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.