It makes more sense if you think of const as “read-only”. Volatile just means the compiler can’t make the assumption that the compiler is the only thing that can modify the variable. A const volatile variable can return different results when read different times.
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 (?)
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.
I will not tell my kids regular scary stories. I will tell them about embedded systems
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.
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.
I’ve never really thought about this before, but const volatile value types don’t really make sense, do they? const volatile pointers make sense, since const pointers can point to non-const values, but const values are typically placed in read-only memory, in which case the volatile is kind of meaningless, no?
They do in embedded when you are polling a read only register. The cpu can change the register but writing to it does nothing.
That seems like a better fit for an intrinsic, doesn’t it? If it truly is a register, then referencing it through a (presumably global) variable doesn’t semantically align with its location, and if it’s a special memory location, then it should obviously be referenced through a pointer.
Some people hate that C is dangerous, but personally I like its can-do attitude.
“Hey C, can I write over the main function at runtime?”
Sure, if you want to, just disable memory protection and memcpy whatever you want there! I trust you.
It’s a great attitude for a computer to have.
C is dangerous like your uncle who drinks and smokes. Y’wanna make a weedwhacker-powered skateboard? Bitchin’! Nail that fucker on there good, she’ll be right. Get a bunch of C folks together and they’ll avoid all the stupid easy ways to kill somebody, in service to building something properly dangerous. They’ll raise the stakes from “accident” to “disaster.” Whether or not it works, it’s gonna blow people away.
C++ is dangerous like a quiet librarian who knows exactly which forbidden tomes you’re looking for. He and his… associates… will gladly share all the dark magic you know how to ask about. They’ll assure you, oh no no no, the power cosmic would never pull someone inside-out, without sufficient warning. They don’t question why a loving god would allow the powers you crave. They will show you which runes to carve, and then, they will hand you the knife.
Rust is like a paranoid overprotective guardian. A “mom friend”, of sorts. Always the designated driver of the group, keeps you from staying up too late, stops you from eating things that might be choking hazards without proper precaution, and so on and so forth. You’ll never meet a person more concerned with your health and safety – until, that is, you say the magic word “unsafe”. Suddenly the alter ego that their hypnotist implanted gets activated, and their entire demeanor changes on a dime. BMX biking? Bungee jumping? Inline assembly? Sounds like a great idea! Let’s go, man! Rules are for NERDS! Then the minute the unsafe block ends, they’re back to normal, fully cognizant of the adventure they just went on and thinking absolutely nothing of it. “Whitewater rafting with you guys was really fun, especially the part where Jason jumped into the water and I went after him! I’d best go get the first aid kit, though – that scrape he got when he did that looks like it might get infected. I know he said it didn’t hurt, but better safe than sorry!”
They kinda scare you when they’re like that, if you’re honest.
This is sometimes practical, too. For example, hooking and extending functions in compiled code that will never be updated by the original author, while preserving the original executable/library files.
You can do that in memory safe languages too. Kotlin extension functions, for example.
Extension functions are not the same at all. Extension functions are syntactic sugar. For example if you have an extension function like
public static class ObjectExtension
{
public static void DoSomething(this object input) { }
}
You can call that function on an object by doing object.DoSomething() - Yes. But underneath it’s the same as doing ObjectExtension.DoSomething(object)
That function does not actually become part of the object, and you can’t use it to override existing functions
A closer example of how to do something similar in a memory safe language would be - in C# - using something like Castle DynamicProxy - where through a lot of black magic - you can create a DynamicProxy and fool the CLR into thinking it’s talking to an object, while it’s actually talking to a DynamicProxy instead. And so then you can actually intercept invocations to existing methods and overrule them
Generally overruling existing functions at runtime is not that easy
This is actually how you should declare something that you will never change, but something might change externally, like an input pin or status register.
Writing to it might do something completely different or just crash, but you also don’t want the compiler getting creative with reads; You don’t want the compiler optimizing out a check for a button press because the “constant” value is never changed.
If you have a memory-mapped peripheral where there’s a readonly register, I could see it being const volatile.
Just spin the pipe wrench open and slide it up then you can switch it back real quick.
Thank you for watching this OHSA message on bad lockout procedure, now back to your regularly scheduled programming.
