Reference

poll(2)

synchronous I/O multiplexing

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 exclusive 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 interrupted 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), connectx(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.

History