1poll(2)                       System Calls Manual                      poll(2)
2
3
4

NAME

6       poll, ppoll - wait for some event on a file descriptor
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <poll.h>
13
14       int poll(struct pollfd *fds, nfds_t nfds, int timeout);
15
16       #define _GNU_SOURCE         /* See feature_test_macros(7) */
17       #include <poll.h>
18
19       int ppoll(struct pollfd *fds, nfds_t nfds,
20                 const struct timespec *_Nullable tmo_p,
21                 const sigset_t *_Nullable sigmask);
22

DESCRIPTION

24       poll()  performs a similar task to select(2): it waits for one of a set
25       of file descriptors to become ready to perform I/O.  The Linux-specific
26       epoll(7)  API performs a similar task, but offers features beyond those
27       found in poll().
28
29       The set of file descriptors to be monitored is specified in the fds ar‐
30       gument, which is an array of structures of the following form:
31
32           struct pollfd {
33               int   fd;         /* file descriptor */
34               short events;     /* requested events */
35               short revents;    /* returned events */
36           };
37
38       The caller should specify the number of items in the fds array in nfds.
39
40       The  field  fd  contains  a  file descriptor for an open file.  If this
41       field is negative, then the corresponding events field is  ignored  and
42       the revents field returns zero.  (This provides an easy way of ignoring
43       a file descriptor for a single poll() call: simply set the fd field  to
44       its bitwise complement.)
45
46       The  field  events  is  an  input  parameter, a bit mask specifying the
47       events the application is interested in for  the  file  descriptor  fd.
48       This field may be specified as zero, in which case the only events that
49       can be returned in revents are POLLHUP, POLLERR, and POLLNVAL (see  be‐
50       low).
51
52       The field revents is an output parameter, filled by the kernel with the
53       events that actually occurred.  The bits returned in  revents  can  in‐
54       clude  any  of those specified in events, or one of the values POLLERR,
55       POLLHUP, or POLLNVAL.  (These three bits are meaningless in the  events
56       field,  and will be set in the revents field whenever the corresponding
57       condition is true.)
58
59       If none of the events requested (and no error) has occurred for any  of
60       the  file  descriptors,  then poll() blocks until one of the events oc‐
61       curs.
62
63       The timeout argument specifies the number of milliseconds  that  poll()
64       should  block  waiting for a file descriptor to become ready.  The call
65       will block until either:
66
67       •  a file descriptor becomes ready;
68
69       •  the call is interrupted by a signal handler; or
70
71       •  the timeout expires.
72
73       Note that the timeout interval will be rounded up to the  system  clock
74       granularity, and kernel scheduling delays mean that the blocking inter‐
75       val may overrun by a small amount.   Specifying  a  negative  value  in
76       timeout means an infinite timeout.  Specifying a timeout of zero causes
77       poll() to return immediately, even if no file descriptors are ready.
78
79       The bits that may be set/returned in events and revents are defined  in
80       <poll.h>:
81
82       POLLIN There is data to read.
83
84       POLLPRI
85              There  is  some  exceptional  condition  on the file descriptor.
86              Possibilities include:
87
88              •  There is out-of-band data on a TCP socket (see tcp(7)).
89
90              •  A pseudoterminal master in  packet  mode  has  seen  a  state
91                 change on the slave (see ioctl_tty(2)).
92
93              •  A cgroup.events file has been modified (see cgroups(7)).
94
95       POLLOUT
96              Writing  is  now possible, though a write larger than the avail‐
97              able space in a socket or pipe will still block  (unless  O_NON‐
98              BLOCK is set).
99
100       POLLRDHUP (since Linux 2.6.17)
101              Stream  socket peer closed connection, or shut down writing half
102              of connection.  The _GNU_SOURCE feature test macro must  be  de‐
103              fined  (before  including  any  header files) in order to obtain
104              this definition.
105
106       POLLERR
107              Error condition (only returned in revents; ignored  in  events).
108              This  bit  is  also  set  for a file descriptor referring to the
109              write end of a pipe when the read end has been closed.
110
111       POLLHUP
112              Hang up (only returned in revents;  ignored  in  events).   Note
113              that  when  reading  from  a  channel such as a pipe or a stream
114              socket, this event merely indicates that the peer closed its end
115              of the channel.  Subsequent reads from the channel will return 0
116              (end of file) only after all outstanding data in the channel has
117              been consumed.
118
119       POLLNVAL
120              Invalid  request: fd not open (only returned in revents; ignored
121              in events).
122
123       When compiling with _XOPEN_SOURCE defined, one also has the  following,
124       which convey no further information beyond the bits listed above:
125
126       POLLRDNORM
127              Equivalent to POLLIN.
128
129       POLLRDBAND
130              Priority band data can be read (generally unused on Linux).
131
132       POLLWRNORM
133              Equivalent to POLLOUT.
134
135       POLLWRBAND
136              Priority data may be written.
137
138       Linux also knows about, but does not use POLLMSG.
139
140   ppoll()
141       The  relationship  between poll() and ppoll() is analogous to the rela‐
142       tionship between select(2) and pselect(2): like pselect(2), ppoll() al‐
143       lows  an  application to safely wait until either a file descriptor be‐
144       comes ready or until a signal is caught.
145
146       Other than the difference in the precision of the timeout argument, the
147       following ppoll() call:
148
149           ready = ppoll(&fds, nfds, tmo_p, &sigmask);
150
151       is nearly equivalent to atomically executing the following calls:
152
153           sigset_t origmask;
154           int timeout;
155
156           timeout = (tmo_p == NULL) ? -1 :
157                     (tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
158           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
159           ready = poll(&fds, nfds, timeout);
160           pthread_sigmask(SIG_SETMASK, &origmask, NULL);
161
162       The  above  code  segment  is  described  as  nearly equivalent because
163       whereas a negative timeout value for poll() is interpreted as an  infi‐
164       nite  timeout, a negative value expressed in *tmo_p results in an error
165       from ppoll().
166
167       See the description of pselect(2) for an explanation of why ppoll()  is
168       necessary.
169
170       If  the  sigmask argument is specified as NULL, then no signal mask ma‐
171       nipulation is performed (and thus ppoll() differs from poll()  only  in
172       the precision of the timeout argument).
173
174       The  tmo_p argument specifies an upper limit on the amount of time that
175       ppoll() will block.  This argument is a pointer to a timespec(3) struc‐
176       ture.
177
178       If tmo_p is specified as NULL, then ppoll() can block indefinitely.
179

RETURN VALUE

181       On  success,  poll() returns a nonnegative value which is the number of
182       elements in the pollfds whose revents fields have been set to a nonzero
183       value  (indicating an event or an error).  A return value of zero indi‐
184       cates that the system call timed out before any file descriptors became
185       ready.
186
187       On error, -1 is returned, and errno is set to indicate the error.
188

ERRORS

190       EFAULT fds  points outside the process's accessible address space.  The
191              array given as argument was not contained in  the  calling  pro‐
192              gram's address space.
193
194       EINTR  A signal occurred before any requested event; see signal(7).
195
196       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
197
198       EINVAL (ppoll()) The timeout value expressed in *tmo_p is invalid (neg‐
199              ative).
200
201       ENOMEM Unable to allocate memory for kernel data structures.
202

VERSIONS

204       On some other UNIX systems, poll() can fail with the  error  EAGAIN  if
205       the  system  fails  to  allocate kernel-internal resources, rather than
206       ENOMEM as Linux does.  POSIX permits this behavior.  Portable  programs
207       may wish to check for EAGAIN and loop, just as with EINTR.
208
209       Some  implementations  define  the nonstandard constant INFTIM with the
210       value -1 for use as a timeout for poll().  This constant  is  not  pro‐
211       vided in glibc.
212
213   C library/kernel differences
214       The  Linux  ppoll()  system call modifies its tmo_p argument.  However,
215       the glibc wrapper function hides this behavior by using a  local  vari‐
216       able for the timeout argument that is passed to the system call.  Thus,
217       the glibc ppoll() function does not modify its tmo_p argument.
218
219       The raw ppoll() system call has a fifth  argument,  size_t  sigsetsize,
220       which  specifies  the size in bytes of the sigmask argument.  The glibc
221       ppoll() wrapper function specifies  this  argument  as  a  fixed  value
222       (equal  to  sizeof(kernel_sigset_t)).  See sigprocmask(2) for a discus‐
223       sion on the differences between the kernel and the libc notion  of  the
224       sigset.
225

STANDARDS

227       poll() POSIX.1-2008.
228
229       ppoll()
230              Linux.
231

HISTORY

233       poll() POSIX.1-2001.  Linux 2.1.23.
234
235              On  older  kernels  that lack this system call, the glibc poll()
236              wrapper function provides emulation using select(2).
237
238       ppoll()
239              Linux 2.6.16, glibc 2.4.
240

NOTES

242       The operation of poll() and ppoll() is not affected by  the  O_NONBLOCK
243       flag.
244
245       For  a  discussion  of what may happen if a file descriptor being moni‐
246       tored by poll() is closed in another thread, see select(2).
247

BUGS

249       See the discussion of spurious readiness notifications under  the  BUGS
250       section of select(2).
251

EXAMPLES

253       The program below opens each of the files named in its command-line ar‐
254       guments and monitors the resulting file descriptors  for  readiness  to
255       read  (POLLIN).   The program loops, repeatedly using poll() to monitor
256       the file descriptors, printing the number of ready file descriptors  on
257       return.  For each ready file descriptor, the program:
258
259       •  displays the returned revents field in a human-readable form;
260
261       •  if  the  file  descriptor  is readable, reads some data from it, and
262          displays that data on standard output; and
263
264       •  if the file descriptor was not readable, but some  other  event  oc‐
265          curred (presumably POLLHUP), closes the file descriptor.
266
267       Suppose we run the program in one terminal, asking it to open a FIFO:
268
269           $ mkfifo myfifo
270           $ ./poll_input myfifo
271
272       In  a  second terminal window, we then open the FIFO for writing, write
273       some data to it, and close the FIFO:
274
275           $ echo aaaaabbbbbccccc > myfifo
276
277       In the terminal where we are running the program, we would then see:
278
279           Opened "myfifo" on fd 3
280           About to poll()
281           Ready: 1
282             fd=3; events: POLLIN POLLHUP
283               read 10 bytes: aaaaabbbbb
284           About to poll()
285           Ready: 1
286             fd=3; events: POLLIN POLLHUP
287               read 6 bytes: ccccc
288
289           About to poll()
290           Ready: 1
291             fd=3; events: POLLHUP
292               closing fd 3
293           All file descriptors closed; bye
294
295       In the above output, we see that poll() returned three times:
296
297       •  On the first return, the bits returned in  the  revents  field  were
298          POLLIN,  indicating  that the file descriptor is readable, and POLL‐
299          HUP, indicating that the other end of the FIFO has been closed.  The
300          program then consumed some of the available input.
301
302       •  The second return from poll() also indicated POLLIN and POLLHUP; the
303          program then consumed the last of the available input.
304
305       •  On the final return, poll() indicated only POLLHUP on the  FIFO,  at
306          which  point  the  file descriptor was closed and the program termi‐
307          nated.
308
309   Program source
310
311       /* poll_input.c
312
313          Licensed under GNU General Public License v2 or later.
314       */
315       #include <fcntl.h>
316       #include <poll.h>
317       #include <stdio.h>
318       #include <stdlib.h>
319       #include <unistd.h>
320
321       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
322                               } while (0)
323
324       int
325       main(int argc, char *argv[])
326       {
327           int            ready;
328           char           buf[10];
329           nfds_t         num_open_fds, nfds;
330           ssize_t        s;
331           struct pollfd  *pfds;
332
333           if (argc < 2) {
334              fprintf(stderr, "Usage: %s file...\n", argv[0]);
335              exit(EXIT_FAILURE);
336           }
337
338           num_open_fds = nfds = argc - 1;
339           pfds = calloc(nfds, sizeof(struct pollfd));
340           if (pfds == NULL)
341               errExit("malloc");
342
343           /* Open each file on command line, and add it to 'pfds' array. */
344
345           for (nfds_t j = 0; j < nfds; j++) {
346               pfds[j].fd = open(argv[j + 1], O_RDONLY);
347               if (pfds[j].fd == -1)
348                   errExit("open");
349
350               printf("Opened \"%s\" on fd %d\n", argv[j + 1], pfds[j].fd);
351
352               pfds[j].events = POLLIN;
353           }
354
355           /* Keep calling poll() as long as at least one file descriptor is
356              open. */
357
358           while (num_open_fds > 0) {
359               printf("About to poll()\n");
360               ready = poll(pfds, nfds, -1);
361               if (ready == -1)
362                   errExit("poll");
363
364               printf("Ready: %d\n", ready);
365
366               /* Deal with array returned by poll(). */
367
368               for (nfds_t j = 0; j < nfds; j++) {
369                   if (pfds[j].revents != 0) {
370                       printf("  fd=%d; events: %s%s%s\n", pfds[j].fd,
371                              (pfds[j].revents & POLLIN)  ? "POLLIN "  : "",
372                              (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
373                              (pfds[j].revents & POLLERR) ? "POLLERR " : "");
374
375                       if (pfds[j].revents & POLLIN) {
376                           s = read(pfds[j].fd, buf, sizeof(buf));
377                           if (s == -1)
378                               errExit("read");
379                           printf("    read %zd bytes: %.*s\n",
380                                  s, (int) s, buf);
381                       } else {                /* POLLERR | POLLHUP */
382                           printf("    closing fd %d\n", pfds[j].fd);
383                           if (close(pfds[j].fd) == -1)
384                               errExit("close");
385                           num_open_fds--;
386                       }
387                   }
388               }
389           }
390
391           printf("All file descriptors closed; bye\n");
392           exit(EXIT_SUCCESS);
393       }
394

SEE ALSO

396       restart_syscall(2), select(2),  select_tut(2),  timespec(3),  epoll(7),
397       time(7)
398
399
400
401Linux man-pages 6.04              2023-03-30                           poll(2)
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