Sending messages from C server to Java client

Question

I'm trying to send an array of string (char ** topics) from a C server to a Java client. Apparently, the server sends the topics properly, but the client does not receive them.

/* SERVER */ while (*topics != NULL) { printf(" > Sending topic '%s'.. ", *topics); if(write(sd, *topics, sizeof(*topics)) == -1) { perror("write"); exit(1); } printf("[OK]\n"); topics++; }

The client looks like this:

/* CLIENT */ static void server_connection() { String topic = null; try { Socket _sd = new Socket(_server, _port); // Socket Descriptor // Create input stream DataInputStream _in = new DataInputStream(_sd.getInputStream()); BufferedReader _br = new BufferedReader(new InputStreamReader(_in)); System.out.println("s> Current Topics:"); while ((topic = _br.readLine()) != null) { System.out.println(topic); } if(topic == null) { System.out.println("Not topics found"); } // Close socket connection _out.close(); _in.close(); _sd.close(); } catch(IOException e) { System.out.println("Error in the connection to the broker " + _server + ":" + _port); } }

The client shows

s> Current Topics:

and remains waiting... :/

Is the C server sending a newline character over the stream? That is what readLine is blocking on in Java. — rgettman
– rgettman, Commented Mar 25, 2016 at 18:06
"the server sends the topics properly" how did you verify this? — alk
– alk, Commented Mar 25, 2016 at 18:22

user207421 · Accepted Answer · 2016-03-26 09:47:32Z

write(sd, *topics, sizeof (*topics))

topics is a char**, so *topics is a pointer to char. sizeof *topics is therefore the size of that pointer, either 2 or 4 or 8 bytes depending on your architecture. This is not what you want. You want strlen(*topics), assuming these are null-terminated strings.
As you're reading lines in the receiver, you need to send lines in the sender. Unless the data already contains a newline,, you need to add one in the sender.

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