I'm generating machine code to call functions from existing system libraries. Most system libraries were written in C, so I'll take C as an example, but the question probably applies to any other language.
If I understand this answer correctly, C compilers are free to choose the ABI/calling convention of a function as long as they preserve the semantics. For instance they can choose to pass a pointer for the returned value as an argument to obtain copy-elision.
Does this mean that no one can ever truly know what's the right way to call a function from a library, even if its C signature is known?
Is this a real concern in practice? Or is it safe to assume that all the functions with non-mangled names from system libraries always use the system's default calling convention?
What other assumptions or considerations can I make about the ABI/calling convention of functions with non-mangled names in system libraries?
C compilers are free to choose the ABI/calling convention of a function as long as they preserve the semantics.
Well, yes and no. The ABI is often defined by the target system, in which case the compiler has to fall in line. In case there exists no ABI for the target system (often the case in microcontroller programming), the compiler is free to do as it pleases, essentially inventing the ABI.
Does this mean that no one can ever truly know what's the right way to call a function from a library, even if its C signature is known?
No you can't unless you know the target system and calling convention. Some systems have several "de facto" standards such as x86 Windows __cdecl vs __stdcall see https://en.wikipedia.org/wiki/X86_calling_conventions
Is this a real concern in practice?
Not within a program written entirely in C. But it becomes a big problem in case the program links external libs such as Windows DLLs, possibly written in other languages. Then you have to use the right calling convention or the program will soon crash.
It's also a very real concern whenever you attempt to mix assembler and C for the given system - the C compiler will handle stacking according to the calling convention, but in the assembler part you have to write this manually. This can also affect the C code, if it is written with care to suit assembler. You'd then pick parameter and return types that are convenient to use.