NAME
poll, pollts
—
synchronous I/O multiplexing
LIBRARY
library “libc”
SYNOPSIS
#include
<poll.h>
int
poll
(struct
pollfd *fds, nfds_t
nfds, int
timeout);
#include <poll.h>
#include <signal.h>
#include <time.h>
int
pollts
(struct
pollfd * restrict fds,
nfds_t nfds,
const struct timespec *
restrict ts, const
sigset_t * restrict sigmask);
DESCRIPTION
poll
()
and pollts
() examine a set of file descriptors to see
if some of them are ready for I/O. The fds argument is a
pointer to an array of pollfd structures as defined in
<poll.h>
(shown below). The
nfds argument determines the size of the
fds array.
struct pollfd { int fd; /* file descriptor */ short events; /* events to look for */ short revents; /* events returned */ };
The fields of struct pollfd are as follows:
- fd
- File descriptor to poll. If the value in fd is negative, the file descriptor is ignored and revents is set to 0.
- events
- Events to poll for. (See below.)
- revents
- Events which may occur. (See below.)
The event bitmasks in events and 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
- Normal data may be written without blocking.
- POLLWRNORM
- Equivalent to POLLOUT.
- 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 events bitmask.
- POLLHUP
- The device or socket has been disconnected. This flag is always checked,
even if not present in the events bitmask. Note that
POLLHUP and POLLOUT should never be present in the
revents bitmask at the same time. If the remote end
of a socket is closed,
poll
() returns a POLLIN event, rather than a POLLHUP. - POLLNVAL
- The file descriptor is not open. This flag is always checked, even if not present in the events bitmask.
If timeout is neither zero nor
INFTIM (-1), it specifies a maximum interval to wait for any file descriptor
to become ready, in milliseconds. If timeout is INFTIM
(-1), the poll blocks indefinitely. If timeout is
zero, then
poll
() will
return without blocking.
If ts is a non-null pointer, it
references a timespec structure which specifies a maximum interval to wait
for any file descriptor to become ready. If ts is a
null pointer,
pollts
()
blocks indefinitely. If ts is a non-null pointer,
referencing a zero-valued timespec structure, then
pollts
() will return without blocking.
If sigmask is a non-null
pointer, then the
pollts
()
function shall replace the signal mask of the caller by the set of signals
pointed to by sigmask before examining the
descriptors, and shall restore the signal mask of the caller before
returning.
RETURN VALUES
poll
() 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 call, the 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)d descriptor), this implementation instead copies the
events bitmask to the 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:
- [
EFAULT
] - fds points outside the process's allocated address space.
- [
EINTR
] - A signal was delivered before the time limit expired and before any of the selected events occurred.
- [
EINVAL
] - The specified time limit is negative or the number of pollfd structures specified is larger than the current file descriptor resource limit.
SEE ALSO
accept(2), connect(2), read(2), recv(2), select(2), send(2), write(2)
HISTORY
The poll
() function appeared in
AT&T System V Release 3 UNIX. The
pollts
() function first appeared in
NetBSD 3.0.
BUGS
The distinction between some of the fields in the events and revents bitmasks is really not useful without STREAMS. The fields are defined for compatibility with existing software.