#include <poll.h> int
poll (struct pollfd fds[] nfds_t nfds int timeout);
DESCRIPTION
The
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
#include <poll.h>
(shown below).
The
Fa nfds
argument determines the size of the
Fa fds
array.
struct pollfd {
int fd; /* file descriptor */
short events; /* events to look for */
short revents; /* events returned */
};
The fields of
Fa struct pollfd
are as follows:
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.
events
Events to poll for.
(See below.)
revents
Events which may occur.
(See below.)
The event bitmasks in
Fa events
and
Fa revents
have the following bits:
POLLIN
Data other than high priority data may be read without blocking.
POLLRDNORM
Normal data may be read without blocking.
POLLRDBAND
Data with a non-zero priority may be read without blocking.
POLLPRI
High priority data may be read without blocking.
POLLOUT
POLLWRNORM
Normal data may be written without blocking.
POLLWRBAND
Data with a non-zero priority may be written without blocking.
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.
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.
POLLNVAL
The file descriptor is not open.
This flag is always checked, even
if not present in the
Fa events
bitmask.
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
poll ();
will return without blocking.
RETURN VALUES
The
poll ();
system call
returns the number of descriptors that are ready for I/O, or -1 if an
error occurred.
If the time limit expires,
poll ();
returns 0.
If
poll ();
returns with an error,
including one due to an interrupted system call,
the
Fa fds
array will be unmodified.
COMPATIBILITY
This implementation differs from the historical one in that a given
file descriptor may not cause
poll ();
to return with an error.
In cases where this would have happened in
the historical implementation (e.g. trying to poll a
revoke(2)Nsed
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.
ERRORS
An error return from
poll ();
indicates:
Bq Er EFAULT
The
Fa fds
argument
points outside the process's allocated address space.
Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
The
poll ();
function appeared in
AT&T System
V .
This manual page and the core of the implementation was taken from
Nx .
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.
