a new eventfd file


       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.


       eventfd() and eventfd2() are Linux-specific.


       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).)

   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

   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.


       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)

       main(int argc, char *argv[])
           int efd, j;
           uint64_t u;
           ssize_t s;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <num>...
", argv[0]);

           efd = eventfd(0, 0);
           if (efd == -1)

           switch (fork()) {
           case 0:
               for (j = 1; j < argc; j++) {
                   printf("Child writing %s to efd
", argv[j]);
                   u = strtoull(argv[j], NULL, 0);
                           /* strtoull() allows various bases */
                   s = write(efd, &u, sizeof(uint64_t));
                   if (s != sizeof(uint64_t))
               printf("Child completed write loop



               printf("Parent about to read
               s = read(efd, &u, sizeof(uint64_t));
               if (s != sizeof(uint64_t))
               printf("Parent read %llu (0x%llx) from efd
                       (unsigned long long) u, (unsigned long long) u);

           case -1:


       futex(2),    pipe(2),   poll(2),   read(2),   select(2),   signalfd(2),
       timerfd_create(2), write(2), epoll(7), sem_overview(7)


       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

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