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.Dd February 27, 2005
.Dt POLL 2
.Os
.Sh NAME
.Nm poll
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.In poll.h
.Ft int
.Fo poll
.Fa "struct pollfd fds[]"
.Fa "nfds_t nfds"
.Fa "int timeout"
.Fc
.Sh DESCRIPTION
.Fn Poll
examines a set of file descriptors
to see if some of them are ready for I/O
or if certain events have occurred on them.
The
.Fa fds
argument is a pointer to an array of pollfd structures,
as defined in
.Aq Pa poll.h
(shown below).  The
.Fa nfds
argument specifies 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 XXXPOLLWRNORM
.It fd
File descriptor to poll.
.It events
Events to poll for.  (See below.)
.It revents
Events which may occur or have occurred.  (See below.)
.El
.Pp
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
.Bl -tag -width XXXPOLLWRNORM
.\" ===========
.It POLLERR
An exceptional condition has occurred on the device or socket.
This flag is output only, and ignored if present in the input
.Fa events
bitmask.
.\" ===========
.It POLLHUP
The device or socket has been disconnected.
This flag is output only,
and ignored if present in the input
.Fa events
bitmask.
Note that POLLHUP and POLLOUT
are mutually exclusive and should never be present in the
.Fa revents
bitmask at the same time.
.\" ===========
.It POLLIN
Data other than high priority data may be read without blocking.
This is equivalent to ( POLLRDNORM | POLLRDBAND ).
.\" ===========
.It POLLNVAL
The file descriptor is not open.
This flag is output only, and ignored if present in the input
.Fa events
bitmask.
.\" ===========
.It POLLOUT
Normal data may be written without blocking.
This is equivalent to POLLWRNORM.
.\" ===========
.It POLLPRI
High priority data may be read without blocking.
.\" ===========
.It POLLRDBAND
Priority data may be read without blocking.
.\" ===========
.It POLLRDNORM
Normal data may be read without blocking.
.\" ===========
.It POLLWRBAND
Priority data may be written without blocking.
.\" ===========
.It POLLWRNORM
Normal data may be written without blocking.
.El
.Pp
The distinction between normal, priority, and high-priority data
is specific to particular file types or devices.
.Pp
If
.Fa timeout
is greater than zero,
it specifies a maximum interval (in milliseconds)
to wait for any file descriptor to become ready.
If
.Fa timeout
is zero, then
.Fn poll
will return without blocking. If the value of
.Fa timeout
is -1, the poll blocks indefinitely.
.Sh RETURN VALUES
.Fn Poll
returns the number of descriptors that are ready for I/O,
or -1 if an error occurred.
If the time limit expires,
.Fn poll
returns 0.
If
.Fn poll
returns with an error,
including one due to an interrupted call,
the
.Fa fds
array will be unmodified and the global variable
.Va errno
will be set to indicate the error.
.Sh ERRORS
.Fn Poll
will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
Allocation of internal data structures fails.
A subsequent request may succeed.
.\" ===========
.It Bq Er EFAULT
.Fa Fds
points outside the process's allocated address space.
.\" ===========
.It Bq Er EINTR
A signal is delivered before the time limit expires
and before any of the selected events occurs.
.\" ===========
.It Bq Er EINVAL
The
.Fa nfds
argument is greater than OPEN_MAX or the
.Fa timeout
argument is less than -1.
.El
.Sh BUGS
The
.Fn poll
system call currently does not support devices.
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr kevent 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn poll
function call appeared in
.At V .