Delphi in a Nutshell

Name

Threadvar Keyword

Syntax

threadvar Name = Type; ...

Description

The threadvar keyword starts a variable declaration in the same manner as the var keyword, except that threadvar can be used only at the unit level. Variables declared using threadvar have distinct values in each thread. Ordinary variables, on the other hand, are shared among all threads in a process.

Tips and Tricks

• If you are using the TThread class (Classes unit), you will probably find it more convenient to use fields to store per-thread data.

• If you are writing a reusable subroutine or class that might be used in multiple threads, you can use threadvar variables to keep track of per-thread information.

• Threadvar declarations apply only to the variable itself. If the variable is a pointer or encapsulates a pointer (e.g., object, long string, dynamic array, Variant), the data might be accessible from multiple threads.

• Threadvar variables use Windows thread-local storage (TLS). Every variable access requires a function call, so another reason to use the TThread class is that access to the object’s fields is faster than using threadvar variables.

• Every module that uses a threadvar variable requires a TLS slot, but Windows has a small number of TLS slots available. A large project should use packages to ensure the availability of this limited resource.

  Every module (application, DLL, package) that contains a threadvar variable needs its own, separate TLS slot. If you link a DLL statically, the DLL contains its own ...