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           #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 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 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:
                  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--that is, 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.
                  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) and ppoll(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


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

    Linux 2010-08-30 EVENTFD(2)


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