Strange things with Winsock2 and system start command

There is a program

#include <Winsock2.h>
#include <stdint.h>
#include <thread>

const uint16_t ANY_VALID_PORT = 12345;
SOCKET sock = INVALID_SOCKET;

void listenPort()
{
    sockaddr_in addr {};
    int addrSize = sizeof(addr);
    accept(sock, (SOCKADDR*)&addr, &addrSize);
}

// #define CORRECT_SEQUENCE

int main()
{
    WSADATA wsaData = { 0 };
    WSAStartup(MAKEWORD(2, 2), &wsaData);
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    sockaddr_in clientService {};
    clientService.sin_family = AF_INET;
    clientService.sin_port = htons(ANY_VALID_PORT);
    clientService.sin_addr.s_addr = INADDR_ANY;
    bind(sock, (SOCKADDR*)&clientService, sizeof(clientService));
    listen(sock, 2);

#ifdef CORRECT_SEQUENCE
    system("start cmd");
    std::thread listening { listenPort };
#else
    std::thread listening { listenPort };
    system("start cmd"); //could be virtually any program
#endif

    closesocket(sock);
    listening.join();

    WSACleanup();
}

Expected behavior: the program finishes while cmd continues to run in a separate window.
Real behavior: the program doesn't finish until the new cmd is closed.
When CORRECT_SEQUENCE is defined, it works just fine. But without it, accept() doesn't return even if sock is closed. The debugger tells us that we are stuck up on join().

What is the reason for this dependency? cmd (or even empty program with only while(1); in the main()) have nothing with our port.

> A socket handle created by the WSASocket or the socket
> function is inheritable by default.

the system start process with CreateProcessA with bInheritHandles = TRUE. as result sock is duplicated to child process.

accept return control, when last handle to sock closed, but after system("start cmd"); exist another sock handle in cmd. only after you close cmd - accept return with error (probably WSAEINTR )

also the the both variants of your code is wrong. and both not work. you also can call closesocket(sock); before call accept. so all is wrong.

also system("start cmd"); is extermally not efficient. this call CreateProcessA for cmd.exe with command line /c start cmd - so first cmd run second cmd and exit..

1. use WSASocketW with WSA_FLAG_NO_HANDLE_INHERIT but never socket
2. never use system. use CreateProcessW.
3. not call cmd /c start cmd. if you need cmd - just exec cmd
4. not pass handle ( sock ) to another thread and in parallel close it