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poll.2: Add an example program
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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man2/poll.2
179
man2/poll.2
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.\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
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.\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" and Copyright (C) 2006, 2019 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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@ -470,6 +470,183 @@ notion of the sigset.
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See the discussion of spurious readiness notifications under the
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BUGS section of
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.BR select (2).
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.SH EXAMPLE
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The program below opens each of the files named in its command-line
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arguments and monitors the resulting file descriptors for readiness to read
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.RB ( POLLIN ).
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The program loops, repeatedly using
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.BR poll ()
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to monitor the file descriptors,
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printing the number of ready file descriptors on return.
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For each ready file descriptor, the program:
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.IP \(bu 2
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displays the returned
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.I revents
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field in a human-readable form;
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.IP \(bu
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if the file descriptor is readable, reads some data from it,
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and displays that data on standard output; and
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.IP \(bu
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if the file descriptors was not readable,
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but some other event occurred (presumably
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.BR POLLHUP ),
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closes the file descriptor.
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.PP
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Suppose we run the program in one terminal, asking it to open a FIFO:
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.PP
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.in +4n
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.EX
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$ \fBmkfifo myfifo\fP
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$ \fB./poll_input myfifo\fP
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.EE
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.in
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.PP
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In a second terminal window, we then open the FIFO for writing,
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write some data to it, and close the FIFO:
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.PP
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.in +4n
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.EX
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$ \fBecho aaaaabbbbbccccc > myfifo\fP
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.EE
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.in
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.PP
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In the terminal where we are running the program, we would then see:
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.PP
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.in +4n
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.EX
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Opened "myfifo" on fd 3
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About to poll()
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Ready: 1
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fd=3; events: POLLIN POLLHUP
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read 10 bytes: aaaaabbbbb
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About to poll()
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Ready: 1
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fd=3; events: POLLIN POLLHUP
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read 6 bytes: ccccc
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About to poll()
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Ready: 1
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fd=3; events: POLLHUP
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closing fd 3
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All file descriptors closed; bye
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.EE
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.in
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.PP
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In the above output, we see that
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.BR poll ()
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returned three times:
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.IP \(bu 3
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On the first return, the bits returned in the
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.I revents
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field were
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.BR POLLIN ,
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indicating that the file descriptor is readable, and
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.BR POLLHUP ,
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indicating that the other end of the FIFO has been closed.
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The program then consumed some of the available input.
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.IP \(bu
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The second return from
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.BR poll ()
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also indicated
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.BR POLLIN
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and
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.BR POLLHUP ;
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the program then consumed the last of the available input.
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.IP \(bu
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On the final return,
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.BR poll ()
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indicated only
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.BR POLLHUP
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on the FIFO,
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at which point the file descriptor was closed and the program terminated.
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.\"
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.SS Program source
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\&
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.nf
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/* poll_input.c */
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#include <poll.h>
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#include <fcntl.h>
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#include <sys/types.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
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} while (0)
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int
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main(int argc, char *argv[])
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{
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int nfds, num_open_fds;
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struct pollfd *pfds;
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if (argc < 2) {
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fprintf(stderr, "Usage: %s file...\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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num_open_fds = nfds = argc \- 1;
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pfds = calloc(nfds, sizeof(struct pollfd));
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if (pfds == NULL)
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errExit("malloc");
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/* Open each file on command line, and add it \(aqpfds\(aq array */
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for (int j = 0; j < nfds; j++) {
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pfds[j].fd = open(argv[j + 1], O_RDONLY);
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if (pfds[j].fd == \-1)
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errExit("open");
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printf("Opened \e"%s\e" on fd %d\en", argv[j + 1], pfds[j].fd);
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pfds[j].events = POLLIN;
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}
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/* Keep calling poll() as long as at least one file descriptor is
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open */
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while (num_open_fds > 0) {
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int ready;
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printf("About to poll()\en");
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ready = poll(pfds, nfds, \-1);
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if (ready == \-1)
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errExit("poll");
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printf("Ready: %d\en", ready);
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/* Deal with array returned by poll() */
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for (int j = 0; j < nfds; j++) {
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char buf[10];
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if (pfds[j].revents != 0) {
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printf(" fd=%d; events: %s%s%s\en", pfds[j].fd,
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(pfds[j].revents & POLLIN) ? "POLLIN " : "",
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(pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
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(pfds[j].revents & POLLERR) ? "POLLERR " : "");
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if (pfds[j].revents & POLLIN) {
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ssize_t s = read(pfds[j].fd, buf, sizeof(buf));
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if (s == \-1)
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errExit("read");
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printf(" read %zd bytes: %.*s\en",
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s, (int) s, buf);
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} else { /* POLLERR | POLLHUP */
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printf(" closing fd %d\en", pfds[j].fd);
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if (close(pfds[j].fd) == \-1)
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errExit("close");
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num_open_fds\-\-;
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}
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}
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}
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}
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printf("All file descriptors closed; bye\en");
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exit(EXIT_SUCCESS);
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}
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.fi
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.SH SEE ALSO
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.BR restart_syscall (2),
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.BR select (2),
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