[DUG] A Super Simple Summary of Thread Safe

[Dennis Chuah]

I'll try not to be a cheer leader ...

1.  Reading and writing to memory is *DEFINITELY* not thread safe, however 
..

a.  Reading from a memory location that is never written to after it has 
been initialised is thread safe (just remember to bracket the initialisation 
code with a critical section).

b.  If you want to share memory, consider using thread safe classes such as 
TThreadList.

2.  The whole VCL (not just screen controls) is not thread safe.  You think 
that you have gotten away with threaded access to some parts, but bugs will 
eventually show up, and they will be very hard to debug, and if you get an 
AV, the stack trace will most probably not show you where the bug is. 
Example problem:  We have a piece of code that generates metafiles to disk. 
It needs a window handle, and so it creates a hidden TForm object and uses 
its handle.  This is all done on a worker thread, ie., not the main thread. 
The code works fine 99.999% of the time, but occasionally, we get a list 
index out of bounds error.  After a lot of debugging, I finally managed to 
track down the problem to TScreen.  The list error happens when the form is 
freed, and TScreen is in the middle of updating a list of forms, the for 
loop runs past the end!  In this sense, it is not only not thread safe, it 
is thread deadly!  Actually, the problem is the VCL is not re-entrant safe.

3.  Classes such as TStringList, TStream, TList, TCollection are not thread 
safe, but OK if you only use them from one thread.  Beware of TStrings - it 
is an abstract class and some of the subclasses may not be thread safe - eg. 
if the subclass is the Lines of a TMemo or Items of a TListbox / TCombobox.

Dennis.

----- Original Message ----- 
From: "Matt Comb" <matt at ferndigital.com>
Sent: Monday, June 20, 2005 11:17 PM


>I would agree with everything you just wrote, but I'd be labelled a 
>cheerleader again ;)
>
> Matt.
> ----- Original Message ----- 
> From: "Kyley Harris" <KyleyHarris at gmail.com>
> To: <delphi at ns3.123.co.nz>
> Sent: Monday, June 20, 2005 11:06 PM
> Subject: [DUG] A Super Simple Summary of Thread Safe
>
>
>>A couple of simple things related to some of the conversations going on.
>>
>> 1/ Reading Memory is always threadsafe, unless the memory was freed. If 
>> you don't use critical sections, you may read the wrong thing, because 
>> something else overwrote the memory while you are reading it, in full or 
>> part.
>> 2/ Writing to Memory is not threadsafe, because you are altering the 
>> memory structure, especially in more complex structures such as lists, 
>> strings, objects etc. Writing to an integer could be considered 
>> threadsafe, but it probably depends on the compiler, the computer, the 
>> stack size, the natural integer size, etc etc.. The Purpose of a Critical 
>> Section is much like a database transaction, to ensure that what you read 
>> and write is correct at any given time, and to create a hands off 
>> approach. if you do lots of reading, with few writes, and need speed, use 
>> a TMultiReadWriteSyncronizer.
>>
>> 3/ Writing to the VCL GUI controls is only ever safe if the 
>> CurrentThreadID = MainThreadID. It does not matter if you are reading or 
>> writing. Why? Most of the controls like TEdit etc are TWinControls. 
>> TWinControls have a Handle. if you use the handle, or create the handle 
>> in any thread other than the main one windows will throw silent tantrums 
>> and simply ruin the stability of your application. This is a design 
>> issue. Don't expect to find it in the help files, or anywhere really :)
>>
>> There are plenty more things that you need to do to make sure a GUI 
>> accessing threaded processing doesn't go down the toilet but I hope this 
>> helps.
>>
>> regards
>>  Kyley.
>>
>> PS. This may not be 100% semantically accurate, but who has time for that 
>> in these posts :D
>>
>> _______________________________________________
>> Delphi mailing list
>> Delphi at ns3.123.co.nz
>> http://ns3.123.co.nz/mailman/listinfo/delphi
>>
>>
>
> _______________________________________________
> Delphi mailing list
> Delphi at ns3.123.co.nz
> http://ns3.123.co.nz/mailman/listinfo/delphi
>