nghttp2/doc/sources/tutorial-hpack.rst

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Tutorial: HPACK API
===================
In this tutorial, we describe basic use of nghttp2's HPACK API. We
briefly describe the APIs for deflating and inflating header fields.
The full example of using these APIs, `deflate.c`_, is attached at the
end of this page. It also resides in the examples directory in the
archive or repository.
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Deflating (encoding) headers
----------------------------
First we need to initialize a :type:`nghttp2_hd_deflater` object using
the `nghttp2_hd_deflate_new()` function::
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int nghttp2_hd_deflate_new(nghttp2_hd_deflater **deflater_ptr,
size_t deflate_hd_table_bufsize_max);
This function allocates a :type:`nghttp2_hd_deflater` object,
initializes it, and assigns its pointer to ``*deflater_ptr``. The
*deflate_hd_table_bufsize_max* is the upper bound of header table size
the deflater will use. This will limit the memory usage by the
deflater object for the dynamic header table. If in doubt, just
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specify 4096 here, which is the default upper bound of dynamic header
table buffer size.
To encode header fields, use the `nghttp2_hd_deflate_hd()` function::
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ssize_t nghttp2_hd_deflate_hd(nghttp2_hd_deflater *deflater,
uint8_t *buf, size_t buflen,
const nghttp2_nv *nva, size_t nvlen);
The *deflater* is the deflater object initialized by
`nghttp2_hd_deflate_new()` described above. The encoded byte string is
written to the buffer *buf*, which has length *buflen*. The *nva* is
a pointer to an array of headers fields, each of type
:type:`nghttp2_nv`. *nvlen* is the number of header fields which
*nva* contains.
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It is important to initialize and assign all members of
:type:`nghttp2_nv`. For security sensitive header fields (such as
cookies), set the :macro:`NGHTTP2_NV_FLAG_NO_INDEX` flag in
:member:`nghttp2_nv.flags`. Setting this flag prevents recovery of
sensitive header fields by compression based attacks: This is achieved
by not inserting the header field into the dynamic header table.
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`nghttp2_hd_deflate_hd()` processes all headers given in *nva*. The
*nva* must include all request or response header fields to be sent in
one HEADERS (or optionally following (multiple) CONTINUATION
frame(s)). The *buf* must have enough space to store the encoded
result, otherwise the function will fail. To estimate the upper bound
of the encoded result length, use `nghttp2_hd_deflate_bound()`::
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size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater,
const nghttp2_nv *nva, size_t nvlen);
Pass this function the same parameters (*deflater*, *nva*, and
*nvlen*) which will be passed to `nghttp2_hd_deflate_hd()`.
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Subsequent calls to `nghttp2_hd_deflate_hd()` will use the current
encoder state and perform differential encoding, which yields HPAC's
fundamental compression gain.
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If `nghttp2_hd_deflate_hd()` fails, the failure is fatal and any
further calls with the same deflater object will fail. Thus it's very
important to use `nghttp2_hd_deflate_bound()` to determine the
required size of the output buffer.
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To delete a :type:`nghttp2_hd_deflater` object, use the
`nghttp2_hd_deflate_del()` function.
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Inflating (decoding) headers
----------------------------
A :type:`nghttp2_hd_inflater` object is used to inflate compressed
header data. To initialize the object, use
`nghttp2_hd_inflate_new()`::
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int nghttp2_hd_inflate_new(nghttp2_hd_inflater **inflater_ptr);
To inflate header data, use `nghttp2_hd_inflate_hd()`::
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ssize_t nghttp2_hd_inflate_hd(nghttp2_hd_inflater *inflater,
nghttp2_nv *nv_out, int *inflate_flags,
uint8_t *in, size_t inlen, int in_final);
`nghttp2_hd_inflate_hd()` reads a stream of bytes and outputs a single
header field at a time. Multiple calls are normally required to read a
full stream of bytes and output all of the header fields.
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The *inflater* is the inflater object initialized above. The *nv_out*
is a pointer to a :type:`nghttp2_nv` into which one header field may
be stored. The *in* is a pointer to input data, and *inlen* is its
length. The caller is not required to specify the whole deflated
header data via *in* at once: Instead it can call this function
multiple times as additional data bytes become available. If
*in_final* is nonzero, it tells the function that the passed data is
the final sequence of deflated header data.
The *inflate_flags* is an output parameter; on success the function
sets it to a bitset of flags. It will be described later.
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This function returns when each header field is inflated. When this
happens, the function sets the :macro:`NGHTTP2_HD_INFLATE_EMIT` flag
in *inflate_flags*, and a header field is stored in *nv_out*. The
return value indicates the number of bytes read from *in* processed so
far, which may be less than *inlen*. The caller should call the
function repeatedly until all bytes are processed. Processed bytes
should be removed from *in*, and *inlen* should be adjusted
appropriately.
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If *in_final* is nonzero and all given data was processed, the
function sets the :macro:`NGHTTP2_HD_INFLATE_FINAL` flag in
*inflate_flags*. When you see this flag set, call the
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`nghttp2_hd_inflate_end_headers()` function.
If *in_final* is zero and the :macro:`NGHTTP2_HD_INFLATE_EMIT` flag is
not set, it indicates that all given data was processed. The caller
is required to pass additional data.
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It is important to note that the function may produce one or more
header fields even if *inlen* is 0 when *in_final* is nonzero, due to
differential encoding.
Example usage of `nghttp2_hd_inflate_hd()` is shown in the
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`inflate_header_block()` function in `deflate.c`_.
Finally, to delete a :type:`nghttp2_hd_inflater` object, use
`nghttp2_hd_inflate_del()`.