What is the difference between QProcess::start and QProcess::startDetached?

Question

The Qt documentation gives this explanation:

• QProcess::start:

  Starts the given program in a new process, if none is already running, passing the command line arguments in arguments.

• QProcess::startDetached:

  Starts the program program with the arguments arguments in a new process, and detaches from it.

What is the difference between the two? Is the difference only that you can start just one instance of a program using QProcess::start and many instances using QProcess::startDetached?

Answer 1

If you use start, termination of the caller process will cause the termination of the called process as well. If you use startDetached, after the caller is terminated, the child will continue to live. For example:

QProcess * p = new QProcess();
p->start("some-app");
delete p;// <---some-app will be terminated

QProcess * p = new QProcess();
p->startDetached("some-app");
delete p;// <---some-app will continue to live

Answer 2

The start() function is a member function, while startDetached is a static class function.

If you look at the documentation of QProcess, you'll see that there are functions to allow you to do things with the process such as: -

• Receive the output or error streams from the running process (readAllStandardOutput / readAllStandardError)
• Redirect the output to a file (setStandardOutputFile)
• Use a file for standard input into the process (setStandardInputFile)
• Communicate via channels
• Get notified when the process has finished

These are just some of the things that you can only do with an instance of a QProcess. If, however, you want a simple and quick way of starting a process without having to create an instance and you don't need the extra functionality, you can simply call QProcess::startDetached.

Also, as the docs state for startDetached: -

If the calling process exits, the detached process will continue to live.