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 userspace applications, and by the  kernel  to
16       notify  userspace  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       Starting with Linux 2.6.27, the following values may be bitwise ORed in
22       flags to change the behaviour of eventfd():
23
24       EFD_NONBLOCK  Set  the O_NONBLOCK file status flag on the new open file
25                     description.   Using  this  flag  saves  extra  calls  to
26                     fcntl(2) to achieve the same result.
27
28       EFD_CLOEXEC   Set  the  close-on-exec (FD_CLOEXEC) flag on the new file
29                     descriptor.  See the description of the O_CLOEXEC flag in
30                     open(2) for reasons why this may be useful.
31
32       In  Linux  up to version 2.6.26, the flags argument is unused, and must
33       be specified as zero.
34
35       As its return value, eventfd() returns a new file descriptor  that  can
36       be  used  to refer to the eventfd object.  The following operations can
37       be performed on the file descriptor:
38
39       read(2)
40              If the eventfd counter has  a  nonzero  value,  then  a  read(2)
41              returns  8  bytes containing that value, and the counter's value
42              is reset to zero.  (The returned value is in  host  byte  order,
43              i.e., the native byte order for integers on the host machine.)
44
45              If the counter is zero at the time of the read(2), then the call
46              either blocks until the counter becomes nonzero, or  fails  with
47              the  error EAGAIN if the file descriptor has been made nonblock‐
48              ing.
49
50              A read(2) will fail with the error EINVAL if  the  size  of  the
51              supplied buffer is less than 8 bytes.
52
53       write(2)
54              A  write(2)  call  adds the 8-byte integer value supplied in its
55              buffer to the counter.  The maximum value that may be stored  in
56              the  counter is the largest unsigned 64-bit value minus 1 (i.e.,
57              0xfffffffffffffffe).  If the addition would cause the  counter's
58              value  to  exceed  the  maximum, then the write(2) either blocks
59              until a read(2) is performed on the file  descriptor,  or  fails
60              with  the error EAGAIN if the file descriptor has been made non‐
61              blocking.
62
63              A write(2) will fail with the error EINVAL if the  size  of  the
64              supplied  buffer  is less than 8 bytes, or if an attempt is made
65              to write the value 0xffffffffffffffff.
66
67       poll(2), select(2) (and similar)
68              The returned file descriptor supports poll(2)  (and  analogously
69              epoll(7)) and select(2), as follows:
70
71              *  The  file descriptor is readable (the select(2) readfds argu‐
72                 ment; the poll(2) POLLIN flag) if the  counter  has  a  value
73                 greater than 0.
74
75              *  The file descriptor is writable (the select(2) writefds argu‐
76                 ment; the poll(2) POLLOUT flag) if it is possible to write  a
77                 value of at least "1" without blocking.
78
79              *  If  an  overflow  of  the  counter  value  was detected, then
80                 select(2) indicates the file descriptor as being  both  read‐
81                 able  and  writable, and poll(2) returns a POLLERR event.  As
82                 noted above, write(2) can never overflow the  counter.   How‐
83                 ever  an  overflow  can  occur if 2^64 eventfd "signal posts"
84                 were performed by the KAIO subsystem (theoretically possible,
85                 but practically unlikely).  If an overflow has occurred, then
86                 read(2)  will  return  that  maximum  uint64_t  value  (i.e.,
87                 0xffffffffffffffff).
88
89              The  eventfd  file  descriptor  also  supports  the  other file-
90              descriptor  multiplexing   APIs:   pselect(2),   ppoll(2),   and
91              epoll(7).
92
93       close(2)
94              When  the  file  descriptor  is  no longer required it should be
95              closed.  When all file  descriptors  associated  with  the  same
96              eventfd  object  have  been closed, the resources for object are
97              freed by the kernel.
98
99       A copy of the file descriptor created by eventfd() is inherited by  the
100       child produced by fork(2).  The duplicate file descriptor is associated
101       with the same eventfd object.  File descriptors  created  by  eventfd()
102       are preserved across execve(2).
103

RETURN VALUE

105       On success, eventfd() returns a new eventfd file descriptor.  On error,
106       -1 is returned and errno is set to indicate the error.
107

ERRORS

109       EINVAL flags is invalid; or, in  Linux  2.6.26  or  earlier,  flags  is
110              nonzero.
111
112       EMFILE The per-process limit on open file descriptors has been reached.
113
114       ENFILE The system-wide limit on the total number of open files has been
115              reached.
116
117       ENODEV Could not mount (internal) anonymous inode device.
118
119       ENOMEM There was insufficient memory  to  create  a  new  eventfd  file
120              descriptor.
121

VERSIONS

123       eventfd()  is  available on Linux since kernel 2.6.22.  Working support
124       is provided in glibc since version 2.8.   The  eventfd2()  system  call
125       (see  NOTES)  is available on Linux since kernel 2.6.27.  Since version
126       2.9, the glibc eventfd() wrapper  will  employ  the  eventfd2()  system
127       call, if it is supported by the kernel.
128

CONFORMING TO

130       eventfd() and eventfd2() are Linux-specific.
131

NOTES

133       Applications  can use an eventfd file descriptor instead of a pipe (see
134       pipe(2)) in all cases where a pipe is used  simply  to  signal  events.
135       The  kernel  overhead  of an eventfd file descriptor is much lower than
136       that of a pipe, and only one file descriptor is  required  (versus  the
137       two required for a pipe).
138
139       When  used in the kernel, an eventfd file descriptor can provide a ker‐
140       nel-userspace bridge allowing, for example, functionalities  like  KAIO
141       (kernel AIO) to signal to a file descriptor that some operation is com‐
142       plete.
143
144       A key point about an eventfd file descriptor is that it  can  be  moni‐
145       tored  just like any other file descriptor using select(2), poll(2), or
146       epoll(7).  This means that an application  can  simultaneously  monitor
147       the  readiness of "traditional" files and the readiness of other kernel
148       mechanisms that support the eventfd interface.  (Without the  eventfd()
149       interface,  these  mechanisms  could  not be multiplexed via select(2),
150       poll(2), or epoll(7).)
151
152   Underlying Linux system calls
153       There are two underlying Linux system calls:  eventfd()  and  the  more
154       recent  eventfd2().   The former system call does not implement a flags
155       argument.  The latter system call implements the flags values described
156       above.   The  glibc  wrapper  function  will use eventfd2() where it is
157       available.
158
159   Additional glibc features
160       The GNU C library defines an additional type, and  two  functions  that
161       attempt  to  abstract  some of the details of reading and writing on an
162       eventfd file descriptor:
163
164           typedef uint64_t eventfd_t;
165
166           int eventfd_read(int fd, eventfd_t *value);
167           int eventfd_write(int fd, eventfd_t value);
168
169       The functions perform the read and write operations on an eventfd  file
170       descriptor, returning 0 if the correct number of bytes was transferred,
171       or -1 otherwise.
172

EXAMPLE

174       The following program creates an eventfd file descriptor and then forks
175       to  create a child process.  While the parent briefly sleeps, the child
176       writes each of the integers  supplied  in  the  program's  command-line
177       arguments to the eventfd file descriptor.  When the parent has finished
178       sleeping, it reads from the eventfd file descriptor.
179
180       The following shell session shows a sample run of the program:
181
182           $ ./a.out 1 2 4 7 14
183           Child writing 1 to efd
184           Child writing 2 to efd
185           Child writing 4 to efd
186           Child writing 7 to efd
187           Child writing 14 to efd
188           Child completed write loop
189           Parent about to read
190           Parent read 28 (0x1c) from efd
191
192   Program source
193
194       #include <sys/eventfd.h>
195       #include <unistd.h>
196       #include <stdlib.h>
197       #include <stdio.h>
198       #include <stdint.h>             /* Definition of uint64_t */
199
200       #define handle_error(msg) \
201           do { perror(msg); exit(EXIT_FAILURE); } while (0)
202
203       int
204       main(int argc, char *argv[])
205       {
206           int efd, j;
207           uint64_t u;
208           ssize_t s;
209
210           if (argc < 2) {
211               fprintf(stderr, "Usage: %s <num>...\n", argv[0]);
212               exit(EXIT_FAILURE);
213           }
214
215           efd = eventfd(0, 0);
216           if (efd == -1)
217               handle_error("eventfd");
218
219           switch (fork()) {
220           case 0:
221               for (j = 1; j < argc; j++) {
222                   printf("Child writing %s to efd\n", argv[j]);
223                   u = strtoull(argv[j], NULL, 0);
224                           /* strtoull() allows various bases */
225                   s = write(efd, &u, sizeof(uint64_t));
226                   if (s != sizeof(uint64_t))
227                       handle_error("write");
228               }
229               printf("Child completed write loop\n");
230
231               exit(EXIT_SUCCESS);
232
233           default:
234               sleep(2);
235
236               printf("Parent about to read\n");
237               s = read(efd, &u, sizeof(uint64_t));
238               if (s != sizeof(uint64_t))
239                   handle_error("read");
240               printf("Parent read %llu (0x%llx) from efd\n",
241                       (unsigned long long) u, (unsigned long long) u);
242               exit(EXIT_SUCCESS);
243
244           case -1:
245               handle_error("fork");
246           }
247       }
248

SEE ALSO

250       futex(2),   pipe(2),   poll(2),   read(2),   select(2),    signalfd(2),
251       timerfd_create(2), write(2), epoll(7), sem_overview(7)
252

COLOPHON

254       This  page  is  part of release 3.25 of the Linux man-pages project.  A
255       description of the project, and information about reporting  bugs,  can
256       be found at http://www.kernel.org/doc/man-pages/.
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260Linux                             2009-01-26                        EVENTFD(2)
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