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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 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.

       Starting with Linux 2.6.27, the following values may be bitwise ORed in
       flags to change the behaviour of eventfd():

       EFD_NONBLOCK  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_CLOEXEC   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.

       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:

	      If the eventfd counter has a  non-zero  value,  then  a  read(2)
	      returns  8  bytes containing that value, and the counter's value
	      is reset to zero.	 (The returned value is in  host  byte	order,
	      i.e., the native byte order for integers on the host machine.)

	      If the counter is zero at the time of the read(2), then the call
	      either blocks until the counter becomes non-zero, or fails  with
	      the error EAGAIN if the file descriptor has been made non-block‐

	      A read(2) will fail with the error EINVAL if  the	 size  of  the
	      supplied buffer is less than 8 bytes.

	      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) 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.,

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

	      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).

       On success, eventfd() returns a new eventfd file descriptor.  On error,
       -1 is returned and errno is set to indicate the error.

       EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is  non-

       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

       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.

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

       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

   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)

       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 3.22 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				  2009-01-26			    EVENTFD(2)

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