получить сообщение от сокета (receive a message from a socket)
Возвращаемое значение (Return value)
These calls return the number of bytes received, or -1 if an
error occurred. In the event of an error, errno is set to
indicate the error.
When a stream socket peer has performed an orderly shutdown, the
return value will be 0 (the traditional "end-of-file" return).
Datagram sockets in various domains (e.g., the UNIX and Internet
domains) permit zero-length datagrams. When such a datagram is
received, the return value is 0.
The value 0 may also be returned if the requested number of bytes
to receive from a stream socket was 0.
