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Start an asynchronous operation to read a message from the stream.

    class DynamicBuffer,
    class ReadHandler>
async_return_type< ReadHandler, void(error_code)>
    opcode& op,
    DynamicBuffer& buffer,
    ReadHandler&& handler);

This function is used to asynchronously read a message from the stream. The function call always returns immediately. The asynchronous operation will continue until one of the following is true:

This operation is implemented in terms of one or more calls to the next layer's async_read_some and async_write_some functions, and is known as a composed operation. The program must ensure that the stream performs no other reads until this operation completes.

Upon a success, op is set to either binary or text depending on the message type, and the input area of the stream buffer will hold all the message payload bytes (which may be zero in length).

During reads, the implementation handles control frames as follows:

Because of the need to handle control frames, read operations can cause writes to take place. These writes are managed transparently; callers can still have one active asynchronous read and asynchronous write operation pending simultaneously (a user initiated call to async_close counts as a write).



A value to receive the message type. This object must remain valid until the handler is called.


A dynamic buffer to hold the message data after any masking or decompression has been applied. This object must remain valid until the handler is called.


The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

void handler(
    error_code const& ec     // Result of operation

Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using boost::asio::io_service::post.