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Content Encoding Support for libcurl
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* About content encodings:
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HTTP/1.1 [RFC 2616] specifies that a client may request that a server encode
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its response. This is usually used to compress a response using one of a set
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of commonly available compression techniques. These schemes are `deflate' (the
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zlib algorithm), `gzip' and `compress' [sec 3.5, RFC 2616]. A client requests
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that the sever perform an encoding by including an Accept-Encoding header in
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the request document. The value of the header should be one of the recognized
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tokens `deflate', ... (there's a way to register new schemes/tokens, see sec
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3.5 of the spec). A server MAY honor the client's encoding request. When a
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response is encoded, the server includes a Content-Encoding header in the
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response. The value of the Content-Encoding header indicates which scheme was
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used to encode the data.
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A client may tell a server that it can understand several different encoding
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schemes. In this case the server may choose any one of those and use it to
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encode the response (indicating which one using the Content-Encoding header).
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It's also possible for a client to attach priorities to different schemes so
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that the server knows which it prefers. See sec 14.3 of RFC 2616 for more
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information on the Accept-Encoding header.
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* Current support for content encoding:
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Support for the 'deflate' and 'gzip' content encoding are supported by
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libcurl. Both regular and chunked transfers should work fine. The library
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zlib is required for this feature. 'deflate' support was added by James
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Gallagher, and support for the 'gzip' encoding was added by Dan Fandrich.
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* The libcurl interface:
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To cause libcurl to request a content encoding use:
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curl_easy_setopt(curl, CURLOPT_ENCODING, <string>)
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where <string> is the intended value of the Accept-Encoding header.
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Currently, libcurl only understands how to process responses that use the
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"deflate" or "gzip" Content-Encoding, so the only values for CURLOPT_ENCODING
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that will work (besides "identity," which does nothing) are "deflate" and
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"gzip" If a response is encoded using the "compress" or methods, libcurl will
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return an error indicating that the response could not be decoded. If
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<string> is NULL no Accept-Encoding header is generated. If <string> is a
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zero-length string, then an Accept-Encoding header containing all supported
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encodings will be generated.
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The CURLOPT_ENCODING must be set to any non-NULL value for content to be
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automatically decoded. If it is not set and the server still sends encoded
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content (despite not having been asked), the data is returned in its raw form
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and the Content-Encoding type is not checked.
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Use the --compressed option with curl to cause it to ask servers to compress
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responses using deflate.
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James Gallagher <jgallagher@gso.uri.edu>
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Dan Fandrich <dan@coneharvesters.com>
added
removed
lib/README.encoding
