When should I use std::thread::detach?
In the destructor of std::thread, std::terminate is called if:
- the thread was not joined (with
t.join()) - and was not detached either (with
t.detach())
Thus, you should always either join or detach a thread before the flows of execution reaches the destructor.
When a program terminates (ie, main returns) the remaining detached threads executing in the background are not waited upon; instead their execution is suspended and their thread-local objects destructed.
Crucially, this means that the stack of those threads is not unwound and thus some destructors are not executed. Depending on the actions those destructors were supposed to undertake, this might be as bad a situation as if the program had crashed or had been killed. Hopefully the OS will release the locks on files, etc... but you could have corrupted shared memory, half-written files, and the like.
So, should you use join or detach ?
- Use
join - Unless you need to have more flexibility AND are willing to provide a synchronization mechanism to wait for the thread completion on your own, in which case you may use
detach
You should call detach if you're not going to wait for the thread to complete with join but the thread instead will just keep running until it's done and then terminate without having the main thread waiting for it specifically.
detach basically will release the resources needed to be able to implement join.
It is a fatal error if a thread object ends its life and neither join nor detach has been called; in this case terminate is invoked.
When you detach thread it means that you don't have to join() it before exiting main().
Thread library will actually wait for each such thread below-main, but you should not care about it.
detach() is mainly useful when you have a task that has to be done in background, but you don't care about its execution. This is usually a case for some libraries. They may silently create a background worker thread and detach it so you won't even notice it.