[fpc-devel] volatile variables

Michael Schnell mschnell at lumino.de
Tue Jun 28 13:14:54 CEST 2011

A similar discussion is going on in Lazarus-develop, but this obviously 
is a compiler question.

In C, there is the volatile keyword that ensures that after the code 
sequence enters the next C instruction after that which modified this 
variable, another thread sees the correct state of the variable.

This is ensured by not caching the value in registers and on SMP systems 
additionally by low level stuff like memory barriers.

Of course this does not protect concurrent read-modify-write accesses 
(same need to be protected by low level atomic instructions or MUTEX and 

For variables not defined as volatile, (e.g.) pthread_mutex (and similar 
stuff on Windows) can be used to protect them.

AFAIK, in Pascal all Global (Static) variables are considered to be 
handled as volatile, so no problem here (other than handling all of them 
them as volatiles might decrease performance slightly).

But what about variables on the heap ? If class instance variables or - 
say - a liked list done with records created by "new" are accessed by 
multiple threads, how can said consistency be enforced ?

Even if you use a critical section to do the protection, this does not 
help if the compiler decides to cache the variable in a register while 
entering or exiting the critical section.

But even without dedicated protection, a volatile variable should be 
able to be be monitored from another thread. Can this be enforced in 
Pascal for help variables ?


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