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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 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 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:

       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, 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 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), 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.

ERRORS

       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 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.

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 kernel-userspace bridge
       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 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 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/.