A good reference on strict aliasing: https://gist.github.com/shafik/848ae25ee209f698763cffee272a5...
malloc() returns an address. The user code writes a value, and then reads a value of the same type. No problem.
Later, malloc() returns the same address. The user code writes a value of another type then reads a value of that same type. Again, no problem. There's no aliasing going on here.
The act of writing a value of a different type tells the compiler that the lifetime of the previous object has ended. There's no special magic required.
Strict aliasing is about situations where we write a value of one type, and then attempt to read a value of a different type from the same location. If you want to do that, then you have to be extremely cautious. But memory allocators don't do that, so it's not an issue.
(Obviously just talking about C here, or POD in C++ terms. If you are dealing with C++ objects with destructors, then you have an extra layer of complexity to deal with, but again that's nothing to do with aliasing.)
afaik only memcpy has that magic property, so I think parent is almost correct.
void *p = malloc(n);
*(int *)p = 42; // ok, *p is now an int.
//*(float *)p = 3.14f; // I think this is not allowed, p points to an int object, regular stores do not change effective type
float x = 3.14f;
memcpy(p, &x, sizeof(float)); // but this is fine, *p now has effective type float
pool->chunk_arr[i].next = &pool->chunk_arr[i + 1];
Later in pool_alloc:
Chunk* result = pool->free_chunk;
...
return result;
In user code:
int *x = pool_alloc();
*x = 42; // aliasing violation, *x has effective type 'Chunk' but tried to access it as an int*
int *x = pool_alloc();
memcpy(x, &(int){0}, sizeof(int)); // establish new effective type as 'int'
// now we can do
*x = 42;**C should issue a defect report and get rid of that nonsense from the standard.
This enables security analysis like valgrind/ASan and secure hardware like MTE/CHERI so it's very important and you can't get rid of it.
However, it's not possible to implement malloc() in C because malloc() is defined as returning new "memory objects" and there is no C operation which creates "memory objects" except malloc() itself. So it only works as long as you can't see into the implementation, or if the compiler gives you special forgiveness somehow.
C++ has such an operation called "placement new", so you want something like that.
It gets complicated when you have virtual memory and an OS involved but even then you can override the system malloc with a simple implementation that allocates from a large static array.
This was never true because of something called provenance: https://www.ralfj.de/blog/2020/12/14/provenance.html. Though it usually doesn't matter and I think it annoys anyone who finds out about it.
But in practice it's not always true on Apple A12 or later because they support PAC (so pointers of different type to the same address can be not equal bit-wise) and is even less true on very latest Android because it supports the really big gun MTE. And MTE is great; you don't want to miss out on it. No explainer here because there's no Wikipedia article for it(!).
Also becomes not true on any system if you use -fbounds-safety or some of the sanitizers.
(The difference here is that system malloc() works with valgrind, -fbounds-safety, theoretical secure hardware with bounds checking etc., and this one doesn't.)
> In particular getting valid addresses to other objects from out-of-bounds address arithmetic is not just conformant with the C standard but by far the most common conforming behavior.
One reason calculating out of bounds addresses might not work out is the calculation might cause the pointer to overflow, and then surprising things might happen like comparisons failing or tag bits in the high bytes getting corrupted.
This doesn't make any sense. How do you know its effective type if you don't have access to the definition of `pool_alloc()'?
It's really quite ridiculous that compiler implementers have managed to overzealously nitpick the C standard so that you can't implement a memory allocator in C.
This is good because it's also what gives you valgrind and CHERI. Take one away and you can't have the other.
(Because if you define all undefined behavior, then programs will rely on it and you can't assume it's an error anymore.)
I think strictly speaking in C this is only true for anonymous memory, i.e. memory you got from some allocator-like function. So if your custom allocator gets its memory from malloc (or sbrk, or mmap), everything is fine, but, for example, you are allocating from a static char array, it is formally UB.
In C++ you can use placement new to change the type of anything.
C++ relatively has fixes to allow allocators to work, it requires calls to std::launder.
If you look at 'pool_free' code you can see that it receives used block from user as 'ptr' then casts it to 'Chunk pointer' and writes to into Chunk member value of type 'Chunk pointer'. I had to change that line to memcpy when I did it last time around 15 years ago in C++ with GCC 4.something. In short if you are writing your own allocators you need either to disable strict aliasing like Linux does or do the dance of 'memcpy' when you access memory as 'internal allocator structures' right after it was used as user type. When it happened to me write was reordered and I observed code that writes into Chunk* next executed first and write of zeros into user type second.