pollexample.c

/* poll_input.c

   Licensed under GNU General Public License v2 or later.
*/
#include <fcntl.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

#define errExit(msg)                                                           \
  do {                                                                         \
    perror(msg);                                                               \
    exit(EXIT_FAILURE);                                                        \
  } while (0)

int main(int argc, char *argv[]) {
  int ready;
  char buf[10];
  nfds_t num_open_fds, nfds;
  ssize_t s;
  struct pollfd *pfds;

  if (argc < 2) {
    fprintf(stderr, "Usage: %s file...\n", argv[0]);
    exit(EXIT_FAILURE);
  }

  num_open_fds = nfds = argc - 1;
  pfds = calloc(nfds, sizeof(struct pollfd));
  if (pfds == NULL)
    errExit("malloc");

  /* Open each file on command line, and add it to 'pfds' array. */

  for (nfds_t j = 0; j < nfds; j++) {
    pfds[j].fd = open(argv[j + 1], O_RDONLY);
    if (pfds[j].fd == -1)
      errExit("open");

    printf("Opened \"%s\" on fd %d\n", argv[j + 1], pfds[j].fd);

    pfds[j].events = POLLIN;
  }

  /* Keep calling poll() as long as at least one file descriptor is
     open. */

  while (num_open_fds > 0) {
    printf("About to poll()\n");
    ready = poll(pfds, nfds, 10000);
    if (ready == -1)
      errExit("poll");

    printf("Ready: %d\n", ready);

    /* Deal with array returned by poll(). */

    for (nfds_t j = 0; j < nfds; j++) {
      if (pfds[j].revents != 0) {
        printf("  fd=%d; events: %s%s%s\n", pfds[j].fd,
               (pfds[j].revents & POLLIN) ? "POLLIN " : "",
               (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
               (pfds[j].revents & POLLERR) ? "POLLERR " : "");

        if (pfds[j].revents & POLLIN) {
          s = read(pfds[j].fd, buf, sizeof(buf));
          if (s == -1)
            errExit("read");
          printf("    read %zd bytes: %.*s\n", s, (int)s, buf);
        } else { /* POLLERR | POLLHUP */
          printf("    closing fd %d\n", pfds[j].fd);
          if (close(pfds[j].fd) == -1)
            errExit("close");
          num_open_fds--;
        }
      }
    }
  }

  printf("All file descriptors closed; bye\n");
  exit(EXIT_SUCCESS);
}

polltest.sh

#!/bin/sh
# compile modified example from man 2 poll
gcc -g -O2 -o pollexample pollexample.c

## set up fifo
rm fifo
mkfifo fifo

# open it for writing, so that opening it for reading in the example works
sleep 500 > fifo &

# start the example in the background in strace, so we can see the timestamps
strace -tt ./pollexample fifo &

# sleep one second so we're sure to be in the 10s polling timeout period
sleep 1

# STOP!
killall -STOP pollexample
# sleep a bit
sleep 2
# CONTINUE!
killall -CONT pollexample

# now let's wait for the timeout to have definitely occurred and kill the cat
sleep 10
rm fifo
killall pollexample