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POLL(2) | System Calls Manual | POLL(2) |
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¶
Thepoll
() 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
<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 */ };
- 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.)
- 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.
poll
() will return without
blocking.
RETURN VALUES¶
Thepoll
() 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 causepoll
() 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 frompoll
() 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¶
Thepoll
() 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.July 8, 2002 | Debian |