xref: /libuv/docs/src/poll.rst (revision 3e1733a0)
1
2.. _poll:
3
4:c:type:`uv_poll_t` --- Poll handle
5===================================
6
7Poll handles are used to watch file descriptors for readability,
8writability and disconnection similar to the purpose of :man:`poll(2)`.
9
10The purpose of poll handles is to enable integrating external libraries that
11rely on the event loop to signal it about the socket status changes, like
12c-ares or libssh2. Using uv_poll_t for any other purpose is not recommended;
13:c:type:`uv_tcp_t`, :c:type:`uv_udp_t`, etc. provide an implementation that is faster and
14more scalable than what can be achieved with :c:type:`uv_poll_t`, especially on
15Windows.
16
17It is possible that poll handles occasionally signal that a file descriptor is
18readable or writable even when it isn't. The user should therefore always
19be prepared to handle EAGAIN or equivalent when it attempts to read from or
20write to the fd.
21
22It is not okay to have multiple active poll handles for the same socket, this
23can cause libuv to busyloop or otherwise malfunction.
24
25The user should not close a file descriptor while it is being polled by an
26active poll handle. This can cause the handle to report an error,
27but it might also start polling another socket. However the fd can be safely
28closed immediately after a call to :c:func:`uv_poll_stop` or :c:func:`uv_close`.
29
30.. note::
31    On windows only sockets can be polled with poll handles. On Unix any file
32    descriptor that would be accepted by :man:`poll(2)` can be used.
33
34.. note::
35    On AIX, watching for disconnection is not supported.
36
37Data types
38----------
39
40.. c:type:: uv_poll_t
41
42    Poll handle type.
43
44.. c:type:: void (*uv_poll_cb)(uv_poll_t* handle, int status, int events)
45
46    Type definition for callback passed to :c:func:`uv_poll_start`.
47
48.. c:enum:: uv_poll_event
49
50    Poll event types
51
52    ::
53
54        enum uv_poll_event {
55            UV_READABLE = 1,
56            UV_WRITABLE = 2,
57            UV_DISCONNECT = 4,
58            UV_PRIORITIZED = 8
59        };
60
61
62Public members
63^^^^^^^^^^^^^^
64
65N/A
66
67.. seealso:: The :c:type:`uv_handle_t` members also apply.
68
69
70API
71---
72
73.. c:function:: int uv_poll_init(uv_loop_t* loop, uv_poll_t* handle, int fd)
74
75    Initialize the handle using a file descriptor.
76
77    .. versionchanged:: 1.2.2 the file descriptor is set to non-blocking mode.
78
79.. c:function:: int uv_poll_init_socket(uv_loop_t* loop, uv_poll_t* handle, uv_os_sock_t socket)
80
81    Initialize the handle using a socket descriptor. On Unix this is identical
82    to :c:func:`uv_poll_init`. On windows it takes a SOCKET handle.
83
84    .. versionchanged:: 1.2.2 the socket is set to non-blocking mode.
85
86.. c:function:: int uv_poll_start(uv_poll_t* handle, int events, uv_poll_cb cb)
87
88    Starts polling the file descriptor. `events` is a bitmask made up of
89    `UV_READABLE`, `UV_WRITABLE`, `UV_PRIORITIZED` and `UV_DISCONNECT`. As soon
90    as an event is detected the callback will be called with `status` set to 0,
91    and the detected events set on the `events` field.
92
93    The `UV_PRIORITIZED` event is used to watch for sysfs interrupts or TCP
94    out-of-band messages.
95
96    The `UV_DISCONNECT` event is optional in the sense that it may not be
97    reported and the user is free to ignore it, but it can help optimize the
98    shutdown path because an extra read or write call might be avoided.
99
100    If an error happens while polling, `status` will be < 0 and corresponds
101    with one of the `UV_E*` error codes (see :ref:`errors`). The user should
102    not close the socket while the handle is active. If the user does that
103    anyway, the callback *may* be called reporting an error status, but this is
104    **not** guaranteed. If `status == UV_EBADF` polling is discontinued for the
105    file handle and no further events will be reported. The user should
106    then call :c:func:`uv_close` on the handle.
107
108    .. note::
109        Calling :c:func:`uv_poll_start` on a handle that is already active is
110        fine. Doing so will update the events mask that is being watched for.
111
112    .. note::
113        Though `UV_DISCONNECT` can be set, it is unsupported on AIX and as such
114        will not be set on the `events` field in the callback.
115
116    .. note::
117        If one of the events `UV_READABLE` or `UV_WRITABLE` are set, the
118        callback will be called again, as long as the given fd/socket remains
119        readable or writable accordingly. Particularly in each of the following
120        scenarios:
121
122        * The callback has been called because the socket became
123          readable/writable and the callback did not conduct a read/write on
124          this socket at all.
125        * The callback committed a read on the socket, and has not read all the
126          available data (when `UV_READABLE` is set).
127        * The callback committed a write on the socket, but it remained
128          writable afterwards (when `UV_WRITABLE` is set).
129        * The socket has already became readable/writable before calling
130          :c:func:`uv_poll_start` on a poll handle associated with this socket,
131          and since then the state of the socket did not changed.
132
133        In all of the above listed scenarios, the socket remains readable or
134        writable and hence the callback will be called again (depending on the
135        events set in the bitmask). This behaviour is known as level
136        triggering.
137
138    .. versionchanged:: 1.9.0 Added the `UV_DISCONNECT` event.
139    .. versionchanged:: 1.14.0 Added the `UV_PRIORITIZED` event.
140
141.. c:function:: int uv_poll_stop(uv_poll_t* poll)
142
143    Stop polling the file descriptor, the callback will no longer be called.
144
145    .. note::
146        Calling :c:func:`uv_poll_stop` is effective immediately: any pending
147        callback is also canceled, even if the socket state change notification
148        was already pending.
149
150.. seealso:: The :c:type:`uv_handle_t` API functions also apply.
151