in reply to
Re^2: Decode perl cgi response in C
in thread Decode perl cgi response in C
Maybe you want to learn about "sockets"? I assume that Unix Network Programming still is the authoritative book about this.
Basically, a "socket" is like a "file", except you can't seek on it, and it goes over the network. From C, you can read/write the socket and you only need to know about the protocol that is spoken over that socket.
The basic idea is that "download data" vs. "read data" is only a frontend issue and exists only for users, not for programs.