Where all to use volatile keyword in C

Question

I know volatile keyword prevents compiler from optimizing a variable and read it from memory whenever it is read. Apart from memory mapped registers, what are all the situat

Answer 1

In neither of your examples is volatile necessary.

volatile is necessary:

1. anywhere a variable may be changed outside of the control of a single thread of execution,
2. anywhere the variable access is required to occur even when it semantically has no effect.

Case 1 includes:

• memory mapped I/O registers,
• memory used for DMA transfers,
• memory shared between interrupt and/or thread contexts,
• memory shared between independent processors (such as dual port RAM)

Case 2 includes:

• loop counters used for empty delay loops, where the entire loop may otherwise be optimised away completely and take no time,
• Variables written to but never read for observation in a debugger.

The above examples may not be exhaustive, but it is the semantics of volatile that are key; the language only has to perform an explicit access as indicated by the source code.