How to stop a thread blocked on std::getline()?

Your current approach may work quite fine with normal files, as these getline() would return in a relatively predictable time and each occurence of the loop is an opportunity to leave it. The only issue is that you don't check if the read was successful or if we're aleady at the end fo the file.

If getline() ought to read from an input device like cin, you cannot prevent the function from being blocked waiting for input in absence of any fatal error. The approach is then to read the input only if we're sure there is some input to read.

Unfortunately, there is no universal and portable way to deal with such situations. The simplest approach could just be to check for available input as explained in this SO answer or here. If no input is available, you'd just loop once more giving the opportunity of stopping the thread.

thread inputThread([&]{
    while (true) {
        wstring message;
        if (!getline(wcin, message)) break
        if (!message.empty()) {
            send(sock, message.data(), (message.size()+1) * sizeof(wchar_t), 0);
        }
    }
});
running = false;
close(sock);
inputThread.detach(); // don't wait on getline