eventfd(2)                                              System Calls Manual                                              eventfd(2)

       eventfd - create a file descriptor for event notification

       Standard C library (libc, -lc)

       #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.

       As its return value, eventfd() returns a new file descriptor that can be used to refer to the eventfd object.

       The following values may be bitwise ORed in flags to change the behavior 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 open file description (see open(2)) referred to by the new  file  descrip‐
              tor.  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.

       Up to Linux 2.6.26, the flags argument is unused, and must be specified as zero.

       The following operations can be performed on the file descriptor returned by eventfd():

              Each successful read(2) returns an 8-byte integer.  A read(2) fails 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_SEM‐
              APHORE 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  con‐
                 taining 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.

              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) fails 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 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.
                 However  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).

              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  de‐
       scriptor  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.

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

       EINVAL An unsupported value was specified in flags.

       EMFILE The per-process limit on the number of 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.

       eventfd()  is  available  since Linux 2.6.22.  Working support is provided since glibc 2.8.  The eventfd2() system call (see
       NOTES) is available since Linux 2.6.27.  Since glibc 2.9, the eventfd() wrapper will employ the eventfd2() system  call,  if
       it is supported by the kernel.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                                                                        │ Attribute     │ Value   │
       │eventfd()                                                                                        │ Thread safety │ MT-Safe │

       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  de‐
       scriptor 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  se‐
       lect(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).)

       The  current  value of an eventfd counter can be viewed via the entry for the corresponding file descriptor in the process's
       /proc/pid/fdinfo directory.  See proc(5) for further details.

   C library/kernel differences
       There are two underlying Linux system calls: eventfd() and the more recent eventfd2().  The former system call does not  im‐
       plement  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.

       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 descrip‐
       tor.  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 <err.h>
       #include <inttypes.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/eventfd.h>
       #include <unistd.h>

       main(int argc, char *argv[])
           int       efd;
           uint64_t  u;
           ssize_t   s;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <num>...\n", argv[0]);

           efd = eventfd(0, 0);
           if (efd == -1)
               err(EXIT_FAILURE, "eventfd");

           switch (fork()) {
           case 0:
               for (size_t 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))
                       err(EXIT_FAILURE, "write");
               printf("Child completed write loop\n");



               printf("Parent about to read\n");
               s = read(efd, &u, sizeof(uint64_t));
               if (s != sizeof(uint64_t))
                   err(EXIT_FAILURE, "read");
               printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u);

           case -1:
               err(EXIT_FAILURE, "fork");

       futex(2), pipe(2), poll(2), read(2), select(2), signalfd(2), timerfd_create(2), write(2), epoll(7), sem_overview(7)

Linux man-pages 6.03                                         2023-02-10                                                  eventfd(2)