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.Dd February 27, 2005 .Dt POLL 2 .Os .Sh NAME .Nm poll .Nd synchronous I/O multiplexing .Sh SYNOPSIS n 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. d -literal struct pollfd { int fd; /* file descriptor */ short events; /* events to look for */ short revents; /* events returned */ }; .Ed
p The fields of .Fa struct pollfd are as follows: l -tag -width XXXPOLLWRNORM t fd File descriptor to poll. t events Events to poll for. (See below.) t revents Events which may occur or have occurred. (See below.) .El
p
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
l -tag -width XXXPOLLWRNORM ===========
t 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.
===========
t 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.
===========
t POLLIN Data other than high priority data may be read without blocking.
This is equivalent to ( POLLRDNORM | POLLRDBAND ).
===========
t POLLNVAL The file descriptor is not open.
This flag is output only, and ignored if present in the input
.Fa events
bitmask.
===========
t POLLOUT Normal data may be written without blocking.
This is equivalent to POLLWRNORM.
===========
t POLLPRI High priority data may be read without blocking.
===========
t POLLRDBAND Priority data may be read without blocking.
===========
t POLLRDNORM Normal data may be read without blocking.
===========
t POLLWRBAND Priority data may be written without blocking.
===========
t POLLWRNORM Normal data may be written without blocking.
.El
p The distinction between normal, priority, and high-priority data is specific to particular file types or devices.
p
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:
l -tag -width Er ===========
t Bq Er EAGAIN Allocation of internal data structures fails.
A subsequent request may succeed.
===========
t Bq Er EFAULT .Fa Fds
points outside the process's allocated address space.
===========
t Bq Er EINTR A signal is delivered before the time limit expires
and before any of the selected events occurs.
===========
t 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 .