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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
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).
In amd64/x86 kernel space you can dereference null as well. My hobby kernel keeps critical kernel structures there XD.
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 mean, changing a const is itself a questionable move (the question being whether the one doing it is insane)
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.
Maybe there's a signal handler or some other outside force that knows where that variable lives on the stack (maybe through DWARF) and can pause your program to modify it asynchronously. Very niche. More practical is purely to inhibit certain compiler optimizations.
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.
Agreed. It's a very adult approach. C hands you a running chainsaw and whatever happens after that is your responsibility. It is also your responsibility to decide when it's not the right time to use C.
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
Ah my bad, misunderstood the use case.
I thought you were talking about keeping an unmaintained library intact but building onto it.
I thought C was a really dangerous way to use that syntactic sugar pattern. Actual manipulation of the bytecode to maintain and extend a compiled binary is wild
Actual manipulation of the bytecode to maintain and extend a compiled binary is wild
Just wait until you learn about machine code. :)
That actually sounds pretty cool
Sometimes what I'd like to be able to do is treat part of an app as a core and the rest like user provided scripts, but written and evaluated in the host language and not running an embedded scripting language like lua with all the extra burden.
E.g. you have an image editor and you want the user to be able to write native functions to process the image. Or you have a game engine and you want to inject new game code from the user without the engine being a compiler or the game logic being bundled scripts.
You'd probably use a different approach for that. Like you'd make your program dynamically load all the .dlls in a "plugins" folder -
Then you'd provide some plugin interface for the users to create plugins, for example:
public interface IImageEditorPlugin
{
public void BeforeImageEdit(int[,] imageData);
public void AfterImageEdit(int[,] imageData);
}
And then you can load plugin classes from all the dlls with dependency injection, and execute them though something like this:
public class ImageEditor(IEnumerable<IImageEditorPlugin> plugins)
{
public void EditImage(int[,] imageData)
{
foreach (var imageEditorPlugin in plugins)
{
imageEditorPlugin.BeforeImageEdit(imageData);
// Do internal image edit function
imageEditorPlugin.AfterImageEdit(imageData);
}
}
}
This is a very simple example obviously, normally you'd send more meta-data to the plugins, or have multiple different interfaces depending on the kinda plugin it is, or have some methods to ask plugins when they're suitable to be used. But this way a user can provide compiled versions of their plugins (in the same language as the core application) - instead of having to provide something like lua scripts
I loved C/C++ in university, finally the damn piece of rock we forced into thinking was doing exactly what I told him to do, no more and no less.
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.
Yeah I stumbled on this too. Surely the joke should be const mutable, not const volatile.
If you have a memory-mapped peripheral where there's a readonly register, I could see it being const volatile.
What is the context of the original image?
Could be simply a way to make sure the button never moves again. I would have simply taken out the knob, personally.
It could be about sending a message.
A missing knob is easy to fix. Bolting a wrench to the housing holding the knob in place is very explicit. It screams "don't touch"
Idk to me it screams "solve this puzzle and win a free wrench" /s
I like the creativity of it, and it does solve the problem in a way that's user-safe. I thought of removing the knob because that's what I do with my barbecue as I store items on the grill when not in use. Remove knobs, put on grill, close barbecue, cover.
I work on industrial controls. Very likely that the switch is momentary, meaning it'll go back when released.
Sometimes there's a little piece of plastic in them to remove the momentary setting, but this works too lol. Fuck it, it's maintenance.
That actually makes sense, thank you for the tidbit!
Still kind of an overkill solution, but at least it's funny
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.
Is this wrench made of chocolate?
Forbidden chocolate
Looks like they didn't want anybody using the secondary tank. Probably haven't had time to pull Dave's body out yet.
const...ish
constn’t
I’m giggling like a kid that finally got the candy from the top drawer. It’s beautiful.
I've used it in the past when having flash memory blocks that could change but you need the compiler to put them into flash memory and not RAM. It's mainly to get the compiler to stop assuming that it can optimize using the default value.
laughs in evil PLC programmer A little forces enabled, a change here, and maybe just move this wire over there while I am at it...
I see a Java programmer evolves into a C programmer
This has 14 (Peter Cline) energy here for the photo. Keep the dials at zero!
When you set the port speed to no negotiate.
volatile int blackhole;
blackhole = 1;
const int X = blackhole;
const int Y = blackhole;
Compiler is forbidden to assume that X == 1 would be true. It's also forbidden to assume that X == Y. const just means the address and/or the data at the address is read only. const volatile int* const hwreg; -> "read only volatile value at read only address hwreg". Compiler can assume the hwreg address won't magically change, but can't assume the value read from that address won't.