Error codes used in this library.  The code range is [-999, -500],
inclusive. The following values are defined:

.. enum:: NGHTTP2_ERR_INVALID_ARGUMENT

    (``-501``) 
    Invalid argument passed.
.. enum:: NGHTTP2_ERR_BUFFER_ERROR

    (``-502``) 
    Out of buffer space.
.. enum:: NGHTTP2_ERR_UNSUPPORTED_VERSION

    (``-503``) 
    The specified protocol version is not supported.
.. enum:: NGHTTP2_ERR_WOULDBLOCK

    (``-504``) 
    Used as a return value from :type:`nghttp2_send_callback2`,
    :type:`nghttp2_recv_callback` and
    :type:`nghttp2_send_data_callback` to indicate that the operation
    would block.
.. enum:: NGHTTP2_ERR_PROTO

    (``-505``) 
    General protocol error
.. enum:: NGHTTP2_ERR_INVALID_FRAME

    (``-506``) 
    The frame is invalid.
.. enum:: NGHTTP2_ERR_EOF

    (``-507``) 
    The peer performed a shutdown on the connection.
.. enum:: NGHTTP2_ERR_DEFERRED

    (``-508``) 
    Used as a return value from
    :func:`nghttp2_data_source_read_callback2` to indicate that data
    transfer is postponed.  See
    :func:`nghttp2_data_source_read_callback2` for details.
.. enum:: NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE

    (``-509``) 
    Stream ID has reached the maximum value.  Therefore no stream ID
    is available.
.. enum:: NGHTTP2_ERR_STREAM_CLOSED

    (``-510``) 
    The stream is already closed; or the stream ID is invalid.
.. enum:: NGHTTP2_ERR_STREAM_CLOSING

    (``-511``) 
    RST_STREAM has been added to the outbound queue.  The stream is
    in closing state.
.. enum:: NGHTTP2_ERR_STREAM_SHUT_WR

    (``-512``) 
    The transmission is not allowed for this stream (e.g., a frame
    with END_STREAM flag set has already sent).
.. enum:: NGHTTP2_ERR_INVALID_STREAM_ID

    (``-513``) 
    The stream ID is invalid.
.. enum:: NGHTTP2_ERR_INVALID_STREAM_STATE

    (``-514``) 
    The state of the stream is not valid (e.g., DATA cannot be sent
    to the stream if response HEADERS has not been sent).
.. enum:: NGHTTP2_ERR_DEFERRED_DATA_EXIST

    (``-515``) 
    Another DATA frame has already been deferred.
.. enum:: NGHTTP2_ERR_START_STREAM_NOT_ALLOWED

    (``-516``) 
    Starting new stream is not allowed (e.g., GOAWAY has been sent
    and/or received).
.. enum:: NGHTTP2_ERR_GOAWAY_ALREADY_SENT

    (``-517``) 
    GOAWAY has already been sent.
.. enum:: NGHTTP2_ERR_INVALID_HEADER_BLOCK

    (``-518``) 
    The received frame contains the invalid header block (e.g., There
    are duplicate header names; or the header names are not encoded
    in US-ASCII character set and not lower cased; or the header name
    is zero-length string; or the header value contains multiple
    in-sequence NUL bytes).
.. enum:: NGHTTP2_ERR_INVALID_STATE

    (``-519``) 
    Indicates that the context is not suitable to perform the
    requested operation.
.. enum:: NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE

    (``-521``) 
    The user callback function failed due to the temporal error.
.. enum:: NGHTTP2_ERR_FRAME_SIZE_ERROR

    (``-522``) 
    The length of the frame is invalid, either too large or too small.
.. enum:: NGHTTP2_ERR_HEADER_COMP

    (``-523``) 
    Header block inflate/deflate error.
.. enum:: NGHTTP2_ERR_FLOW_CONTROL

    (``-524``) 
    Flow control error
.. enum:: NGHTTP2_ERR_INSUFF_BUFSIZE

    (``-525``) 
    Insufficient buffer size given to function.
.. enum:: NGHTTP2_ERR_PAUSE

    (``-526``) 
    Callback was paused by the application
.. enum:: NGHTTP2_ERR_TOO_MANY_INFLIGHT_SETTINGS

    (``-527``) 
    There are too many in-flight SETTING frame and no more
    transmission of SETTINGS is allowed.
.. enum:: NGHTTP2_ERR_PUSH_DISABLED

    (``-528``) 
    The server push is disabled.
.. enum:: NGHTTP2_ERR_DATA_EXIST

    (``-529``) 
    DATA or HEADERS frame for a given stream has been already
    submitted and has not been fully processed yet.  Application
    should wait for the transmission of the previously submitted
    frame before submitting another.
.. enum:: NGHTTP2_ERR_SESSION_CLOSING

    (``-530``) 
    The current session is closing due to a connection error or
    `nghttp2_session_terminate_session()` is called.
.. enum:: NGHTTP2_ERR_HTTP_HEADER

    (``-531``) 
    Invalid HTTP header field was received and stream is going to be
    closed.
.. enum:: NGHTTP2_ERR_HTTP_MESSAGING

    (``-532``) 
    Violation in HTTP messaging rule.
.. enum:: NGHTTP2_ERR_REFUSED_STREAM

    (``-533``) 
    Stream was refused.
.. enum:: NGHTTP2_ERR_INTERNAL

    (``-534``) 
    Unexpected internal error, but recovered.
.. enum:: NGHTTP2_ERR_CANCEL

    (``-535``) 
    Indicates that a processing was canceled.
.. enum:: NGHTTP2_ERR_SETTINGS_EXPECTED

    (``-536``) 
    When a local endpoint expects to receive SETTINGS frame, it
    receives an other type of frame.
.. enum:: NGHTTP2_ERR_TOO_MANY_SETTINGS

    (``-537``) 
    When a local endpoint receives too many settings entries
    in a single SETTINGS frame.
.. enum:: NGHTTP2_ERR_FATAL

    (``-900``) 
    The errors < :enum:`nghttp2_error.NGHTTP2_ERR_FATAL` mean that
    the library is under unexpected condition and processing was
    terminated (e.g., out of memory).  If application receives this
    error code, it must stop using that :type:`nghttp2_session`
    object and only allowed operation for that object is deallocate
    it using `nghttp2_session_del()`.
.. enum:: NGHTTP2_ERR_NOMEM

    (``-901``) 
    Out of memory.  This is a fatal error.
.. enum:: NGHTTP2_ERR_CALLBACK_FAILURE

    (``-902``) 
    The user callback function failed.  This is a fatal error.
.. enum:: NGHTTP2_ERR_BAD_CLIENT_MAGIC

    (``-903``) 
    Invalid client magic (see :macro:`NGHTTP2_CLIENT_MAGIC`) was
    received and further processing is not possible.
.. enum:: NGHTTP2_ERR_FLOODED

    (``-904``) 
    Possible flooding by peer was detected in this HTTP/2 session.
    Flooding is measured by how many PING and SETTINGS frames with
    ACK flag set are queued for transmission.  These frames are
    response for the peer initiated frames, and peer can cause memory
    exhaustion on server side to send these frames forever and does
    not read network.
.. enum:: NGHTTP2_ERR_TOO_MANY_CONTINUATIONS

    (``-905``) 
    When a local endpoint receives too many CONTINUATION frames
    following a HEADER frame.

The flags for header field name/value pair.

.. enum:: NGHTTP2_NV_FLAG_NONE

    (``0``) 
    No flag set.
.. enum:: NGHTTP2_NV_FLAG_NO_INDEX

    (``0x01``) 
    Indicates that this name/value pair must not be indexed ("Literal
    Header Field never Indexed" representation must be used in HPACK
    encoding).  Other implementation calls this bit as "sensitive".
.. enum:: NGHTTP2_NV_FLAG_NO_COPY_NAME

    (``0x02``) 
    This flag is set solely by application.  If this flag is set, the
    library does not make a copy of header field name.  This could
    improve performance.
.. enum:: NGHTTP2_NV_FLAG_NO_COPY_VALUE

    (``0x04``) 
    This flag is set solely by application.  If this flag is set, the
    library does not make a copy of header field value.  This could
    improve performance.

The frame types in HTTP/2 specification.

.. enum:: NGHTTP2_DATA

    (``0``) 
    The DATA frame.
.. enum:: NGHTTP2_HEADERS

    (``0x01``) 
    The HEADERS frame.
.. enum:: NGHTTP2_PRIORITY

    (``0x02``) 
    The PRIORITY frame.
.. enum:: NGHTTP2_RST_STREAM

    (``0x03``) 
    The RST_STREAM frame.
.. enum:: NGHTTP2_SETTINGS

    (``0x04``) 
    The SETTINGS frame.
.. enum:: NGHTTP2_PUSH_PROMISE

    (``0x05``) 
    The PUSH_PROMISE frame.
.. enum:: NGHTTP2_PING

    (``0x06``) 
    The PING frame.
.. enum:: NGHTTP2_GOAWAY

    (``0x07``) 
    The GOAWAY frame.
.. enum:: NGHTTP2_WINDOW_UPDATE

    (``0x08``) 
    The WINDOW_UPDATE frame.
.. enum:: NGHTTP2_CONTINUATION

    (``0x09``) 
    The CONTINUATION frame.  This frame type won't be passed to any
    callbacks because the library processes this frame type and its
    preceding HEADERS/PUSH_PROMISE as a single frame.
.. enum:: NGHTTP2_ALTSVC

    (``0x0a``) 
    The ALTSVC frame, which is defined in `RFC 7383
    <https://tools.ietf.org/html/rfc7838#section-4>`_.
.. enum:: NGHTTP2_ORIGIN

    (``0x0c``) 
    The ORIGIN frame, which is defined by `RFC 8336
    <https://tools.ietf.org/html/rfc8336>`_.
.. enum:: NGHTTP2_PRIORITY_UPDATE

    (``0x10``) 
    The PRIORITY_UPDATE frame, which is defined by :rfc:`9218`.

The flags for HTTP/2 frames.  This enum defines all flags for all
frames.

.. enum:: NGHTTP2_FLAG_NONE

    (``0``) 
    No flag set.
.. enum:: NGHTTP2_FLAG_END_STREAM

    (``0x01``) 
    The END_STREAM flag.
.. enum:: NGHTTP2_FLAG_END_HEADERS

    (``0x04``) 
    The END_HEADERS flag.
.. enum:: NGHTTP2_FLAG_ACK

    (``0x01``) 
    The ACK flag.
.. enum:: NGHTTP2_FLAG_PADDED

    (``0x08``) 
    The PADDED flag.
.. enum:: NGHTTP2_FLAG_PRIORITY

    (``0x20``) 
    The PRIORITY flag.

The SETTINGS ID.

.. enum:: NGHTTP2_SETTINGS_HEADER_TABLE_SIZE

    (``0x01``) 
    SETTINGS_HEADER_TABLE_SIZE
.. enum:: NGHTTP2_SETTINGS_ENABLE_PUSH

    (``0x02``) 
    SETTINGS_ENABLE_PUSH
.. enum:: NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS

    (``0x03``) 
    SETTINGS_MAX_CONCURRENT_STREAMS
.. enum:: NGHTTP2_SETTINGS_INITIAL_WINDOW_SIZE

    (``0x04``) 
    SETTINGS_INITIAL_WINDOW_SIZE
.. enum:: NGHTTP2_SETTINGS_MAX_FRAME_SIZE

    (``0x05``) 
    SETTINGS_MAX_FRAME_SIZE
.. enum:: NGHTTP2_SETTINGS_MAX_HEADER_LIST_SIZE

    (``0x06``) 
    SETTINGS_MAX_HEADER_LIST_SIZE
.. enum:: NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL

    (``0x08``) 
    SETTINGS_ENABLE_CONNECT_PROTOCOL
    (`RFC 8441 <https://tools.ietf.org/html/rfc8441>`_)
.. enum:: NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES

    (``0x09``) 
    SETTINGS_NO_RFC7540_PRIORITIES (:rfc:`9218`)

The status codes for the RST_STREAM and GOAWAY frames.

.. enum:: NGHTTP2_NO_ERROR

    (``0x00``) 
    No errors.
.. enum:: NGHTTP2_PROTOCOL_ERROR

    (``0x01``) 
    PROTOCOL_ERROR
.. enum:: NGHTTP2_INTERNAL_ERROR

    (``0x02``) 
    INTERNAL_ERROR
.. enum:: NGHTTP2_FLOW_CONTROL_ERROR

    (``0x03``) 
    FLOW_CONTROL_ERROR
.. enum:: NGHTTP2_SETTINGS_TIMEOUT

    (``0x04``) 
    SETTINGS_TIMEOUT
.. enum:: NGHTTP2_STREAM_CLOSED

    (``0x05``) 
    STREAM_CLOSED
.. enum:: NGHTTP2_FRAME_SIZE_ERROR

    (``0x06``) 
    FRAME_SIZE_ERROR
.. enum:: NGHTTP2_REFUSED_STREAM

    (``0x07``) 
    REFUSED_STREAM
.. enum:: NGHTTP2_CANCEL

    (``0x08``) 
    CANCEL
.. enum:: NGHTTP2_COMPRESSION_ERROR

    (``0x09``) 
    COMPRESSION_ERROR
.. enum:: NGHTTP2_CONNECT_ERROR

    (``0x0a``) 
    CONNECT_ERROR
.. enum:: NGHTTP2_ENHANCE_YOUR_CALM

    (``0x0b``) 
    ENHANCE_YOUR_CALM
.. enum:: NGHTTP2_INADEQUATE_SECURITY

    (``0x0c``) 
    INADEQUATE_SECURITY
.. enum:: NGHTTP2_HTTP_1_1_REQUIRED

    (``0x0d``) 
    HTTP_1_1_REQUIRED

The flags used to set in *data_flags* output parameter in
:type:`nghttp2_data_source_read_callback2`.

.. enum:: NGHTTP2_DATA_FLAG_NONE

    (``0``) 
    No flag set.
.. enum:: NGHTTP2_DATA_FLAG_EOF

    (``0x01``) 
    Indicates EOF was sensed.
.. enum:: NGHTTP2_DATA_FLAG_NO_END_STREAM

    (``0x02``) 
    Indicates that END_STREAM flag must not be set even if
    NGHTTP2_DATA_FLAG_EOF is set.  Usually this flag is used to send
    trailer fields with `nghttp2_submit_request2()` or
    `nghttp2_submit_response2()`.
.. enum:: NGHTTP2_DATA_FLAG_NO_COPY

    (``0x04``) 
    Indicates that application will send complete DATA frame in
    :type:`nghttp2_send_data_callback`.

The category of HEADERS, which indicates the role of the frame.  In
HTTP/2 spec, request, response, push response and other arbitrary
headers (e.g., trailer fields) are all called just HEADERS.  To
give the application the role of incoming HEADERS frame, we define
several categories.

.. enum:: NGHTTP2_HCAT_REQUEST

    (``0``) 
    The HEADERS frame is opening new stream, which is analogous to
    SYN_STREAM in SPDY.
.. enum:: NGHTTP2_HCAT_RESPONSE

    (``1``) 
    The HEADERS frame is the first response headers, which is
    analogous to SYN_REPLY in SPDY.
.. enum:: NGHTTP2_HCAT_PUSH_RESPONSE

    (``2``) 
    The HEADERS frame is the first headers sent against reserved
    stream.
.. enum:: NGHTTP2_HCAT_HEADERS

    (``3``) 
    The HEADERS frame which does not apply for the above categories,
    which is analogous to HEADERS in SPDY.  If non-final response
    (e.g., status 1xx) is used, final response HEADERS frame will be
    categorized here.

The flags for header inflation.

.. enum:: NGHTTP2_HD_INFLATE_NONE

    (``0``) 
    No flag set.
.. enum:: NGHTTP2_HD_INFLATE_FINAL

    (``0x01``) 
    Indicates all headers were inflated.
.. enum:: NGHTTP2_HD_INFLATE_EMIT

    (``0x02``) 
    Indicates a header was emitted.

State of stream as described in RFC 7540.

.. enum:: NGHTTP2_STREAM_STATE_IDLE

    (``1``) 
    idle state.
.. enum:: NGHTTP2_STREAM_STATE_OPEN

    open state.
.. enum:: NGHTTP2_STREAM_STATE_RESERVED_LOCAL

    reserved (local) state.
.. enum:: NGHTTP2_STREAM_STATE_RESERVED_REMOTE

    reserved (remote) state.
.. enum:: NGHTTP2_STREAM_STATE_HALF_CLOSED_LOCAL

    half closed (local) state.
.. enum:: NGHTTP2_STREAM_STATE_HALF_CLOSED_REMOTE

    half closed (remote) state.
.. enum:: NGHTTP2_STREAM_STATE_CLOSED

    closed state.