poll()

Multiplex input/output over a set of file descriptors

Synopsis:

#include <sys/poll.h>

int poll( struct pollfd fds*,
          nfds_t nfds,
          int timeout );

Arguments:

fds

An array of pollfd structures that defines the file descriptors of interest.

nfds

The number of elements in the fds array.

timeout

The timeout, in milliseconds.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The poll() function provides applications with a mechanism for multiplexing input/output over a set of file descriptors.

The fds argument is an array of pollfd structures:

struct pollfd {
    int fd;
    short events;
    short revents;
};

The members are:

fd

The file descriptor to be examined.

events

Flags that indicate the type of events to look for.

revents

Returned event flags.

For each member of the array pointed to by fds, poll() examines the given file descriptor for the event(s) specified in events. The number of pollfd structures in the fds array is specified by nfds. The array's members are pollfd structures within which fd specifies an open file descriptor, and events and revents are bitmasks constructed by ORing a combination of the following event flags:

POLLERR

An error has occurred on the device. This flag is valid only in the revents bitmask; it's ignored in the events member.

POLLHUP

The device has been disconnected. This event and POLLOUT are mutually exclusive; a device can never be writable if a hangup has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutually exclusive. If the remote end of a socket is closed, poll() indicates a POLLIN event rather than POLLHUP. This flag is valid only in the revents bitmask; it's ignored in the events member.

POLLIN

Data other than high-priority data may be read without blocking. This is equivalent to POLLRDNORM | POLLRDBAND.

POLLNVAL

The specified fd value is invalid. This flag is valid only in the revents member; it's ignored in the events member.

POLLOUT

Normal data may be written without blocking.

POLLPRI

High-priority data may be read without blocking.

POLLRDBAND

Priority data may be read without blocking.

POLLRDNORM

Normal data may be read without blocking.

POLLWRBAND

Priority data may be written.

POLLWRNORM

Equivalent to POLLOUT.

The significance and semantics of normal, priority, and high-priority data are file- and device-specific.

If the value of fd is less than 0, events is ignored, and revents is set to 0 in that entry on return from poll().

In each pollfd structure, poll() clears the revents member, except that where the application requested a report on a condition by setting one of the bits of events listed above, poll() sets the corresponding bit in revents if the requested condition is true. In addition, poll() sets the POLLHUP, POLLERR, and POLLNVAL flag in revents if the condition is true, even if the application didn't set the corresponding bit in events.

If none of the defined events occurs on any selected file descriptor, poll() waits at least timeout milliseconds for an event to occur on any of the selected file descriptors. If the value of timeout is 0, poll() returns immediately. If the value of timeout is -1, poll() blocks until a requested event occurs or until the call is interrupted.

The poll() function isn't affected by the O_NONBLOCK flag.

The poll() function reports regular files, terminal and pseudo-terminal devices, FIFOs, and pipes.

Regular files always poll TRUE for reading and writing.

A file descriptor for a socket that's listening for connections indicates that it's ready for reading, once connections are available. A file descriptor for a socket that connects asynchronously indicates that it's ready for writing, once a connection has been established.

Returns:

> 0

The total number of file descriptors that have been selected.

0

The call timed out, and no file descriptor has been selected.

-1

An error occurred (errno is set).

Errors:

EAGAIN

The allocation of internal data structures failed, but a subsequent request may succeed.

EINTR

A signal was caught during poll().

EFAULT

The fds argument pointed to a nonexistent portion of the calling process's address space.

Examples:

#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/poll.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

struct sockaddr_in sad;

void *
client(void *arg)
{
   int s;
   const char *p = "Some data\n";

   if ((s = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
      perror("socket");
      return NULL;
   }

   if (connect(s, (struct sockaddr *)&sad, sizeof(sad)) == -1) {
       perror("connect");
       return NULL;
   }

   write(s, p, strlen(p));
   close(s);

   return NULL;
}

int 
main(void)
{
   struct pollfd fds;
   int s = -1, s2 = -1, done_accept = 0, oflags, ret;
   char buf[100];

   if ((s = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
      perror("socket");
      return 1;
   }

   sad.sin_family      = AF_INET;
   sad.sin_len         = sizeof(sad);
   sad.sin_addr.s_addr = inet_addr("127.0.0.1");
   sad.sin_port        = htons(1234);

   fds.fd = s;
   fds.events = POLLRDNORM;

   oflags = fcntl(s, F_GETFL);
   oflags |= O_NONBLOCK;
   fcntl(s, F_SETFL, oflags);

   if (bind(s, (struct sockaddr *)&sad, sizeof(sad)) == -1) {
      perror("bind");
      return 1;
   }

   listen(s, 5);

   if ((ret = pthread_create(NULL, NULL, client, NULL)) != EOK) {
      fprintf(stderr, "pthread_create: %s\n", strerror(ret));
      return 1;
   }

   for (;;) {
      if ((ret = poll(&fds, 1, -1)) == -1) {
         perror("poll");
         break;
      }
      
      else if (ret != 1 || (fds.revents & POLLRDNORM) == 0) {
         break;
      }

      if (done_accept) {
         if ((ret = read(s2, buf, sizeof(buf))) <= 0) {
            break;
         }
            
         printf("%s", buf);
      
      }
      else { 
         if ((s2 = accept(s, NULL, 0)) == -1) {
            perror("accept");
            break;
         }
                        
         fds.fd = s2;
         done_accept = 1;
      }
   }

   close(s);
   close(s2);

   return 0;
}

Classification:

POSIX 1003.1 XSI

Caveats:

Not all managers support POLLPRI, POLLPRI, POLLERR, and POLLHUP.

See also:

read(), select(), write()

<sys/poll.h>