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


     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  behav-
     ior 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:
            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 contain-

               ing 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  non-
            A  write(2)  fails with the error EINVAL if the size of the sup-
            plied 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.,
            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 the number of 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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     |Interface | Attribute     | Value   |
     |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,  functionali-
     ties  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  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 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 details.
 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
 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)
     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]);
         efd = eventfd(0, 0);
         if (efd == -1)
         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))
             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 %llu (0x%llx) from efd\n",
                     (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 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 EVENTFD(2)

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