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Parsing Chunks
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__:
Chunking Parse Callbacks
158
Name
158
Description
[link beast.ref.boost__beast__http__parser.on_chunk_header `on_chunk_header`]
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:
[link beast.ref.boost__beast__http__parser.on_chunk_body `on_chunk_body`]
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:
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.
Given the HTTP response as input on the left, the output of the function shown above is shown on the right:
Chunk Parsing Example Output
Input
Output
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
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
English Actions
Chunked Encoding
For message payloads whose size is not known ahead of time, HTTP version 1.1 defines the [@https://tools.ietf.org/html/rfc7230#section-4.1 ['chunked]] transfer coding. This coding consists of zero or more [@https://tools.ietf.org/html/rfc7230#section-4.1 ['chunked bodies]], followed by a [@https://tools.ietf.org/html/rfc7230#section-4.1 ['last chunk]]. Each chunked body may contain optional application-defined, connection-specific [@https://tools.ietf.org/html/rfc7230#section-4.1.1 ['chunk-extensions]]. The last chunk may contain additional HTTP field values in a section of the last chunk called a [@https://tools.ietf.org/html/rfc7230#section-4.1.2 ['chunk-trailer]]. The field values are "promised" in the header as a comma delimited list of field names in the [@https://tools.ietf.org/html/rfc7230#section-4.4 [*Trailer]] field value. Clients indicate their willingness to accept trailers by including the "trailers" token in the [@https://tools.ietf.org/html/rfc7230#section-4.3 [*TE]] field value.
Serializing Chunks
The __serializer__ automatically applies the chunked transfer encoding when a message returns `true` from [link beast.ref.boost__beast__http__message.chunked.overload1 `message::chunked`]. The boundaries between chunks emitted by the serializer are implementation defined. Chunk extensions and trailers are omitted. Applications which need precise control over the chunk boundaries, extensions, and trailers may use a set of helper classes which enable manual emission of message payloads using chunk encoding.
To use these helper classes, first serialize the header portion of the message using the standard interface. Then prepare the buffers, chunk extensions, and desired trailers, and use them with these helpers:
Chunking Helpers
[link beast.ref.boost__beast__http__chunk_body `chunk_body`]
A buffer sequence representing a complete chunk body.
[link beast.ref.boost__beast__http__chunk_crlf `chunk_crlf`]
A buffer sequence representing the CRLF (`"\r\n"`) delimiter. This class is used when the caller desires to emit the chunk body in two or more individual stream operations.
[link beast.ref.boost__beast__http__chunk_extensions `chunk_extensions`]

[link beast.ref.boost__beast__http__basic_chunk_extensions `basic_chunk_extensions`]
Input
Output
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
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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String added in the repository
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
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
06/05/2026
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String information

Component
Boost Beast Translation (zh_Hans)/Doc / Qbk / 04 Http / 08 Chunked Encoding (qbk)
Source string description
type: table code
Flags
read-only
Unit identifier
57417
Source string location
string ID 224
String age
06/05/2026
Last updated
06/05/2026
Source string age
06/05/2026
Translation file QuickBook file
doc/qbk/04_http/08_chunked_encoding.qbk, string 44