At the same time the C ABI is not part of the C standard. It's a GNU thing (or a Microsoft thing for Windows, or an Apple thing for Mac, or...). While they requ

At the same time the C ABI is not part of the C standard. It's a GNU thing (or a Microsoft thing for Windows, or an Apple thing for Mac, or...). While they require the compiler to provide a stable ABI there's nothing preventing C20 (or future versions) from breaking existing C ABIs. It's not really right to talk about "the" C ABI, rather there's the x86 GCC ABI and the x86_64 GCC ABI and the x86 MSVC ABI and the MIPS GCC ABI and the... That's why compilers have target "triples" (now more than 3 items): <CPU architecture><subarchitecture>-<vendor>-<os/system>-<abi>. So you might have ARMv7m-st-none-eabi for some embedded STM32 bare-metal code and x86_64-pc-linux-gnu for Linux. All C, all different ABIs.