POLL(2) BSD System Calls Manual POLL(2)
NAME
poll -- synchronous I/O multiplexing
SYNOPSIS
#include <poll.h>
int
poll(struct pollfd fds[], nfds_t nfds, int timeout);
DESCRIPTION
Poll() examines a set of file descriptors to see if some of them are ready for I/O or if certain events
have occurred on them. The fds argument is a pointer to an array of pollfd structures, as defined in
<poll.h> (shown below). The nfds argument specifies 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.
events Events to poll for. (See below.)
revents Events which may occur or have occurred. (See below.)
The event bitmasks in events and revents have the following bits:
POLLERR An exceptional condition has occurred on the device or socket. This flag is output
only, and ignored if present in the input events bitmask.
POLLHUP The device or socket has been disconnected. This flag is output only, and ignored if
present in the input events bitmask. Note that POLLHUP and POLLOUT are mutually exclu-sive exclusive
sive and should never be present in the revents bitmask at the same time.
POLLIN Data other than high priority data may be read without blocking. This is equivalent to
( POLLRDNORM | POLLRDBAND ).
POLLNVAL The file descriptor is not open. This flag is output only, and ignored if present in
the input events bitmask.
POLLOUT Normal data may be written without blocking. This is equivalent to POLLWRNORM.
POLLPRI High priority data may be read without blocking.
POLLRDBAND Priority data may be read without blocking.
POLLRDNORM Normal data may be read without blocking.
POLLWRBAND Priority data may be written without blocking.
POLLWRNORM Normal data may be written without blocking.
The distinction between normal, priority, and high-priority data is specific to particular file types
or devices.
If timeout is greater than zero, it specifies a maximum interval (in milliseconds) to wait for any file
descriptor to become ready. If timeout is zero, then poll() will return without blocking. If the value
of timeout is -1, the poll blocks indefinitely.
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 inter-rupted interrupted
rupted call, the fds array will be unmodified and the global variable errno will be set to indicate the
error.
ERRORS
Poll() will fail if:
[EAGAIN] Allocation of internal data structures fails. A subsequent request may succeed.
[EFAULT] Fds points outside the process's allocated address space.
[EINTR] A signal is delivered before the time limit expires and before any of the selected
events occurs.
[EINVAL] The nfds argument is greater than OPEN_MAX or the timeout argument is less than -1.
BUGS
The poll() system call currently does not support devices.
SEE ALSO
accept(2), connect(2), kevent(2), read(2), recv(2), select(2), send(2), write(2)
HISTORY
The poll() function call appeared in AT&T System V UNIX.
BSD February 27, 2005 BSD
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