即使进程终止，waitpid始终返回-1

Question

I'm on Mac and I use Qt 5. I have a button in my QMainWindow that execute a program when the it s clicked.

I used this static function to execute a detached process :QProcess::startDetached ( http://doc.qt.io/qt-5/qprocess.html#startDetached ).

I have no problem to start / kill a process with this function on Mac.

I would like to determine if the user terminates the process (an OpenGL application). I used this code

void MyProgram::startApplication()
{
bool ret = QProcess::startDetached(program,arguments, workingDirectory, &m_PID);
if (ret && m_PID)
{
    printf("m_PID = %d (started)\n", (int)m_PID);
    QThread* thread = new QThread;
    connect(thread, &QThread::started, [this]() {
        qint64 pid = m_PID;
        printf("pid = %d (started)\n", (int)pid);
        QThread::msleep(2000);
        while (pid)
        {
            int wstatus = 0;
            pid_t ret = waitpid(pid, &wstatus, WNOHANG);
            if (ret > 0)
            {
                if (WIFSIGNALED(wstatus) || WIFSTOPPED(wstatus) || WIFEXITED(wstatus))
                {
                    // dont kill or stop cuz the process is already finished
                    printf("pid = %d ret = %d (stopped)\n", (int)pid, (int)ret);
                    break;
                }
            }
            else if (ret < 0)
            {
                printf("pid = %d ret = %d(error)\n", (int)pid, (int)ret);
                //break;
            }
            QThread::msleep(200);
        }
        m_PID = 0;
    });
    }

   bool MyProgram::IsRunning()
   {
       return m_PID != 0;
   }

This problem is that waitpid() always returns -1 and I can't never know when the started process is finished or killed.

I think that waitpid works only when you re the parent no in detached process but kill(m_PID, SIGINT) works (stop the process)

I need only to know if the process is finished or not

Answer 1

QProcess::startDetached documents that:

Unix: The started process will run in its own session and act like a daemon.

(so I guess it calls daemon(3) or do something equivalent)

If you really want to use waitpid(2) you'll better use the usual fork(2) and execve(2) in your own code (but that is not the Qt way of doing).

BTW, why don't you use QProcess the usual way? It should be more Qt friendly (and more portable to other OSes). You'll use (ie connect to some of your slot ) the QProcess::finished qt-signal.

but kill(m_PID, SIGINT) works

Of course, kill(2) can be used on some non-child process.