540
you are viewing a single comment's thread
view the rest of the comments
[-] zea_64@lemmy.blahaj.zone 97 points 8 months ago

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.

[-] fl42v@lemmy.ml 10 points 8 months ago* (last edited 8 months ago)

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@programming.dev 66 points 8 months ago

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.

[-] humbletightband@lemmy.dbzer0.com 27 points 8 months ago

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

[-] suzune@ani.social 22 points 8 months ago* (last edited 8 months ago)

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@programming.dev 14 points 8 months ago

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

[-] humbletightband@lemmy.dbzer0.com 2 points 8 months ago* (last edited 8 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

[-] mox@lemmy.sdf.org 19 points 8 months ago* (last edited 8 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.

[-] TheEntity@lemmy.world 2 points 8 months ago

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.

[-] fl42v@lemmy.ml 2 points 8 months ago

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

[-] QuaternionsRock@lemmy.world 6 points 8 months ago

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?

[-] rooster_butt@lemm.ee 23 points 8 months ago* (last edited 8 months ago)

They do in embedded when you are polling a read only register. The cpu can change the register but writing to it does nothing.

[-] QuaternionsRock@lemmy.world 1 points 8 months ago

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.

[-] zea_64@lemmy.blahaj.zone 4 points 8 months ago

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.

this post was submitted on 25 Mar 2024
540 points (98.6% liked)

Programmer Humor

19623 readers
1 users here now

Welcome to Programmer Humor!

This is a place where you can post jokes, memes, humor, etc. related to programming!

For sharing awful code theres also Programming Horror.

Rules

founded 1 year ago
MODERATORS