POLL(2)

NAME
poll - synchronous I/O multiplexing

LIBRARY
libc

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

DESCRIPTION
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:

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.

POLLRDHUP
    Remote peer closed connection, or shut down writing. Unlike POLLHUP, POLLRDHUP must be present in the events bitmask to be reported. Applies only to stream sockets.

POLLNVAL
    The file descriptor is not open, or in capability mode the file descriptor has insufficient rights. 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, unlike poll(), is used to safely wait until either a set of file descriptors becomes ready or until a signal is caught. The fds and nfds arguments are identical to the analogous arguments of poll(). The timeout argument in ppoll() points to a const struct timespec which is defined in <sys/timespec.h> (shown below) 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.

    struct timespec {
    	time_t  tv_sec;         /* seconds */
    	long    tv_nsec;        /* and nanoseconds */
    };

RETURN VALUES
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.

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.

ERRORS
An error return from poll() indicates:

[EFAULT]
    The fds argument 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 invalid. One of its components is negative or too large.

[EINVAL]
    The number of pollfd structures specified by nfds exceeds the system tunable kern.maxfilesperproc and FD_SETSIZE.

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

STANDARDS
The poll() function conforms to p1003.1-2001. The ppoll() function conforms to p1003.1-2024. The POLLRDHUP flag is not specified by POSIX, but is compatible with Linux and illumos.

HISTORY
The poll() function appeared in AT&T UNIX. This manual page and the core of the implementation was taken from NetBSD. The ppoll() function first appeared in FreeBSD 10.2

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.