timeout and select() not working as expected - c++

Question

I'm trying to communicate with a server using an rcon protocol to take control over a gamesever. So far I was using an exisiting C# library for this but this library is sometimes quite buggy and therefore I'm was writing my own application in C++ so that I'm able to use it both on windows and on my linux server.
So far this worked out pretty well but now I'm running into problems when I try to set a timeout using select() to find out if the server still started and responding to my commands. During the first run of the application select() always returns the right value, but after closing and running it again, results get weird.
My code pretty much looks like this:
#include <stdio.h>
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string.h>
#include <unistd.h>
// Socket Includes
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
// Other Includes
#include "main.h"
#include "crc32.h"
int main() {
struct sockaddr_in server;
int mysocket, slen = sizeof(server);
char buffer[2048];
if ((mysocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
printf("error creating socket");
memset((char *) &server, 0, sizeof(server));
server.sin_family = AF_INET;
server.sin_addr.s_addr = inet_addr("xxx.xxx.xxx.xxx");
server.sin_port = htons(1234);
if(connect(mysocket, (sockaddr*) &server, sizeof(server)) < 0)
printf("error connecting the socket");
bool isConnected = false;
/*
Creating the Packet to send, cut to save some space
*/
sendto(mysocket, loginPacket.c_str(), loginPacket.length(), 0, (sockaddr *) &server, slen);
while(1) {
// clearing the buffer
bzero(buffer, 2048);
// Timeout Settings
int selectSize = 0;
struct timeval timeout;
timeout.tv_sec = 5;
timeout.tv_usec = 0;
fd_set fds;
FD_ZERO(&fds);
FD_SET(mysocket, &fds);
selectSize = select(mysocket + 1, &fds, 0, 0, &timeout);
std::cout << "Size is: " selectSize << std::endl; // just for testing :)
if (selectSize == 1) {
int recvLength = recvfrom(mysocket, buffer, sizeof(buffer), 0, (sockaddr *) &server, (socklen_t*) &slen);
if (buffer[7] == 0x00) {
if (buffer[8] == 0x01) {
//password correct
isConnected = true;
break;
}
if (buffer[8] == 0x00) {
//password wrong, do sth.
}
}
}
}
// tests
sayHello();
close(mysocket);
return 0;
}
When I start the script for the first time and the server is not started, everything works as expected, after 5 seconds selectSize returns the value 0. It loops further on until I start the the server, then it will return value 1 and break the while loop. Afterwards I quit the application, turn off the server and start the script again. Instead of return value 0 after 5 seconds, it immediately returns value 1 (even though the server is offline and there's not packet to receive) and then after 5 seconds it will return value 0. Running the same script (with some adjustments) on windows gave me pretty much the same result, but selectSize pretty much always returned 1, even though the server was offline and the value should have been 0. I read tons of sites about using select() the right way but none of them helped me so far as select() is not returning reliable results after quitting and restarting the application.
P.S.: There was some confusing information about how to use select() the right way, I used pretty much every solution provided on the internet like:
selectSize = select(mysocket + 1, &fds, 0, 0, &timeout);
selectSize = select(0, &fds, 0, 0, &timeout);
selectSize = select(1, &fds, 0, 0, &timeout);
But none of them gave me a reliable result.