3.14. VARIADIC FUNCTIONS
mov r8d, 1
mov QWORD PTR [rsp-56], rax
jmp .L5
.L7:
; work out saved arguments
lea rax, [rsp-48]
mov ecx, esi
add esi, 8
add rcx, rax
mov ecx, DWORD PTR [rcx]
cmp ecx, -1
je .L4
.L8:
add edi, ecx
add r8d, 1
.L5:
; decide, which part we will work out now.
; is current argument number less or equal 6?
cmp esi, 47
jbe .L7 ; no, process saved arguments then
; work out arguments from stack
mov rcx, rdx
add rdx, 8
mov ecx, DWORD PTR [rcx]
cmp ecx, -1
jne .L8
.L4:
mov eax, edi
cdq
idiv r8d
ret
.LC1:
.string "%d\n"
main:
sub rsp, 8
mov edx, 7
mov esi, 2
mov edi, 1
mov r9d, -1
mov r8d, 15
mov ecx, 10
xor eax, eax
call arith_mean
mov esi, OFFSET FLAT:.LC1
mov edx, eax
mov edi, 1
xor eax, eax
add rsp, 8
jmp __printf_chk
By the way, a similar usage of theShadow Spaceis also considered here:6.1.8 on page 740.
Using pointer to the first function argument
The example can be rewritten withoutva_argmacro:
#include <stdio.h>
int arith_mean(int v, ...)
{
int i=&v;
int sum=i, count=1;
i++;
while(1)
{
if ((*i)==-1) // terminator