C++ TCP socket multiple write() on server side treated as single read on client side

Question

I'm developing a program with TCP connection, which consists of a server and multiple clients.

Server side

write(sockFd,message,strlen(message));
write(sockFd,message2,strlen(message2));

Client side

char msg[256];

bzero(msg, 256);
n = read (listenFd, msg, 255);
cout << "Msg1 " << msg << endl;

bzero(msg, 256);
n = read (listenFd, msg, 255);
cout << "Msg2 " << msg << endl;

The problem is after the server write() the two messages, the first read() on the client(s) may read all of messages from server. For example, the output is Msg 1 content-of-msg1content-of-msg2. Why it can happen?

Answer 1

TCP is a streaming protocol so data comes as a stream, this means that each read will read as much data as there is in the buffer (or as much as you requested).

So if you want to break the data down into packets or datagrams, you will need to do so with your own session based protocol. One way of doing this is prefixing the outgoing data with 4 bytes of length. This way the reader can first read the length and that way know how big the rest of the datagram is.