Exit Zen
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Parsing Chunks
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The __parser__ automatically removes the chunked transfer coding when it is the last encoding in the list. However, it also discards the chunk extensions and does not provide a way to determine the boundaries between chunks. Advanced applications which need to access the chunk extensions or read complete individual chunks may use a callback interface provided by __parser__:
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Chunking Parse Callbacks
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Name
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Description
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[link beast.ref.boost__beast__http__parser.on_chunk_header `on_chunk_header`]
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Set a callback to be invoked on each chunk header.

The callback will be invoked once for every chunk in the message payload, as well as once for the last chunk. The invocation happens after the chunk header is available but before any body octets have been parsed.

The extensions are provided in raw, validated form, use [link beast.ref.boost__beast__http__basic_chunk_extensions.parse `chunk_extensions::parse`] to parse the extensions into a structured container for easier access. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
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[link beast.ref.boost__beast__http__parser.on_chunk_body `on_chunk_body`]
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Set a callback to be invoked on chunk body data.

The callback will be invoked one or more times to provide buffers corresponding to the chunk body for the current chunk. The callback receives the number of octets remaining in this chunk body including the octets in the buffer provided.

The callback must return the number of octets actually consumed. Any octets not consumed will be presented again in a subsequent invocation of the callback. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
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This example will read a message header from the stream, and then manually read each chunk. It recognizes the chunk boundaries and outputs the contents of each chunk as it comes in. Any chunk extensions are printed, each extension on its own line. Finally, any trailers promised in the header are printed.
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Given the HTTP response as input on the left, the output of the function shown above is shown on the right:
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Chunk Parsing Example Output
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Input
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Output
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HTTP/1.1 200 OK\r\n
Server: test\r\n
Trailer: Expires, Content-MD5\r\n
Transfer-Encoding: chunked\r\n
\r\n
5\r\n
First\r\n
d;quality=1.0\r\n
Hello, world!\r\n
e;file=abc.txt;quality=0.7\r\n
The Next Chunk\r\n
8;last\r\n
Last one\r\n
0\r\n
Expires: never\r\n
Content-MD5: f4a5c16584f03d90\r\n
\r\n
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Chunk Body: First
Extension: quality = 1.0
Chunk Body: Hello, world!
Extension: file = abc.txt
Extension: quality = 0.7
Chunk Body: The Next Chunk
Extension: last
Chunk Body: Last one
Expires: never
Content-MD5: f4a5c16584f03d90
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