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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 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	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),	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  ker‐
       nel-userspace  bridge  allowing, for example, functionalities like KAIO
       (kernel AIO) to signal to a file descriptor that some operation is com‐
       plete.

       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 3.27 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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