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Threadsafe function

Posted on 2001-07-11
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Last Modified: 2012-05-04
Lets say I have a simple function which don't interacts with the VCL, e.g. adding two numbers.
If I call this function the same time from two threads, would that cause an error.
Would it make a differens if the function creates an local object and frees it after use. Then two objects should be created I guess.

Please higher my thread knowledge :)
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Question by:koger
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3 Comments
 
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Expert Comment

by:Epsylon
ID: 6274999
You can call the same function from 2 different threads as long as the function is thread-safe. Functions like Pos(), IntToStr() and Length() are safe. The same counts for Objects. Each thread can create its own objects. E.g. TStringlist and TMemoryStream.
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by:alx512
ID: 6275785
Use threadvar directive instead of var.
A thread-local variable is like a global variable, except that each thread of execution gets its own private copy of the variable, which cannot be accessed from other threads.

Thread-variable declarations
  cannot occur within a procedure or function.
  cannot include initializations.
  cannot specify the absolute directive.

Reference-counted variables (such as long strings, dynamic arrays, or interfaces) are not thread-safe, even if they are declared with threadvar. Do not use dynamic thread variables, since there is in general no way to free the heap-allocated memory created by each thread of execution. Finally, do not create pointer- or procedural-type thread variables.
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Accepted Solution

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Madshi earned 200 total points
ID: 6275903
alx512 is right, but only if you need something like global variables. THEN threadvar is one solution, and THEN dynamic reference-counted variables are not thread safe in the meaning, that threadvar don't secure them automatically.
If you need global variables, you can also not use threadvar, but normal global variables and secure them with e.g. critical sections. In that case you can also use dynamic reference-counted variables. Of course its more work than threadvars, because you have to secure the access to the variables yourself.

But I think koger was only talking about local variables, and there you can use whatever you like, it's all thread safe. Epsylon is right there. Also you can let 1000 threads run the same code at the same time, no problem at all... Look, in win9x the system dlls are all shared between all processes. Nevertheless all threads can theoretically run through the system APIs at the same time, no problem. Each thread has its own stack and its own code pointer.

Regards, Madshi.
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