C++ – I’ve heard i++ isn’t thread safe, is ++i thread-safe


I've heard that i++ isn't a thread-safe statement since in assembly it reduces down to storing the original value as a temp somewhere, incrementing it, and then replacing it, which could be interrupted by a context switch.

However, I'm wondering about ++i. As far as I can tell, this would reduce to a single assembly instruction, such as 'add r1, r1, 1' and since it's only one instruction, it'd be uninterruptable by a context switch.

Can anyone clarify? I'm assuming that an x86 platform is being used.

Best Solution

You've heard wrong. It may well be that "i++" is thread-safe for a specific compiler and specific processor architecture but it's not mandated in the standards at all. In fact, since multi-threading isn't part of the ISO C or C++ standards (a), you can't consider anything to be thread-safe based on what you think it will compile down to.

It's quite feasible that ++i could compile to an arbitrary sequence such as:

load r0,[i]  ; load memory into reg 0
incr r0      ; increment reg 0
stor [i],r0  ; store reg 0 back to memory

which would not be thread-safe on my (imaginary) CPU that has no memory-increment instructions. Or it may be smart and compile it into:

lock         ; disable task switching (interrupts)
load r0,[i]  ; load memory into reg 0
incr r0      ; increment reg 0
stor [i],r0  ; store reg 0 back to memory
unlock       ; enable task switching (interrupts)

where lock disables and unlock enables interrupts. But, even then, this may not be thread-safe in an architecture that has more than one of these CPUs sharing memory (the lock may only disable interrupts for one CPU).

The language itself (or libraries for it, if it's not built into the language) will provide thread-safe constructs and you should use those rather than depend on your understanding (or possibly misunderstanding) of what machine code will be generated.

Things like Java synchronized and pthread_mutex_lock() (available to C/C++ under some operating systems) are what you need to look into (a).

(a) This question was asked before the C11 and C++11 standards were completed. Those iterations have now introduced threading support into the language specifications, including atomic data types (though they, and threads in general, are optional, at least in C).