Two programs blocking each other while communicating in local loop - c++

Question

I've programmed a simple c++ client which should communicate with another program using a listener thread (a downloaded EchoServer in Java) over the local loop on Ubuntu 14.04. However, there's a problem in the following situation:
Client connects to server
Server sends greeting message
Client receives message and sends a new message to server
Client waits for an answer; the main thread sleeps and the listener thread waits for an answer using recv()
In the next step, the server should receive the message sent by the server, but it doesn't. Instead it first receives the message once the client terminates.
I think that the problem is that the client blocks resources and thus not allowing the server to receive the message, but I'm not sure. Unfortunately I don't have the option to test this on two separate machines.
Code snippets:
// main method
int main(void) {
Client client("127.0.0.1", 13050);
std::cout << client.open() << std::endl;
client.attachListener(foo);
usleep(1000 * 1000 * 2);
std::cout << client.send("hello") << std::endl;
usleep(1000 * 1000 * 5);
}
// send method
int Client::send(const char* msg) {
return write(sockfd, msg, strlen(msg));
}
// listener function
void* Client::listen() {
char buffer[256];
unsigned int receive_size = 0;
while(true) {
receive_size = 0;
while((receive_size = recv(sockfd, &buffer, 256, 0)) > 0) {
buffer[receive_size] = '\0';
msgHandler(buffer);
bzero(&buffer, 256);
}
if(receive_size == 0) {
msgHandler("Server disconnected");
} else if(receive_size == 1) {
msgHandler("Connection failure!");
}
}
return NULL;
}
Output:
1
Welcome to the Java EchoServer. Type 'bye' to close.
6

Answer 1

The EchoServer implementations typically want to see a newline on the message you send before they'll echo it back. Instead of client.send("hello") try client.send("hello\n").
Also, though this isn't really necessary for an application you're just experimenting with, you might want to turn off the Nagle algorithm on your client socket so that small messages get sent immediately. Add code like this just after the point where you call connect with client socket:
int flag = 1;
int res = setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, &flag, sizeof flag);
if (res < 0) // handle setsockopt failure here...
This code requires these header files:
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>