POLL(2)                   OpenBSD Programmer's Manual                  POLL(2)

NAME
     poll - synchronous I/O multiplexing



SYNOPSIS
     #include <poll.h>

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

DESCRIPTION
     poll() provides a mechanism for reporting I/O conditions across a set of
     file descriptors.

     The arguments are as follows:

     fds      Points to an array of pollfd structures, which are defined as:

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

              The fd member is an open file descriptor.  The events and
              revents members are bitmasks of conditions to monitor and condi-
              tions found, respectively.

     nfds     The number of pollfd structures in the array.

     timeout  Maximum interval to wait for the poll to complete, in millisec-
              onds.  If this value is 0, then poll() will return immediately.
              If this value is less than 0, poll() will block indefinitely un-
              til a condition is found.

     The calling process sets the events bitmask and poll() sets the revents
     bitmask.  Each call to poll() resets the revents bitmask for accuracy.
     The condition flags in the bitmasks are defined as:

     POLLIN      Data is available on the file descriptor for reading.

     POLLNORM    Same as POLLIN.

     POLLPRI     Same as POLLIN.

     POLLOUT     Data can be written to the file descriptor without blocking.

     POLLERR     This flag is not used in this implementation and is provided
                 only for source code compatibility.

     POLLHUP     The file descriptor was valid before the polling process and
                 invalid after.  Presumably, this means that the file descrip-
                 tor was closed sometime during the poll.

     POLLNVAL    The corresponding file descriptor is invalid.

     POLLRDNORM  Same as POLLIN.

     POLLRDBAND  Same as POLLIN.

     POLLWRNORM  Same as POLLOUT.



     POLLWRBAND  Same as POLLOUT.

     POLLMSG     This flag is not used in this implementation and is provided
                 only for source code compatibility.

     All flags except POLLIN, POLLOUT, and their synonyms are for use only in
     the revents member of the pollfd structure.  An attempt to set any of
     these flags in the events member will generate an error condition.

     In addition to I/O multiplexing, poll() can be used to generate simple
     timeouts.  This functionality may be achieved by passing a null pointer
     for fds.

WARNINGS
     The POLLHUP flag is only a close approximation and may not always be ac-
     curate.

RETURN VALUES
     Upon error, poll() returns a -1 and sets the global variable errno to in-
     dicate the error.  If the timeout interval was reached before any events
     occurred, a 0 is returned.  Otherwise, poll() returns the number of file
     descriptors for which revents is non-zero.

ERRORS
     poll() will fail if:

     [EINVAL]   nfds was either a negative number or greater than the number
                of available file descriptors.

     [EINVAL]   An invalid flags was set in the events member of the pollfd
                structure.

     [EINVAL]   The timeout passed to poll() was too large.

     [EAGAIN]   Resource allocation failed inside of poll().  Subsequent calls
                to poll() may succeed.

     [EINTR]    poll() caught a signal during the polling process.

SEE ALSO
     poll(2),  select(2),  sysconf(3)

HISTORY
     A poll() system call appeared in AT&T System V UNIX

OpenBSD 2.6                    December 13, 1994                             2