There’s ABI and ABI

With all this talk about x32 there are people who might not know what we’re referring to when we talk about ABI. Indeed this term, much alike its sibling API, is so overloaded with multiple meanings that the only way to know what one is referring to is understanding in which of the many contexts it’s being used.

It’s not that I haven’t talked about ABI before but I think it’s the first time I talk about it in this context.

Let’s start from the meaning of the two acronyms:

  • API stands for Application Programming Interface;
  • ABI stands for Application Binary Interface.

The whole idea is that the API is what the humans are concerned with, and ABI what the computers are concerned with. But I have to repeat that what these two mean depends vastly on the context you refer to them.

For instance what I usually talk about is the ABI of a shared object which is a very limited subset of what we talk about in the context of x32. In that context, the term ABI refers to the “compiled API”, which often is mistaken for the object’s symbol table although it includes more details such as the ordered content of the transparent structures, the order and size of the parameters in functions’ signatures and the meaning of said parameters and the return value (that’s why recently we had trouble due to libnetlink changing the return values, which caused NetworkManager to fail).

When we call x32 and amd64 ABIs of the x86-64 architecture, instead, we refer to the interface between a few more components… while I don’t know of a sure, all-covering phrase, the interfaces involved in this kind of ABI are those between kernel and userspace (the syscalls), the actual ELF variant used (in this case it’s a 32-bit class, x86-64 arch ELF file), the size of primitive types as declared by the compiler (long, void*, int, …), the size of typedefs from the standard library, the ordered content of the standard transparent structures, and probably most importantly the calling convention for functions. Okay there are a few more things in the mix such as the symbol resolution and details like those, but the main points are here, I think.

Now in all the things I noted above there is one thing that can be extracted without having to change the whole architecture ABI — it’s the C library ABI: the symbol table, typedefs, ordered content of transparent structures, and so on. That is the limited concept of shared object ABI applied to the C library object itself. This kind of change still require a lot of work, among others because of the way glibc works, which will likely require replacing a number of libraries, modules, the loader and more and more.

Why do I single out this kind of change? Well, while this would have also caused trouble with binaries the same way the introduction of a new architecture did, there is an interesting “What if” scenario: What if Ryan’s FatELF and Intel’s x32 ABI happened nine years ago, and people would have been keen on breaking the C library ABI for the good old x86 at the time?

In such a condition, with the two ABIs being both ILP32 style (which means that int, long and void* are 32-bit), if the rest of the C library ABI was the same between the two, a modified version of Ryan’s FatELF approach – one where the data sections are shared – could have been quite successful!

But let it be clear, this is not going to happen as it is now. Changing the C library ABI for x86 at this point is a script worth of Monty Python and the new x32 ABI corrects some of the obvious problems present in x86 itself — namely the use of 32-bit off_t (which restricts the size of files) and time_t (which could cause the Y2K38 bug), with the two of them having widely incompatible data structures.

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