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)
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.
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
I’m giggling like a kid that finally got the candy from the top drawer. It’s beautiful.
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.
