Sockets - recv returning -1

#include "stdafx.h"
#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include <winsock.h>
#include <vector>
#include "Player.h"

#pragma comment(lib, "Ws2_32.lib")

std::vector<Player> players;

int main()
{
	printf("Server started\r\n");

	SOCKET connection_socket;
	
	WSADATA wsaData;
	sockaddr_in server;

	// start winsock
	int ret = WSAStartup(MAKEWORD(2, 2), &wsaData);

	if (ret != 0) return 0;

	// fill in winsock struct ... 
	server.sin_family = AF_INET;
	server.sin_addr.s_addr = INADDR_ANY;
	server.sin_port = htons(123);

	connection_socket = socket(AF_INET, SOCK_STREAM, 0);

	if (connection_socket == INVALID_SOCKET) return 0;

	// bind our socket to a port(port 123) 
	if (bind(connection_socket, (sockaddr*)&server, sizeof(server)) != 0) return 0;
	// listen for a connection  
	if (listen(connection_socket, 5) != 0) return 0;

	// socket that we send and receive data on
	SOCKET client;

	sockaddr_in from;
	int fromlen = sizeof(from);

	// loop forever 
	while (true)
	{
		// accept connections
		client = accept(connection_socket, (struct sockaddr*)&from, &fromlen);
		printf("Client connected\r\n");

		Player p;
		PlayerConnection* connection = new PlayerConnection();
		connection->init(client);
		p.init(connection);
		players.push_back(p);
	}

	closesocket(connection_socket);
	WSACleanup();

	return 0;
}

#include "stdafx.h"
#include "PlayerConnection.h"

#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include <winsock.h>
#include <vector>
#include <iostream>

struct ThreadData
{
	PlayerConnection* instance;
	SOCKET& socket;
};

DWORD WINAPI ThreadProcedure(LPVOID lpParam)
{
	ThreadData* data = (ThreadData*)lpParam;
	PlayerConnection* instance = data->instance;
	SOCKET socket = data->socket;

	char buf[100];
	int res;

	// if buffer = 'hello': strstr(buf, "hello")

	// receive messages
	while (true)
	{
		res = recv(socket, buf, sizeof(buf), 0); // read data

		Sleep(10);
		printf("res: %i\n", res);
		if (res <= 0)
		{
			printf("EXITING THREAD");
			closesocket(socket);
			ExitThread(0);
		}

		if (strstr(buf, "exit"))
		{
			instance->disconnect();
		}
		else
		{
			printf("Client said: %s\n", buf);
			Sleep(10);

			char txbuf[100];
			strcpy_s(txbuf, "You said: ");
			strcat_s(txbuf, buf);
			instance->send_message(txbuf);
		}

		strcpy_s(buf, "");
	}

	return 0;
}

void PlayerConnection::init(SOCKET& socket)
{
	this->socket = socket;

	ThreadData td = { this, socket };
	HANDLE threadHandle = CreateThread(NULL, 0, ThreadProcedure, &td, 0, NULL);
}

void PlayerConnection::send_message(char* buf)
{
	send(socket, buf, strlen(buf), 0);
}

void PlayerConnection::disconnect()
{
	closesocket(socket);
	ExitThread(0);
}