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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.

       As its return value, eventfd() returns a new file descriptor that can be used to refer  to
       the eventfd object.

       The following values may be bitwise ORed in flags to change the behavior 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  open  file  description  (see  open(2))
              referred  to  by  the  new  file  descriptor.  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

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

       The following operations can be performed on the file descriptor returned by eventfd():

              Each successful read(2) returns an 8-byte integer.  A read(2) fails 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 zero.

              *  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 nonblocking.

              A write(2) fails 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

              *  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) 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 set.


       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 the number of 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.


       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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue   │
       │eventfd() │ Thread safety │ MT-Safe │


       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

       The  current value of an eventfd counter can be viewed via the entry for the corresponding
       file descriptor in the process's /proc/[pid]/fdinfo directory.  See  proc(5)  for  further

   C library/kernel differences
       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.


       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 <inttypes.h>           /* Definition of PRIu64 & PRIx64 */
       #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;
           uint64_t u;
           ssize_t s;

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

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

           switch (fork()) {
           case 0:
               for (int 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))
               printf("Child completed write loop\n");



               printf("Parent about to read\n");
               s = read(efd, &u, sizeof(uint64_t));
               if (s != sizeof(uint64_t))
               printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, 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)


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