c++ - Is a write to variable on a single core CPU atomic? -

i have single core cpu (arm cortex m3, 32bit) 2 threads. assuming following situation:

// thread 1: int16_t = 1; double b = 1.0; // other fancy stuff including starting thread 2 (;;) {std::cout << << "," <<b;}  // thread 2: = 2; b = 2.0; 

i can handle following outputs:

• 1,1
• 1,2
• 2,1
• 2,2

can output 1 of (1/2) without using mutex or other locking mechanisms? , more specifically, compiler dependent? , behavior different int16 , double?

it depends on cpu, mostly, though in theory involving multiple threads on pre-c11 @ best implementation defined , @ worst undefined behavior, compiler might anything.

if can ignore crazy compilers silly things, , assume compiler use cpu's facilities in reasonable way, depends on cpu supports.

cortex-m3 32-bit cpu 32-bit bus , no fpu. reads , writes of 32-bit , smaller values atomic. double, however, 64 bits, read/write of double involve 2 instructions , non-atomic. if 1 thread reads while other writing might half 1 value , half other.

now in specific example, values 1.0 , 2.0 both 0 lower half, 'mix' innocuous, other values not have behavior.