.. _poll:

:c:type:`uv_poll_t` --- Poll handle
===================================

Poll handles are used to watch file descriptors for readability,
writability and disconnection similar to the purpose of :man:`poll(2)`.

The purpose of poll handles is to enable integrating external libraries that
rely on the event loop to signal it about the socket status changes, like
c-ares or libssh2. Using uv_poll_t for any other purpose is not recommended;
:c:type:`uv_tcp_t`, :c:type:`uv_udp_t`, etc. provide an implementation that is faster and
more scalable than what can be achieved with :c:type:`uv_poll_t`, especially on
Windows.

It is possible that poll handles occasionally signal that a file descriptor is
readable or writable even when it isn't. The user should therefore always
be prepared to handle EAGAIN or equivalent when it attempts to read from or
write to the fd.

It is not okay to have multiple active poll handles for the same socket, this
can cause libuv to busyloop or otherwise malfunction.

The user should not close a file descriptor while it is being polled by an
active poll handle. This can cause the handle to report an error,
but it might also start polling another socket. However the fd can be safely
closed immediately after a call to :c:func:`uv_poll_stop` or :c:func:`uv_close`.

.. note::
    On windows only sockets can be polled with poll handles. On Unix any file
    descriptor that would be accepted by :man:`poll(2)` can be used.

.. note::
    On AIX, watching for disconnection is not supported.

Data types
----------

.. c:type:: uv_poll_t

    Poll handle type.

.. c:type:: void (*uv_poll_cb)(uv_poll_t* handle, int status, int events)

    Type definition for callback passed to :c:func:`uv_poll_start`.

.. c:type:: uv_poll_event

    Poll event types

    ::

        enum uv_poll_event {
            UV_READABLE = 1,
            UV_WRITABLE = 2,
            UV_DISCONNECT = 4,
            UV_PRIORITIZED = 8
        };


Public members
^^^^^^^^^^^^^^

N/A

.. seealso:: The :c:type:`uv_handle_t` members also apply.


API
---

.. c:function:: int uv_poll_init(uv_loop_t* loop, uv_poll_t* handle, int fd)

    Initialize the handle using a file descriptor.

    .. versionchanged:: 1.2.2 the file descriptor is set to non-blocking mode.

.. c:function:: int uv_poll_init_socket(uv_loop_t* loop, uv_poll_t* handle, uv_os_sock_t socket)

    Initialize the handle using a socket descriptor. On Unix this is identical
    to :c:func:`uv_poll_init`. On windows it takes a SOCKET handle.

    .. versionchanged:: 1.2.2 the socket is set to non-blocking mode.

.. c:function:: int uv_poll_start(uv_poll_t* handle, int events, uv_poll_cb cb)

    Starts polling the file descriptor. `events` is a bitmask made up of
    `UV_READABLE`, `UV_WRITABLE`, `UV_PRIORITIZED` and `UV_DISCONNECT`. As soon
    as an event is detected the callback will be called with `status` set to 0,
    and the detected events set on the `events` field.

    The `UV_PRIORITIZED` event is used to watch for sysfs interrupts or TCP
    out-of-band messages.

    The `UV_DISCONNECT` event is optional in the sense that it may not be
    reported and the user is free to ignore it, but it can help optimize the
    shutdown path because an extra read or write call might be avoided.

    If an error happens while polling, `status` will be < 0 and corresponds
    with one of the `UV_E*` error codes (see :ref:`errors`). The user should
    not close the socket while the handle is active. If the user does that
    anyway, the callback *may* be called reporting an error status, but this is
    **not** guaranteed. If `status == UV_EBADF` polling is discontinued for the
    file handle and no further events will be reported. The user should
    then call :c:func:`uv_close` on the handle.

    .. note::
        Calling :c:func:`uv_poll_start` on a handle that is already active is
        fine. Doing so will update the events mask that is being watched for.

    .. note::
        Though `UV_DISCONNECT` can be set, it is unsupported on AIX and as such
        will not be set on the `events` field in the callback.

    .. note::
        If one of the events `UV_READABLE` or `UV_WRITABLE` are set, the
        callback will be called again, as long as the given fd/socket remains
        readable or writable accordingly. Particularly in each of the following
        scenarios:

        * The callback has been called because the socket became
          readable/writable and the callback did not conduct a read/write on
          this socket at all.
        * The callback committed a read on the socket, and has not read all the
          available data (when `UV_READABLE` is set).
        * The callback committed a write on the socket, but it remained
          writable afterwards (when `UV_WRITABLE` is set).
        * The socket has already became readable/writable before calling
          :c:func:`uv_poll_start` on a poll handle associated with this socket,
          and since then the state of the socket did not changed.

        In all of the above listed scenarios, the socket remains readable or
        writable and hence the callback will be called again (depending on the
        events set in the bitmask). This behaviour is known as level
        triggering.

    .. versionchanged:: 1.9.0 Added the `UV_DISCONNECT` event.
    .. versionchanged:: 1.14.0 Added the `UV_PRIORITIZED` event.

.. c:function:: int uv_poll_stop(uv_poll_t* poll)

    Stop polling the file descriptor, the callback will no longer be called.

    .. note::
        Calling :c:func:`uv_poll_stop` is effective immediately: any pending
        callback is also canceled, even if the socket state change notification
        was already pending.

.. seealso:: The :c:type:`uv_handle_t` API functions also apply.