c - Variadic arguments and x64

Question

How do the va_arg va_start va_list va_end macros work under the hood in x64?

The calling convention in i386 passes parameters on the stack, hence the macro just increments some pointer that points to the stack base and forwards it. However, in x64, all parameters are passed by registers.... so what happens there? How does the called function know which registers were used to pass arguments to ensure it doesn't clobber them?

Answer 1

This is defined by the ABI for the architecture. On the SysV ABI for amd64 (so pretty much everything other than Windows), the ABI document says (page 56 and forward):

The prologue of a function taking a variable argument list and known to call the macro va_start is expected to save the argument registers to the register save area.

Then va_list is a struct with a pointer to the register save area and the place on the stack where some additional arguments could have been passed (not all function arguments fit in registers).