Start an asynchronous operation to read a certain amount of data from a stream.
template< typename AsyncReadStream, typename MutableBufferSequence, typename ReadHandler> void async_read( AsyncReadStream & s, const MutableBufferSequence & buffers, ReadHandler handler);
This function is used to asynchronously read a certain number of bytes of data from a stream. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:
- The supplied buffers are full. That is, the bytes transferred is equal to the sum of the buffer sizes.
- An error occurred.
This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. The program must ensure that the stream performs no other read operations (such as async_read, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.
template< typename MutableBufferSequence, typename ReadHandler> void async_read_some( const MutableBufferSequence & buffers, ReadHandler handler);
This function is used to asynchronously read data from the stream socket. The function call always returns immediately.
总结一下：
asio::async_read 通常用户读取指定长度的数据，读完或出错才返回。
而socket的async_read_some读取到数据或出错就返回，不一定读完了整个包。