Delphi threadvar question

Question

When we have a threadvar declared, when this variable will be initialized (the object is created)? Does it occurs at the first assignment of the var? For example:

threadvar
  myThreadVar : string;

......

//inside a thread
  ...
  myThreadVar := 'my value'; // In this point the var will be initialized?

What happens if I try to use this var outside a thread after the thread has set the value for the var? For example:

//at the main thread (application)
  ...
  //Call the thread;
  //thread finishes execution
  //thread is destroyed
  ShowMessage(myThreadVar); // what happens here?

Answer 1

The threadvars for a thread are initialized the first time their thread accesses any one of them. They are set to a default all-bits-zero value, which for strings is the empty string.

Threadvars may or may not be finalized. It depends on how much notice the RTL gets that a thread is terminating. For that reason, it's probably best not to store any dynamically allocated types (strings included) in threadvars. Instead, use an instance variable of a TThread object to store thread-specific data.

---

The second part of your question is nonsense. It has you executing code on a thread after the thread has already terminated. There is no such thing as running code "outside a thread." All code runs in threads. Every program has at least one thread.

Each thread has its own copy of a threadvar. No thread can read another thread's copy, so once a thread terminates, all its threadvars are inaccessible.

Your ShowMessage call will display the value belonging to the current thread, not the thread that already terminated.

Answer 2

The thread-local storage will be zeroed out (initialized) when the thread is created. So before you run the line myThreadVar := 'my value';, it'll be an empty string.

As for your second question, threadvars are unique to each thread. When you declare a threadvar, you declare a slot in thread-local storage, and each thread gets a copy of the slot. You can think of it as kind of like thread1.myThreadVar, thread2.myThreadVar, mainThread.myThreadVar, etc. So if you set a threadvar in one thread and try to read it in another, you won't read what you set in the other thread; you'll read whatever is assigned to the current thread's version of the threadvar.

Answer 3

threadvar means that you have an instance of the variable per thread. There's no such thing as 'outside a thread' - if you're not running inside an explicit thread that you created, you're running on the process' default thread. if you set a threadvar to a value within an explicit thread, that value is invisible to all other threads.