How to access the metadata that malloc stores for each block?

Question

I know that depending of the implementation of malloc, the algorithm used differs (free linked lists, buckets, binary buddy... - and often it is a mix).

So I was wondering if it is possible to know the size of the metadata stored by malloc for a certain block and access it even as a copy.

I have not found any standard function or method to do it.

Any idea?

There is no standard function for this. You need to look at the individual implementation of the C library you're using. Just curious: what do you want to do with that meta data or its size? — Jabberwocky
– Jabberwocky, Commented Nov 6, 2024 at 12:43
This is a very typical XY problem. Why do you need this information? What actual problem is it supposed to solve? — Some programmer dude
– Some programmer dude, Commented Nov 6, 2024 at 12:45
As you already wrote: "Depending on the implementation..." How would any standard function provide access to data that is not defined by the standard? — Gerhardh
– Gerhardh, Commented Nov 6, 2024 at 12:45
Usually it is accessible through wild & crazy pointer arithmetic as an offset of the address you received from malloc. But you need to know what implementation that is used in order to write a hack to inspect those bytes. — Lundin
– Lundin, Commented Nov 6, 2024 at 12:53
I don't "need" this information, it's purly informative. I am coding my own malloc and I was just wondering how much memory is used by a standard implementation of malloc (I mean, one which is well coded:) for each block. Like, if I malloc(1), how much bytes are really used in memory? — touikss
– touikss, Commented Nov 6, 2024 at 13:03

nneonneo · Accepted Answer · 2024-11-06 13:49:22Z

4

There is no standard function to get the metadata for a malloc block, because as you correctly note, the location and contents of the metadata block are heavily implementation-dependent.

There are implementation-specific functions that you can use to get some information about heap blocks, for example, malloc_usable_size(void *ptr) in GNU libc. Other malloc implementation may support similar extensions.

answered Nov 6, 2024 at 13:49

nneonneo

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