1EVENTFD(2)                 Linux Programmer's Manual                EVENTFD(2)
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NAME

6       eventfd - create a file descriptor for event notification
7

SYNOPSIS

9       #include <sys/eventfd.h>
10
11       int eventfd(unsigned int initval, int flags);
12

DESCRIPTION

14       eventfd()  creates  an  "eventfd  object"  that can be used as an event
15       wait/notify mechanism by user-space applications, and by the kernel  to
16       notify  user-space  applications  of  events.   The  object contains an
17       unsigned 64-bit integer (uint64_t) counter that is  maintained  by  the
18       kernel.   This  counter  is initialized with the value specified in the
19       argument initval.
20
21       The following values may be bitwise ORed in flags to change the  behav‐
22       ior of eventfd():
23
24       EFD_CLOEXEC (since Linux 2.6.27)
25              Set the close-on-exec (FD_CLOEXEC) flag on the new file descrip‐
26              tor.  See the description of the O_CLOEXEC flag in  open(2)  for
27              reasons why this may be useful.
28
29       EFD_NONBLOCK (since Linux 2.6.27)
30              Set  the  O_NONBLOCK  file  status  flag  on  the  new open file
31              description.  Using this flag saves extra calls to  fcntl(2)  to
32              achieve the same result.
33
34       EFD_SEMAPHORE (since Linux 2.6.30)
35              Provide  semaphore-like  semantics  for  reads from the new file
36              descriptor.  See below.
37
38       In Linux up to version 2.6.26, the flags argument is unused,  and  must
39       be specified as zero.
40
41       As  its  return value, eventfd() returns a new file descriptor that can
42       be used to refer to the eventfd object.  The following  operations  can
43       be performed on the file descriptor:
44
45       read(2)
46              Each  successful  read(2)  returns an 8-byte integer.  A read(2)
47              fails with the error EINVAL if the size of the  supplied  buffer
48              is less than 8 bytes.
49
50              The value returned by read(2) is in host byte order—that is, the
51              native byte order for integers on the host machine.
52
53              The semantics of read(2) depend on whether the  eventfd  counter
54              currently has a nonzero value and whether the EFD_SEMAPHORE flag
55              was specified when creating the eventfd file descriptor:
56
57              *  If EFD_SEMAPHORE was not specified and  the  eventfd  counter
58                 has  a nonzero value, then a read(2) returns 8 bytes contain‐
59                 ing that value, and the counter's value is reset to zero.
60
61              *  If EFD_SEMAPHORE was specified and the eventfd counter has  a
62                 nonzero  value, then a read(2) returns 8 bytes containing the
63                 value 1, and the counter's value is decremented by 1.
64
65              *  If the eventfd counter is zero at the time  of  the  call  to
66                 read(2),  then  the  call  either  blocks  until  the counter
67                 becomes nonzero (at  which  time,  the  read(2)  proceeds  as
68                 described  above)  or fails with the error EAGAIN if the file
69                 descriptor has been made nonblocking.
70
71       write(2)
72              A write(2) call adds the 8-byte integer value  supplied  in  its
73              buffer  to the counter.  The maximum value that may be stored in
74              the counter is the largest unsigned 64-bit value minus 1  (i.e.,
75              0xfffffffffffffffe).   If the addition would cause the counter's
76              value to exceed the maximum, then  the  write(2)  either  blocks
77              until  a  read(2)  is performed on the file descriptor, or fails
78              with the error EAGAIN if the file descriptor has been made  non‐
79              blocking.
80
81              A  write(2)  fails with the error EINVAL if the size of the sup‐
82              plied buffer is less than 8 bytes, or if an attempt is  made  to
83              write the value 0xffffffffffffffff.
84
85       poll(2), select(2) (and similar)
86              The  returned  file descriptor supports poll(2) (and analogously
87              epoll(7)) and select(2), as follows:
88
89              *  The file descriptor is readable (the select(2) readfds  argu‐
90                 ment;  the  poll(2)  POLLIN  flag) if the counter has a value
91                 greater than 0.
92
93              *  The file descriptor is writable (the select(2) writefds argu‐
94                 ment;  the poll(2) POLLOUT flag) if it is possible to write a
95                 value of at least "1" without blocking.
96
97              *  If an overflow  of  the  counter  value  was  detected,  then
98                 select(2)  indicates  the file descriptor as being both read‐
99                 able and writable, and poll(2) returns a POLLERR  event.   As
100                 noted  above,  write(2) can never overflow the counter.  How‐
101                 ever an overflow can occur if  2^64  eventfd  "signal  posts"
102                 were performed by the KAIO subsystem (theoretically possible,
103                 but practically unlikely).  If an overflow has occurred, then
104                 read(2)  will  return  that  maximum  uint64_t  value  (i.e.,
105                 0xffffffffffffffff).
106
107              The eventfd  file  descriptor  also  supports  the  other  file-
108              descriptor multiplexing APIs: pselect(2) and ppoll(2).
109
110       close(2)
111              When  the  file  descriptor  is  no longer required it should be
112              closed.  When all file  descriptors  associated  with  the  same
113              eventfd  object  have  been closed, the resources for object are
114              freed by the kernel.
115
116       A copy of the file descriptor created by eventfd() is inherited by  the
117       child produced by fork(2).  The duplicate file descriptor is associated
118       with the same eventfd object.  File descriptors  created  by  eventfd()
119       are  preserved across execve(2), unless the close-on-exec flag has been
120       set.
121

RETURN VALUE

123       On success, eventfd() returns a new eventfd file descriptor.  On error,
124       -1 is returned and errno is set to indicate the error.
125

ERRORS

127       EINVAL An unsupported value was specified in flags.
128
129       EMFILE The per-process limit on the number of open file descriptors has
130              been reached.
131
132       ENFILE The system-wide limit on the total number of open files has been
133              reached.
134
135       ENODEV Could not mount (internal) anonymous inode device.
136
137       ENOMEM There  was  insufficient  memory  to  create  a new eventfd file
138              descriptor.
139

VERSIONS

141       eventfd() is available on Linux since kernel 2.6.22.   Working  support
142       is  provided  in  glibc  since version 2.8.  The eventfd2() system call
143       (see NOTES) is available on Linux since kernel 2.6.27.   Since  version
144       2.9,  the  glibc  eventfd()  wrapper  will employ the eventfd2() system
145       call, if it is supported by the kernel.
146

ATTRIBUTES

148       For  an  explanation  of  the  terms  used   in   this   section,   see
149       attributes(7).
150
151       ┌──────────┬───────────────┬─────────┐
152Interface Attribute     Value   
153       ├──────────┼───────────────┼─────────┤
154eventfd() │ Thread safety │ MT-Safe │
155       └──────────┴───────────────┴─────────┘
156

CONFORMING TO

158       eventfd() and eventfd2() are Linux-specific.
159

NOTES

161       Applications  can use an eventfd file descriptor instead of a pipe (see
162       pipe(2)) in all cases where a pipe is used  simply  to  signal  events.
163       The  kernel  overhead  of an eventfd file descriptor is much lower than
164       that of a pipe, and only one file descriptor is  required  (versus  the
165       two required for a pipe).
166
167       When  used  in  the  kernel,  an  eventfd file descriptor can provide a
168       bridge from kernel to user space, allowing, for  example,  functionali‐
169       ties  like  KAIO  (kernel AIO) to signal to a file descriptor that some
170       operation is complete.
171
172       A key point about an eventfd file descriptor is that it  can  be  moni‐
173       tored  just like any other file descriptor using select(2), poll(2), or
174       epoll(7).  This means that an application  can  simultaneously  monitor
175       the  readiness of "traditional" files and the readiness of other kernel
176       mechanisms that support the eventfd interface.  (Without the  eventfd()
177       interface,  these  mechanisms  could  not be multiplexed via select(2),
178       poll(2), or epoll(7).)
179
180       The current value of an eventfd counter can be viewed via the entry for
181       the  corresponding  file descriptor in the process's /proc/[pid]/fdinfo
182       directory.  See proc(5) for further details.
183
184   C library/kernel differences
185       There are two underlying Linux system calls:  eventfd()  and  the  more
186       recent  eventfd2().   The former system call does not implement a flags
187       argument.  The latter system call implements the flags values described
188       above.   The  glibc  wrapper  function  will use eventfd2() where it is
189       available.
190
191   Additional glibc features
192       The GNU C library defines an additional type, and  two  functions  that
193       attempt  to  abstract  some of the details of reading and writing on an
194       eventfd file descriptor:
195
196           typedef uint64_t eventfd_t;
197
198           int eventfd_read(int fd, eventfd_t *value);
199           int eventfd_write(int fd, eventfd_t value);
200
201       The functions perform the read and write operations on an eventfd  file
202       descriptor, returning 0 if the correct number of bytes was transferred,
203       or -1 otherwise.
204

EXAMPLE

206       The following program creates an eventfd file descriptor and then forks
207       to  create a child process.  While the parent briefly sleeps, the child
208       writes each of the integers  supplied  in  the  program's  command-line
209       arguments to the eventfd file descriptor.  When the parent has finished
210       sleeping, it reads from the eventfd file descriptor.
211
212       The following shell session shows a sample run of the program:
213
214           $ ./a.out 1 2 4 7 14
215           Child writing 1 to efd
216           Child writing 2 to efd
217           Child writing 4 to efd
218           Child writing 7 to efd
219           Child writing 14 to efd
220           Child completed write loop
221           Parent about to read
222           Parent read 28 (0x1c) from efd
223
224   Program source
225
226       #include <sys/eventfd.h>
227       #include <unistd.h>
228       #include <stdlib.h>
229       #include <stdio.h>
230       #include <stdint.h>             /* Definition of uint64_t */
231
232       #define handle_error(msg) \
233           do { perror(msg); exit(EXIT_FAILURE); } while (0)
234
235       int
236       main(int argc, char *argv[])
237       {
238           int efd, j;
239           uint64_t u;
240           ssize_t s;
241
242           if (argc < 2) {
243               fprintf(stderr, "Usage: %s <num>...\n", argv[0]);
244               exit(EXIT_FAILURE);
245           }
246
247           efd = eventfd(0, 0);
248           if (efd == -1)
249               handle_error("eventfd");
250
251           switch (fork()) {
252           case 0:
253               for (j = 1; j < argc; j++) {
254                   printf("Child writing %s to efd\n", argv[j]);
255                   u = strtoull(argv[j], NULL, 0);
256                           /* strtoull() allows various bases */
257                   s = write(efd, &u, sizeof(uint64_t));
258                   if (s != sizeof(uint64_t))
259                       handle_error("write");
260               }
261               printf("Child completed write loop\n");
262
263               exit(EXIT_SUCCESS);
264
265           default:
266               sleep(2);
267
268               printf("Parent about to read\n");
269               s = read(efd, &u, sizeof(uint64_t));
270               if (s != sizeof(uint64_t))
271                   handle_error("read");
272               printf("Parent read %llu (0x%llx) from efd\n",
273                       (unsigned long long) u, (unsigned long long) u);
274               exit(EXIT_SUCCESS);
275
276           case -1:
277               handle_error("fork");
278           }
279       }
280

SEE ALSO

282       futex(2),   pipe(2),   poll(2),   read(2),   select(2),    signalfd(2),
283       timerfd_create(2), write(2), epoll(7), sem_overview(7)
284

COLOPHON

286       This  page  is  part of release 4.16 of the Linux man-pages project.  A
287       description of the project, information about reporting bugs,  and  the
288       latest     version     of     this    page,    can    be    found    at
289       https://www.kernel.org/doc/man-pages/.
290
291
292
293Linux                             2017-09-15                        EVENTFD(2)
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