Do any languages have a notion of "relative pointers"? So in the example if instead of appending "line" as ptr & len, it'd instead be appending an offset & len which could in theory be used to safely compute the actual location even with relocations.

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adzm 31 minutes ago | parent | next [-]

in c++, boost interprocess has offset_ptr which is useful since the shared data structure may be mapped at different locations in memory in each process

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amluto 3 hours ago | parent | prev | next [-]

Languages with dependent types can express things like “this offset is in bounds relative to this other array”, which is maybe what you’re thinking of.

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afdbcreid an hour ago | parent | prev | next [-]

That is called an index. If you want it to be standalone, you can bundle it with the ArrayList.

	▲	dminik 20 minutes ago \| parent \| next [-]
		I wish languages made it easier (or possible) to track index ownership at compile time.
	▲	sparkie 30 minutes ago \| parent \| prev [-]
		I think parent was after base+offset+index rather than just base+index. Examples would be eg, `string_view` or `ArraySegment`. They hold some offset relative to a base allocation, and when we index the string_view or ArraySegment we're indexing relative to that offset.

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Someone 4 hours ago | parent | prev | next [-]

If you squeeze your eyes a bit, C compilers for Windows used to have them, with far pointers (https://en.wikipedia.org/wiki/Far_pointer)

Similarly, CPU architectures that use descriptors can (have to?) have languages with that notion.

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sparkie 3 hours ago | parent | next [-]

The FS and GS segment selectors are still used in x86-64, typically for `thread_local` storage, but they can be repurposed.

`thread_local` is an example of a "relative pointer" though. Instructions to access the thread local are prefixed with `fs:` or `gs:`, and point relative to the address in the respective segment register.

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frmdstryr 3 hours ago | parent | prev [-]

After more searching I found this article https://www.gingerbill.org/article/2020/05/17/relative-point...

A far pointer sounds like the global based pointer described in that article. The far pointer Wikipedia article says they are problematic but doesn't give much reasoning as to why.

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sparkie 2 hours ago | parent [-]

Far pointers are for accessing memory in different segments. They're basically obsolete now. They were necessary in older machines with limited sized pointers or address spaces.

GCC still supports `__seg_fs` and `__seg_gs`, which behave similar to `far` in the example on the wiki page, as the FS and GS segment registers are still valid in x86-64 and used for TLS. Clang uses attributes `address_space(257)` and `address_space(256)` for the same thing.

The `__based` pointer in MSVC exploits the addressing modes by pinning the base in eg: `[base+index*scale+displacement]`. It's unrelated to segmentation.

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Joker_vD an hour ago | parent [-]

> Far pointers are for accessing memory in different segments. They're basically obsolete now.

Project CHERI would like to disagree.

	▲	sparkie 36 minutes ago \| parent [-]
		That's Fat pointers, not Far pointers. A fat pointer is a pointer with some other associated data which is stored in the pointer itself - typically by widening the number of bits used to hold a pointer value. The addressable bits usually remain unchanged - the added bits contain the auxiliary data. Segmentation isn't used. There's no separate registers to hold the bounds information in CHERI - the bounds are held in the pointer value, unlike for example, the now obsolete Intel MPX, which held bounds information in separate registers. There's some similarity to segmentation because the CHERI pointer restricts which addresses can be accessed, but I wouldn't compare them to far pointers. Most modern processors have a single linear virtual address space and don't use segmentation, and even where segment registers exist (eg, FS and GS on x86-64), they're only superficial "address spaces" - allocated sections of the process's linear virtual address space which could be accessed without segmentation registers if you knew the base address held in FS or GS.

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geon an hour ago | parent | prev [-]

Array indexing?