POLL(2) MidnightBSD System Calls Manual POLL(2)


poll — synchronous I/O multiplexing


Standard C Library (libc, −lc)


#include <poll.h>


poll(struct pollfd fds[], nfds_t nfds, int timeout);


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 <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:


File descriptor to poll. If fd is equal to -1 then revents is cleared (set to zero), and that pollfd is not checked.


Events to poll for. (See below.)


Events which may occur. (See below.)

The event bitmasks in events and revents have the following bits:


Data other than high priority data may be read without blocking.


Normal data may be read without blocking.


Data with a non-zero priority may be read without blocking.


High priority data may be read without blocking.


Normal data may be written without blocking.


Data with a non-zero priority may be written without blocking.


An exceptional condition has occurred on the device or socket. This flag is always checked, even if not present in the events bitmask.


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.


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.


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.


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:


The fds argument points outside the process’s allocated address space.


A signal was delivered before the time limit expired and before any of the selected events occurred.


The specified time limit is negative.


accept(2), connect(2), kqueue(2), read(2), recv(2), select(2), send(2), write(2)


The poll() function appeared in AT&T System V UNIX. This manual page and the core of the implementation was taken from NetBSD.


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.

MidnightBSD 0.3 July 8, 2002 MidnightBSD 0.3