NAME
poll - synchronous I/O multiplexing
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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 fds argument is a pointer to an array of
pollfd structures as defined in #include <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 fd is equal to -1 then 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 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.
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.
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 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)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.
ERRORS
An error return from poll() indicates:
[EFAULT] The fds argument 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.
SEE ALSO
accept(2), connect(2), kqueue(2), read(2), recv(2), select(2), send(2),
write(2)
HISTORY
The poll() function appeared in AT&T System V UNIX. This manual page and
the core of the implementation was taken from NetBSD.
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.