Saturday, January 14, 2023

(Programming) Combining the constant and volatile keywords in C

Sometimes asked as an interview question in embedded programming, this summary article covers an interesting use case of combining two seemingly opposite keywords in C [1].

Here is a TLDR of what this means:
1. int volatile x; Tells the compiler that x can change independent of the code e.g a register change that happens in Hardware.
2. uint16_t const x; Is used to tell the compiler that the variable x is immutable.
3. So when can we end up using these together? The article [1] talks about 3 cases, but they are essentially the same thing. We can use these keywords together for a pointer where the address of the pointer does not change (constant) but the value contained at that address is volatile (e.g. a register, shared memory etc). 
e.g. Constant address of volatile hardware register / memory. uint8_t volatile * const p_ptr; i.e the p_ptr has a constant address for a volatile (register).
[1] Michael Barr Combining C's volatile and constant