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EVENTFD(2)		   Linux Programmer's Manual		    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	behav‐
       iour 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:

       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 buf‐
	      fer 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 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.

       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  non‐
	      blocking.

	      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	 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.,
		 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 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
       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).)

   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 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2010-08-30			    EVENTFD(2)
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