multithreading - Threadsafe test / decrement -

is code threadsafe, or fcount changed thread before interlockeddecrement executed?

procedure tmyobject.wait; begin   if fcount > 0     interlockeddecrement(fcount);   .. end; 

it's not thread-safe.

• thread 1 reads fcount=1, evaluates condition true.
• thread 2 reads fcount=1, evaluates condition true.
• thread 1 decrements fcount 0
• thread 2 decrements fcount -1

yet assume code intended prevent reducing fcount below zero.

you might want consider following instead:

if interlockeddecrement(fcount) < 0   interlockedincrement(fcount); 

in way, 1 of 2 concurrent threads reduce value -1 , "fix mistake".
however, have side-effect fcount may temporarily < 0.