arrays 是否保证在虚拟 memory 中是连续的？

Question

int main() { 
 char a[3] = {1,2,3};
 return sizeof(a);
}

Is a guaranteed to be in consecutive bytes in virtual memory? I know it may not be consecutive in physical memory as the mapping is done behind the scene by the MMU.

If the compiler notices i'm not taking the address of any of the elements, is it then free to put them on non consecutive addresses in memory or even put them in a register?

Let's assume the optimizer will not fully get rid of it in my example.

Answer 1

Is a guaranteed to be in consecutive bytes in virtual memory? I know it may not be consecutive in physical memory as the mapping is done behind the scene by the MMU.

Your code is guaranteed to behave as-if the array was in consecutive bytes of address space.

If the compiler notices i'm not taking the address of any of the elements, is it then free to put them on non consecutive addresses in memory or even put them in a register?

It is free to do so even if you do take the address. The compiler can compile your code any way it wants to make it efficient so long as the code doesn't break.

Let's assume the optimizer will not fully get rid of it in my example.

Okay. But it's allowed to. C has an "as-if" rule which means that all rules are just about the behavior your code has to observe, they don't constrain how the compiler (or the machine) get that behavior.

Answer 2

The non-pedantic answer is that, yes, for all intents and purposes, arrays are guaranteed to be stored contiguously in C. This is no accident, it's pretty much a fundamental part of the definition of an array.

The whole point of an array is that you can access any element in constant (that is, O(1)) time, notionally by computing an address, and without having to chase any links as you would with almost any other data structure. So if, as some obscure and invisible implementation detail, an array were somehow not actually stored contiguously, it would always have to act exactly as if it were stored contiguously. And since contiguity is the defining property of an array, I don't think there's any harm in thinking consciously about that property, and assuming that it's always true.

Answer 3

Well in this particular code snippet most likely a will be stored in the stack and yes in this particular case it would be contiguous.

However if you were to run at a higher optimization most likely any compiler would not even store a and just do constant propagation and just return the size of a .

If you were to allocate memory in heap then it depends on the allocator most likely if it is Buddy or slab allocator then more often than not they would be contiguous but if i were to have a naive free linked list allocator then it may not be contiguous when there are multiple calls for heap allocation. In case of Arrays this is not possible as you most likely will have single call so it would still be contigous.

Tools like objdump, opt from llvm, gdb etc are great tools if you want to check the disassembly and how the compiler lays out the assembly code across different optimization levels.

Answer 4

C is defined in terms of an abstract machine. The bytes are guaranteed to be contiguous in the abstract machine.

The real machine can do literally anything so long as the observable behaviour of the program matches an allowed output of the program in the abstract machine.

So, there are no guarantees about any placement of data in the real machine.