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NAME

poll — synchronous I/O multiplexing

SYNOPSIS

#include <poll.h>

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

DESCRIPTION

poll() provides a mechanism for multiplexing I/O across a set of file descriptors. It is similar in function to select(2). Unlike select(2), however, it is possible to only pass in data corresponding to the file descriptors for which events are wanted. This makes poll() more efficient than select(2) in most cases.

The arguments are as follows:

fds

Points to an array of pollfd structures, which are defined as:

struct pollfd {
	int fd;
	short events;
	short revents;
};

The fd member is an open file descriptor. If fd is -1, the pollfd structure is considered unused, and revents will be cleared.

The events and revents members are bitmasks of conditions to monitor and conditions found, respectively.

nfds

An unsigned integer specifying the number of pollfd structures in the array.

timeout

Maximum interval to wait for the poll to complete, in milliseconds. If this value is 0, poll() will return immediately. If this value is INFTIM (-1), poll() will block indefinitely until a condition is found.

The calling process sets the events bitmask and poll() sets the revents bitmask. Each call to poll() resets the revents bitmask for accuracy. The condition flags in the bitmasks are defined as:

POLLIN

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

POLLRDNORM

Normal data may be read without blocking.

POLLRDBAND

Priority data may be read without blocking.

POLLNORM

Same as POLLRDNORM. This flag is provided for source code compatibility with older programs and should not be used in new code.

POLLPRI

High-priority data may be read without blocking.

POLLOUT

Normal data may be written without blocking.

POLLWRNORM

Same as POLLOUT.

POLLWRBAND

Priority data may be written.

POLLERR

An error has occurred on the device or socket. This flag is only valid in the revents bitmask; it is ignored in the events member.

POLLHUP

The device or socket has been disconnected. This event and POLLOUT are mutually-exclusive; a descriptor can never be writable if a hangup has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutually-exclusive. This flag is only valid in the revents bitmask; it is ignored in the events member.

POLLNVAL

The corresponding file descriptor is invalid. This flag is only valid in the revents bitmask; it is ignored in the events member.

The significance and semantics of normal, priority, and high-priority data are device-specific.

In addition to I/O multiplexing, poll() can be used to generate simple timeouts. This functionality may be achieved by passing a null pointer for fds.

RETURN VALUES

Upon error, poll() returns -1 and sets the global variable errno to indicate the error. If the timeout interval was reached before any events occurred, poll() returns 0. Otherwise, poll() returns the number of file descriptors for which revents is non-zero.

EXAMPLES

The following example implements a read from the standard input that times out after 60 seconds:

#include <err.h>
#include <poll.h>
#include <stdio.h>
#include <unistd.h>

struct pollfd pfd[1];
char buf[BUFSIZ];
int nfds;

pfd[0].fd = STDIN_FILENO;
pfd[0].events = POLLIN;
nfds = poll(pfd, 1, 60 * 1000);
if (nfds == -1 || (pfd[0].revents & (POLLERR|POLLHUP|POLLNVAL)))
	errx(1, "poll error");
if (nfds == 0)
	errx(1, "time out");
if (read(STDIN_FILENO, buf, sizeof(buf)) == -1)
	errx(1, "read");

ERRORS

poll() will fail if:

[EFAULT]

fds points outside the process's allocated address space.

[EINTR]

poll() caught a signal during the polling process.

[EINVAL]

nfds was greater than the number of available file descriptors.

[EINVAL]

The timeout passed to poll() was too large.

SEE ALSO

getrlimit(2), read(2), select(2), write(2)

STANDARDS

The poll() function is compliant with the specification.

HISTORY

A poll() system call appeared in AT&T System V Release 3 UNIX.

BUGS

The POLLERR and POLLWRBAND flags are accepted but ignored by the kernel.

Because OpenBSD does not implement STREAMS, there is no distinction between some of the fields in the events and revents bitmasks. As a result, the POLLIN, POLLNORM, and POLLRDNORM flags are equivalent.

Internally to the kernel, poll() works poorly if multiple processes wait on the same file descriptor.