In C++, do variadic functions (those with ... at the end of the parameter list) necessarily follow the __cdecl calling convention?

Question

I know that __stdcall functions can't have ellipses, but I want to be sure there are no platforms that support the stdarg.h functions for calling conventions other than __cdecl or __stdcall.

Answer 1

The calling convention has to be one where the caller clears the arguments from the stack (because the callee doesn't know what will be passed).

That doesn't necessarily correspond to what Microsoft calls "__cdecl" though. Just for example, on a SPARC, it'll normally pass the arguments in registers, because that's how the SPARC is designed to work -- its registers basically act as a call stack that gets spilled to main memory if the calls get deep enough that they won't fit into register anymore.

Though I'm less certain about it, I'd expect roughly the same on IA64 (Itanium) -- it also has a huge register set (a couple hundred if memory serves). If I'm not mistaken, it's a bit more permissive about how you use the registers, but I'd expect it to be used similarly at least a lot of the time.

Why does this matter to you? The point of using stdarg.h and its macros is to hide differences in calling convention from your code, so it can work with variable arguments portably.

Edit, based on comments: Okay, now I understand what you're doing (at least enough to improve the answer). Given that you already (apparently) have code to handle the variations in the default ABI, things are simpler. That only leaves the question of whether variadic functions always use the "default ABI", whatever that happens to be for the platform at hand. With "stdcall" and "default" as the only options, I think the answer to that is yes. Just for example, on Windows, wsprintf and wprintf break the rule of thumb, and uses cdecl calling convention instead of stdcall.

Answer 2

The most definitive way that you can determine this is to analyze the calling conventions. For variadic functions to work, your calling convention needs a couple of attributes:

• The callee must be able to access the parameters that aren't part of the variable argument list from a fixed offset from the top of the stack. This requires that the compiler push the parameters onto the stack from right to left. (This includes such things as the first parameter to printf, the format specification. Also, the address of the variable argument list itself must also be derived from a known location.)
• The caller must be responsible for removing the parameters off the stack once the function has returned, because only the compiler, while generating the code for the caller, knows how many parameters were pushed onto the stack in the first place. The variadic function itself does not have this information.

stdcall won't work because the callee is responsible for popping parameters off the stack. In the old 16-bit Windows days, pascal wouldn't work because it pushed parameters onto the stack from left to right.

Of course, as the other answers have alluded to, many platforms don't give you any choice in terms of calling convention, making this question irrelevant for those ones.