POLL(2)

POLL(2) | System Calls Manual | POLL(2)

# NAME

`poll, pollts` —
synchronous I/O multiplexing

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <poll.h>`

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

`#include <poll.h>`
`#include <signal.h>`
`#include <time.h>`

int
`pollts`(struct pollfd * restrict fds, nfds_t nfds, const struct timespec * restrict ts, const sigset_t * restrict sigmask);

# DESCRIPTION

`poll`() and `pollts`() 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 the value in fd is negative, the file descriptor is ignored and revents is set to 0.
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
  Normal data may be written without blocking.
POLLWRNORM
  Equivalent to POLLOUT.
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. If the remote end of a socket is closed, `poll`() returns a POLLIN event, rather than a POLLHUP.
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.

If ts is a non-null pointer, it references a timespec structure which specifies a maximum interval to wait for any file descriptor to become ready. If ts is a null pointer, `pollts`() blocks indefinitely. If ts is a non-null pointer, referencing a zero-valued timespec structure, then `pollts`() will return without blocking.

If sigmask is a non-null pointer, then the `pollts`() function shall replace the signal mask of the caller by the set of signals pointed to by sigmask before examining the descriptors, and shall restore the signal mask of the caller before returning.

# 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)d 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`]
  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 or the number of pollfd structures specified is larger than the current file descriptor resource limit.

# SEE ALSO

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

# HISTORY

The `poll`() function appeared in AT&T System V Release 3 UNIX. The `pollts`() function first appeared in NetBSD 3.0.

# 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.

May 5, 2019 | NetBSD 9.3