Question

i have multiple threads running the following QProcess. Randomly they fail with error state 5. The Qt docs do not give any more details. Has anyone a clue what that error could come from? Thank you very much.

extCmd = new QProcess(this);

QString cmd = "/usr/bin/php";
QStringList argStr;
argStr << "/bin/sleep" << "10"; // changed to ever working command
extCmd->start(cmd, args);
bool suc = extCmd->waitForFinished(-1);
if (!suc) {
   qDebug() << "finishing failed error=" 
            << extCmd.error() 
            << extCmd.errorString();
}

Gives me the output:

finishing failed error= 5 "Unknown error"

Answer 1

Tangential to your problem is the fact that you should not be starting a thread per each process. A QProcess emits a finished(int code, QProcess::ExitStatus status) signal when it’s done. It will also emit started() and error() upon successful and unsuccessful startup, respectively. Connect all those three signals to a slot in a QObject, then start the process, and deal with the results in the slots. You won’t need any extra threads.

If you get a started() signal, then you can be sure that the process’s file name was correct, and the process was started. Whatever exit code you get from finished(int) is then indicative of what the process did, perhaps in response to potentially invalid arguments you might have passed to it. If you get a error() signal, the process has failed to start because you gave a wrong filename to QProcess::start(), or you don’t have correct permissions.

You should not be writing synchronous code where things happen asynchronously. Synchronous code is code that blocks for a particular thing to happen, like calling waitForCmdFinished. I wish that there was a Qt configuration flag that disables all those leftover synchronous blocking APIs, just like there’s a flag to disable/enable Qt 3 support APIs. The mere availability of those blocking APIs promotes horrible hacks like the code above. Those APIs should be disabled by default IMHO. Just as there should be a test for moving QThread and derived classes to another thread. It’s also a sign of bad design in every example of publicly available code I could find, and I did a rather thorough search to convince myself I wasn’t crazy or something.

The only reasonable use I recall for a waitxxx method in Qt is the wait for a QThread to finish. Even then, this should be only called from within the ~QThread, so as to prevent the QThread from being destroyed with the tread still running.