如何在没有标头的TCP协议中使用boost :: asio来告知消息的大小？

Question

The Boost chat server example demonstrates handling a simple TCP message protocol in which each message is preceded by a fixed-size header which tells you the size of the message which follows. This means you always know exactly how many bytes to read in your next call to async_read() ; you alternate between reading a header whose size is always the same, and a message whose size is given in the header. This works well with the Boost i/o service model, which promises to call a handler when exactly the expected number of bytes have been received from the socket.

How can I use Boost to run a TCP protocol which doesn't use a header like this? My client has a protocol which uses special byte sequences to represent the start and end of each message, so I won't know how many bytes to read in each call to async_read() ; I have to just get bytes from the socket as they arrive and watch for the special byte sequences. If I pick a sensible buffer size like 256 bytes, and if my handler will only be called when that many bytes have been read, I believe the i/o service will generally end up receiving the last few bytes of the most recent message from the network, but not passing them to my handler until the next message comes along and brings the byte total up to the number I'm expecting. The next message may not arrive for some time, and I want to handle the current message as soon as it arrives.

Reading one byte at a time isn't a good idea for performance reasons, correct?

http://www.boost.org/doc/libs/1_45_0/doc/html/boost_asio/examples.html

Answer 1

There is few options:

• You can use async_read_until to read until your "ending sequence"(so until end of message).

• If your "ending sequence" depends on "starting sequence", you can make it to read fixed buffer (equal to starting sequence length); calculate the ending sequence; and then setup async_read_until .

• Also, you can make call to async_read_some to read any amount of bytes which arrived into socket buffer. Then check your buffer with your own function for containing complete packet or need to read next part.