POLL(2)

NAME
poll, ppoll - synchronous I/O multiplexing

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <poll.h>
int
poll(struct pollfd *fds, nfds_t nfds, int timeout);
int
ppoll(struct pollfd *fds, nfds_t nfds, const struct timespec *timeout, const sigset_t *newsigmask);

DESCRIPTION
Poll() and ppoll() examine 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:

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.)

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

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.

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 ppoll() system call can be used to safely wait until either a set of file descriptors becomes ready, or until a signal is caught. The timeout argument in ppoll() points to a const struct timespec rather than the int timeout used by poll(). A null pointer may be passed to indicate that ppoll() should wait indefinitely. Finally, newsigmask specifies a signal mask which is set while waiting for input. When ppoll() returns, the original signal mask is restored.

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.

COMPATIBILITY
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.

The ppoll() implementation uses a precise timeout which is intended to mimic the behaviour of this syscall in Linux.

ERRORS
An error return from poll() indicates:

[EFAULT]
    Fds 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), pselect(2), read(2), recv(2), select(2), send(2), write(2)

HISTORY
The poll() function call appeared in AT&T UNIX. This manual page was taken from NetBSD. The ppoll() function first appeared in DragonFly 4.6.

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.