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The Expect field with the value "100-continue" in a request is
special. It indicates that the after sending the message header, a client
desires an immediate informational response before sending the message body,
which presumably may be expensive to compute or large. This behavior is described
in rfc7231
section 5.1.1. Invoking the 100-continue behavior is implemented
easily in a client by constructing a serializer
to send the header first,
then receiving the server response, and finally conditionally send the body
using the same serializer instance. A synchronous, simplified version (no
timeout) of this client action looks like this:
/** Send a request with Expect: 100-continue This function will send a request with the Expect: 100-continue field by first sending the header, then waiting for a successful response from the server before continuing to send the body. If a non-successful server response is received, the function returns immediately. @param stream The remote HTTP server stream. @param buffer The buffer used for reading. @param req The request to send. This function modifies the object: the Expect header field is inserted into the message if it does not already exist, and set to "100-continue". @param ec Set to the error, if any occurred. */ template< class SyncStream, class DynamicBuffer, class Body, class Allocator> void send_expect_100_continue( SyncStream& stream, DynamicBuffer& buffer, request<Body, basic_fields<Allocator>>& req, error_code& ec) { static_assert(is_sync_stream<SyncStream>::value, "SyncStream requirements not met"); static_assert( boost::asio::is_dynamic_buffer<DynamicBuffer>::value, "DynamicBuffer requirements not met"); // Insert or replace the Expect field req.set(field::expect, "100-continue"); // Create the serializer request_serializer<Body, basic_fields<Allocator>> sr{req}; // Send just the header write_header(stream, sr, ec); if(ec) return; // Read the response from the server. // A robust client could set a timeout here. { response<string_body> res; read(stream, buffer, res, ec); if(ec) return; if(res.result() != status::continue_) { // The server indicated that it will not // accept the request, so skip sending the body. return; } } // Server is OK with the request, send the body write(stream, sr, ec); }