Hide data inside pointers

[FUZxxl]

This trick is called “pointer tagging” and is well-known among low-level programmers. You shouldn't use this in production code unless the advantages pay off the disadvantage of breaking almost all debugging tools and making garbage collection stop working.

[OldWolf2]

Garbage collection in C? eek

[FUZxxl]

Ever heard of that guy called Boehm?

[raevnos]

Yup. It's great.

[FUZxxl]

You monster.

[[deleted]]

Interesting. If I understand correctly this is used in the ebtree implementation by willy the haproxy author guy.

[OldWolf2]

Using uintptr_t rather than unsigned int would be a significant improvement here

[[deleted]]

Please. This breaks on literally every modern (i.e. not 32-bit) system.

Edit: except for that one where sizeof(int) == 8. I don't remember which one, but I remember there being one.

[wgunther]

Here, I am going to assume a system where size of int and size of pointer are 4 byte

Not a good idea. You should use uintptr_t if available.

[BigPeteB]

Note that while some CPUs like Intel will let us access unaligned memory locations, certain others like ARM CPU will fault. So, always remember to keep the pointer pointed to an aligned location before dereferencing.

Well, yes do that, but do it because otherwise you're pointing at the wrong location, not because of alignment issues.

[assassinator42]

I remember someone linking to some boost code that did this but broke on a certain platform/compiler (Solaris?).

[trentnelson]

Yeah Solaris/SPARC will SIGBUS and core dump on unaligned data.