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.Dd July 8, 2002
.Dt POLL 2
.Os
.Sh NAME
.Nm poll
.Nd synchronous I/O multiplexing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In poll.h
.Ft int
.Fn poll "struct pollfd fds[]" "nfds_t nfds" "int timeout"
.Sh DESCRIPTION
The
.Fn poll
system call
examines a set of file descriptors to see if some of them are ready for
I/O.
The
.Fa fds
argument is a pointer to an array of pollfd structures as defined in
.In poll.h
(shown below).  The
.Fa nfds
argument determines the size of the
.Fa fds
array.
.Bd -literal
struct pollfd {
    int    fd;       /* file descriptor */
    short  events;   /* events to look for */
    short  revents;  /* events returned */
};
.Ed
.Pp
The fields of
.Fa struct pollfd
are as follows:
.Bl -tag -width XXXrevents
.It fd
File descriptor to poll.  If fd is equal to -1 then
.Fa revents
is cleared (set to zero), and that pollfd is not checked.
.It events
Events to poll for.  (See below.)
.It revents
Events which may occur.  (See below.)
.El
.Pp
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
.Bl -tag -width XXXPOLLWRNORM
.It POLLIN
Data other than high priority data may be read without blocking.
.It POLLRDNORM
Normal data may be read without blocking.
.It POLLRDBAND
Data with a non-zero priority may be read without blocking.
.It POLLPRI
High priority data may be read without blocking.
.It POLLOUT
.It POLLWRNORM
Normal data may be written without blocking.
.It POLLWRBAND
Data with a non-zero priority may be written without blocking.
.It POLLERR
An exceptional condition has occurred on the device or socket.  This
flag is always checked, even if not present in the
.Fa events
bitmask.
.It POLLHUP
The device or socket has been disconnected.  This flag is always
checked, even if not present in the
.Fa events
bitmask.  Note that
POLLHUP
and
POLLOUT
should never be present in the
.Fa revents
bitmask at the same time.
.It POLLNVAL
The file descriptor is not open.  This flag is always checked, even
if not present in the
.Fa events
bitmask.
.El
.Pp
If
.Fa timeout
is neither zero nor INFTIM (-1), it specifies a maximum interval to
wait for any file descriptor to become ready, in milliseconds.  If
.Fa timeout
is INFTIM (-1), the poll blocks indefinitely.  If
.Fa timeout
is zero, then
.Fn poll
will return without blocking.
.Sh RETURN VALUES
The
.Fn poll
system call
returns the number of descriptors that are ready for I/O, or -1 if an
error occured.  If the time limit expires,
.Fn poll
returns 0.
If
.Fn poll
returns with an error,
including one due to an interrupted system call,
the
.Fa fds
array will be unmodified.
.Sh COMPATIBILITY
This implementation differs from the historical one in that a given
file descriptor may not cause
.Fn poll
to return with an error.  In cases where this would have happened in
the historical implementation (e.g. trying to poll a
.Xr revoke 2 Ns ed
descriptor), this implementation instead copies the
.Fa events
bitmask to the
.Fa revents
bitmask.  Attempting to perform I/O on this descriptor will then
return an error.  This behaviour is believed to be more useful.
.Sh ERRORS
An error return from
.Fn poll
indicates:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa fds
argument
points outside the process's allocated address space.
.It Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.It Bq Er EINVAL
The specified time limit is negative.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr kqueue 2 , 
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr write 2
.Sh BUGS
The distinction between some of the fields in the
.Fa events
and
.Fa revents
bitmasks is really not useful without STREAMS.  The fields are
defined for compatibility with existing software.
.Sh HISTORY
The
.Fn poll
function appeared in
.At V .
This manual page and the core of the implementation was taken from
.Nx .