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 i3

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).