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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 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) pro-
		 ceeds	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 block-
		 ing.

	      *  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.   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  over-
		 flow  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, 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  applica-
       tion  can simultaneously monitor the readiness of "traditional" files and the readiness of
       other kernel mechanisms that support the eventfd interface.  (Without the eventfd() inter-
       face, 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 imple-
       ments  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, return-
       ing 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 3.55 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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