The comment about C requiring functions to be able to be called without a prototype in scope is not true, in the case of variadic functions. Calling a variadic

The comment about C requiring functions to be able to be called without a prototype in scope is not true, in the case of variadic functions. Calling a variadic function without a correct prototype in scope is undefined behaviour - so it's open to an ABI to specify a different calling convention for them. (The relevant part of C99 is §6.5.2.2 Function Calls, p6).