C/C++ volatile variable accessed from another module

Question

I know and understand the purpose of volatile variables and optimisation in general (well, I think I do!). This question relates specifically to what happens if a variable is accessed outside the module it is declared in.

In the following scenario, if funcThatWaits was called inside bar.c, it could be optimised and not fetch the value of sTheVar each loop iteration.

However, when GetTheVar is called externally could the same optimisation apply or does the function call ensure sTheVar will always be read each loop iteration?

I am not suggesting this is good code or practice, but an example for the sake of the question.

bar.h

int GetTheVar(void);

bar.c

static /*volatile*/ int sTheVar;

int GetTheVar(void)
{
    return sTheVar;
}

static void someISROrFuncCalledFromAnotherThread(void)
{
    sTheVar = 1;
}

foo.c

#include "bar.h"

void funcThatWaits(void)
{
    while(GetTheVar() != 1) {}
}

Answer 1

when GetTheVar is called externally could the same optimisation apply or does the function call ensure sTheVar will always be read each loop iteration?

The same optimization may apply. For instance, if you are using LTO (Link-Time Optimization), then the compiler knows everything about GetTheVar and will likely decide funcThatWaits is an infinite loop (which, by the way, would be UB).