Getting shell command output in c++ before it ends running

Question

Is there anyway to get the command stdout and stderr output before it ends running? I've been trying popen and here's the code:

string exec(const char* cmd) {
    char buffer[128];
    string result;
    string modCmd = (string)cmd + (string)" 2>&1";
    FILE* pipe = popen(modCmd.c_str(), "r");
    if (!pipe) throw runtime_error("popen() failed!");
    try {
        while (!feof(pipe)) {
            if (fgets(buffer, 128, pipe) != nullptr)
            {
                result += buffer;
                cout << buffer;
            }
        }
    } catch (...) {
        pclose(pipe);
        throw;
    }
    pclose(pipe);
    return result;
}

but the problem is, all the cout s stack up and run after the command's running is over.
PS The command I'm trying to execute is aria2c

Answer 1

如@VTT所述，问题是aria2c不会刷新其输出，但是有一种解决方法是运行stdbuf -o0 aria2c而不是运行aria2c ...