The meaning of “volatile”

Hans-J. Boehm,

Basic View of Volatile/Atomic

• If the only races involve synchronization variables (e.g. Java volatiles), then multithreaded execution can be viewed as interleaving:– “Dekker’s” example (everything initially zero):

Thread 1 Thread 2x = 1; y = 1;r1 = y; r2 = x;

– Might be executed as:x = 1; y = 1; r2 = x; r1 = y; orx = 1; y = 1; r1 = y; r2 = x;

– Outcome r1 = r2 = 0 is impossible• Java volatile, C++0x atomic<T> declare sync. variables.

*Provided certain esoteric library calls are avoided.

Details across languages are confusing

Java volatile y y y n y n

C++0x SC (default) atomic<T> y y y n y y

C++0x low level atomic<T> n y y n s as

j.u.c.atomic def y y n def as

C + posix volatile n n n y m n

Java ordinary variables n y s n n n

C ordinary variables n n n n n n

def=“by default” as=“with alternate syntax” s=“sometimes” m=“maybe”

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