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Thread safe? or not?

Posted on 1998-04-20
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Last Modified: 2010-04-06
easy one.  are strings (and other simple types) thread safe?

if I have a global string variable defined, and two threads reading/writing it, is it safe?  or do I need to make up a thread safe string (similar to a Thread safe list - TThreadList)
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Question by:1750
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extreme earned 80 total points
ID: 1335703
You should not access strings in two thread because strings in Delphi are dynamically allocated. So, in a call to a function in a thread you can pass the address (don't forget that strings since D2 are pointers) of a string that is destroyed in another thread to store a diferent string. That would cause your program to commit an error...Maybe not an access violation, but sure it won't work as spected. Static types *could* be thread safe. I say could because that depends on your logic, in some cases access an integer (for example) between two threads could not cause problems, and sometimes not.

Consider a code where you do something like this:

globalvar:
  i : integer = 0;
 
threadcode:
while i < xx do
  begin
     inc (i, 2);
     Print (i);
  end;

That code executed in parallel won't work as spected, because after one thread modifies the value of i, the other thread inmediatly could modify it too. So, when the program executes the Print line i has a value that is equal to: Last i + 4, not + 2. this can happen, or not... depends on several factors. Do you got the idea?.

All resources that should be accessed by two or more diferent threads should be accessed using some control mechanism. Call it semaphores, critical sections, etc... The only resources that you access directly without control are the control resources, like the variables used as semaphores, or maybe some object that encapsulates the critical section logic.

About the simple types, of course that access them directly in the thread would not cause extra problems to your app besides the wrong logic, but access the strings could cause more problems than expected. When working with control algoritms don't forget to check very well the logic you use, it's a common problem when working with threads to cause a deadlock (when two threads wait infinitely for a shared resource to be available).

I hope this serve you.

Cheers
 
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by:1750
ID: 1335704
Thanks! good answer.  Having looked at the Source for TThreadList, Ive created a simple TThreadString class which uses the same locking mechanism, a "TRTLCriticalSection", and its associated methods.
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