It makes more sense if you think of
constas “read-only”. Volatile just means the compiler can’t make the assumption that the compiler is the only thing that can modify the variable. Aconst volatilevariable can return different results when read different times.I thought of it more in terms of changing constants (by casting the
constaway). AFAIK when it’s notvolatile, the compiler can place it into read-only data segment or make it a part of some other data, etc. So, technically, changing aconst volatilewould be less of a UB compared to changing a regularconst(?)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
NULLpointers at some point.More...
Some platforms can have something interesting at memory address
0x0(it’s oftenNULLin C).In amd64/x86 kernel space you can dereference null as well. My hobby kernel keeps critical kernel structures there XD.
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
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
constis itself a questionable move (the question being whether the one doing it is insane)
I’ve never really thought about this before, but
const volatilevalue types don’t really make sense, do they?const volatilepointers make sense, sinceconstpointers can point to non-constvalues, butconstvalues are typically placed in read-only memory, in which case thevolatileis 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.
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.
You have a talent for metaphor.
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.
I tried thinking of one for Rust, and ‘the mom friend with a safeword’ is alarmingly accurate.
The secret basement is never locked. It’s fine to go down there, alone. You’ll only be scarred on the inside.
It’s when you go down together that all bets are off.
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.
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.
Idk to me it screams “solve this puzzle and win a free wrench” /s
What too many video games does to a mfer 😄
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
I’m sure they just needed a way to lock the selector knob to the primary position, and didn’t want to rewire it.
Drills and taps two holes, adds a metal strap, and sacrifices a tool to save a 5 minute fix of jumping over the contact with a 2" piece of wire lmfao
A lot of people won’t touch electrical, and the problem with modifying the wiring is you need to be able to clearly document or show what was changed in case it needs to be reversed later.
This is ugly, but it’s immediately obvious how to reverse it to anyone who looks at it. And that pipe wrench probably wasn’t being used anymore anyways. I doubt they tapped the holes, those are probably just self-tap screws that both drilled the hole and cut the thread as they screwed in. No one will call this an elegant solution, but if it works it works.
“documenting the change” is a pipe dream.
If you’ve ever worked in maintenance, active production, etc, you’ll be lucky to even have schematics. And trust me, there are a lot of hacks of people fucking with controls for 30+ years straight that soooo much of it is full of “fixes” like this, whether it’s something pushing a button in, or pieces of metal instead of fuses, or wires jumping over what’s “in the way” like whole safety systems and e-stops, contactors forced to run, etc etc etc.
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.
Context is very interesting: https://stackoverflow.com/questions/4592762/difference-between-const-const-volatile
Const flags to the code that you cannot change the value, and volatile flags to the compiler that it’s not safe to change the value.
Volatile means that the value should be read each time its accessed. It can’t be cached in a register or the read be otherwise assumed and optimized away or the instructions around its access be reordered.
const…ish
constn’t
Looks like they didn’t want anybody using the secondary tank. Probably haven’t had time to pull Dave’s body out yet.
Is this wrench made of chocolate?
Forbidden chocolate
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…
This has 14 (Peter Cline) energy here for the photo. Keep the dials at zero!
I see a Java programmer evolves into a C programmer
volatile int blackhole; blackhole = 1; const int X = blackhole; const int Y = blackhole;Compiler is forbidden to assume that
X == 1would be true. It’s also forbidden to assume thatX == Y.constjust 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 thehwregaddress won’t magically change, but can’t assume the value read from that address won’t.
