[fpc-devel] ref count types / threadsave question
fpc at mfriebe.de
Wed Jan 2 20:08:42 CET 2019
Below is for ansistring, but same should apply to dyn arrays.
Procedure fpc_AnsiStr_Incr_Ref (S : Pointer);
[Public,Alias:'FPC_ANSISTR_INCR_REF']; compilerproc; inline;
If S=Nil then
// anytime after this line:
// other thread, and only ref-count holder calls fpc_ansistr_decr_ref
// and finishes before "inclock" gets the lock => so memory gets freed
If PAnsiRec(S-AnsiFirstOff)^.Ref<0 then exit;
what happens if another thread holds the only reference to the string
(ref count = 1), and the other thread calls and finishes
fpc_ansistr_decr_ref before inclocked reaches the point where it
actually gets the lock?
In that case the memory holding the ref-count would be freed, before
inclock acquires the lock?
What I was actually thinking off was an potential optimization proposal.
If a local string var has a refcount of 1, then it can not be accessed
by any other thread. Therefore it needs no lock for decreasing the ref.
- The only one ref has to be hold by the current thread
- Since it is a local var no other thread can see the identifier, and
therefore this var can not be inside a fpc_AnsiStr_Incr_Ref in another
You could have a pointer (no ref count) to the string, but passing that
to another thread would be unsafe anyway, and not get anymore unsafe by
such a change.
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