为什么在发布模式下不能访问 for 循环，但在调试中它工作正常

Question

my problem is that I coded server/client application on windows. Now in debug mode everything works fine and as it should , but in release mode the server doesn´t receive or send messages.

I think it is due to that, that the for loop inside the infinite for loop doesn´t get accessed.I also tried to implement this solution with a while but it didn´t work. I think it might be the problem that i call a function in the condition field, because when i compare the i to an integer it gets accessed. Also interesting is that when i std::cout something right before the inner for loop , the loop gets also accessed , despite the fact that I am calling the function in the condition field.

#include <iostream>
#include <thread>
#include "server.cpp"

//gets defined in server.cpp
void server::acceptConn() {
    u_long mode =1;
    for(;;){
        int len = sizeof(incAddr[connectedSocks]);
        if((inc_sock[connectedSocks] = accept(serv,(struct sockaddr *)&incAddr[connectedSocks],&len))!= SOCKET_ERROR){
            std::cout<<"client connected : "<<inet_ntoa(incAddr[connectedSocks].sin_addr)<<std::endl;
            ioctlsocket(inc_sock[connectedSocks],FIONBIO,&mode);
            connectedSocks++;
        }
    }
}

int main() {
    server ser;
    ser.init_Server();
    std::thread t(&server::acceptConn,&ser);
    char buf[1024];
    for(;;){
        for(int i=0 ; ser.getCounter()>i;i++){
            if (recv(ser.getInc_Sock()[i], buf, sizeof(buf), 0) == SOCKET_ERROR) {

            } else{
                for (int x = 0; x < ser.getCounter(); x++) {
                    if(x==i){//just that the message doesnt get send back to the sender}
                    else{
                        if (send(ser.getInc_Sock()[x], buf, sizeof(buf), 0) == SOCKET_ERROR) {
                            std::cout<<"send failed : "<<WSAGetLastError()<<std::endl;
                        }
                    }
                }
            }
        }
    }
}

int getCounter(){return  connectedSocks;};//in server.h

The result should be that Server is having a List of connected socks and is distributing the messages to everyone. Now when I am running it in debug mode everything works fine. What could be the problem?

Answer 1

Your code lacks any form of synchronization between the two threads. The worker thread writes data, and the main thread reads data. But because there is no synchronization between them (mutex/atomics/etc), every read from the main thread is a data race with the writes from the worker thread.

And data races are undefined behavior. So the compiler is permitted to compile the main thread as if nobody ever writes data .

When you put a std::cout in your loops, this "works" because std::cout requires some form of synchronization to keep multiple threads from writing to stdout at the same time. So you're effectively piggybacking off of that internal synchronization. But you shouldn't rely on that; you should instead use proper synchronization primitives.