I thought of it more in terms of changing constants (by casting the const away). AFAIK when it's not volatile, the compiler can place it into read-only data segment or make it a part of some other data, etc. So, technically, changing a const volatile would be less of a UB compared to changing a regular const (?)

[–] Scoopta 66 points 7 months ago (1 children)

const volatile is used a lot when doing HW programming. Const will prevent your code from editing it and volatile prevents the compiler from making assumptions. For example reading from a read only MMIO region. Hardware might change the value hence volatile but you can't because it's read only so marking it as const allows the compiler to catch it instead of allowing you to try and fail.

[–] [email protected] 27 points 7 months ago (1 children)

I will not tell my kids regular scary stories. I will tell them about embedded systems

[–] [email protected] 22 points 7 months ago* (last edited 7 months ago) (2 children)

When you program embedded you'll also dereference NULL pointers at some point.

More...

Some platforms can have something interesting at memory address 0x0 (it's often NULL in C).

[–] Scoopta 14 points 7 months ago

In amd64/x86 kernel space you can dereference null as well. My hobby kernel keeps critical kernel structures there XD.

[–] [email protected] 2 points 7 months ago* (last edited 7 months ago)

I was thinking about telling them how in embedded systems it's a good practice to allocate the memory by hand, having in mind the backlog, but yours will come first

[–] [email protected] 19 points 7 months ago* (last edited 7 months ago)

AFAIK when it’s not volatile, the compiler can place it into read-only data segment

True, but preventing that is merely a side effect of the volatile qualifier when applied to any random variable. The reason for volatile's existence is that some memory is changed by the underlying hardware, or by an external process, or by the act of accessing it.

The qualifier was a necessary addition to C in order to support such cases, which you might not encounter if you mainly deal with application code, but you'll see quite a bit in domains like hardware drivers and embedded systems.

A const volatile variable is simply one of these that doesn't accept explicit writes. A sensor output, for example.

[–] [email protected] 2 points 7 months ago (1 children)

The very notion of "less of a UB" is against the concept of UB. If you have an UB in your program, all guarantees are out of the window.

[–] [email protected] 2 points 7 months ago

I mean, changing a const is itself a questionable move (the question being whether the one doing it is insane)