poll - synchronous I/O multiplexing
Standard C Library (libc, -lc)
#include <sys/types.h>
#include <poll.h>
int
poll(struct pollfd *fds, nfds_t nfds, int timeout);
poll() examines 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
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 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.
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.
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) ed 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.
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.
accept(2), connect(2), read(2), recv(2), select(2), send(2), write(2)
The poll() function call appeared in AT&T UNIX V.3.
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.
BSD September 7, 1996 BSD
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