Comment by PaulDavisThe1st
2 days ago
There is no C ABI (windows compilers do things quite differently from linux ones, etc) and there is no certainly no C++ equivalent.
2 days ago
There is no C ABI (windows compilers do things quite differently from linux ones, etc) and there is no certainly no C++ equivalent.
C ABI is the system V abi for Unix, since C was literally created for it. And that is the abi followed by pretty much any Unix successor: Linux, Apple's OS, FreeBSD.
Windows has its own ABI.
The different abi is pretty much legacy and the fact that x86_64 ABI was built by AMD + Linux etc, while Microsoft worked with Intel for the Itanium abi.
> And that is the abi followed by pretty much any Unix successor: Linux, Apple's OS, FreeBSD.
Even limiting that to “on x64”, I don’t see how that’s true. To make a syscall, the ABI on Linux says “make the call”, while MacOS (and all the BSDs, I think) says “call the provided library function”.
Also (https://developer.apple.com/documentation/xcode/writing-64-b...): “Apple platforms typically follow the data representation and procedure call rules in the standard System V psABI for AMD64, using the LP64 programming model. However, when those rules are in conflict with the longstanding behavior of the Apple LLVM compiler (Clang) on Apple platforms, then the ABI typically diverges from the standard Processor Specific Application Binary Interface (psABI) and instead follows longstanding behavior”
Some of the exceptions mentioned there are:
- Asynchronous Swift functions receive the address of their async frame in r14. r14 is no longer a callee-saved register for such calls.
- Integer arguments that are smaller than int are required to be promoted to int by the caller, and the callee may assume that this has been done. (This includes enumerations whose underlying type is smaller than int.) For example, if the caller passes a signed short argument in a register, the low 32 bits of the register at the moment of call must represent a value between -32,768 and 32,767 (inclusive). Similar, if the caller passes an unsigned char argument in a register, the low 32 bits of the register at the moment of call must represent a value between 0 and 255 (inclusive). This rule also applies to return values and arguments passed on the stack.
Swift has its own ABI and calling convention, so that makes sense that Apple adapted to it.
The system v abi doesn't say anything about syscall.
Windows x86_64 abi is the same abi for x86, for this reason, you can only pass arguments in 4 registers ( while unix uses 6 ) because x86 only had 8 registers.
I think people have expectations that are misaligned with history and reality about this, to be honest. We can't expect all OS to do things in the same way.
C was created to rewrite the UNIX system, and POSIX compliance is followed by all successors, with minimal differences.
When it became clear that "Itanium" was a failure, Microsoft couldn't just pull an ABI out of the box and break all applications, so they just reused the same x86 ABI.
The C ABI is basically per-platform (+ variations, like 32- vs 64-bit). But you can get by quite well pretending there is something like a C ABI if you use <stdint.h>.
[flagged]
[flagged]