eventfd(2)


NAME

   eventfd - create a file descriptor for event notification

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 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
   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  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
          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.

   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
          nonblocking.

          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.,
             0xffffffffffffffff).

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

   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.

ERRORS

   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.

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.

ATTRIBUTES

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

   
   Interface  Attribute      Value   
   
   eventfd()  Thread safety  MT-Safe 
   

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
   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).)

   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
   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
   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]);
           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 4.09 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
   https://www.kernel.org/doc/man-pages/.


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