va函数测试

[nmvbxcz]

#include <stdio.h>
#define _INTSIZEOF(n)  ((sizeof(n)+sizeof(int)-1)&~(sizeof(int) - 1) )
#define va_start(ap,v) ( ap = (char *)&v + _INTSIZEOF(v) )          //第一个可选参数地址
#define va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) ) //下一个参数地址
#define va_end(ap)   ( ap = (char *)0 )                           // 将指针置为无效

void simple_va_fun(int i, ...)
{
char * arg_ptr;
int j=0,k=0;
va_start(arg_ptr,i);
j=va_arg(arg_ptr,int);
k=va_arg(arg_ptr,int);
va_end(arg_ptr);
printf("%d@%d@%d \n",i,j,k);
return;
}

void main()
{
	simple_va_fun(5,7,9);
}

以下是使用gdb调试的结果，打印了内存，感觉传入的参数ijk并没有依次存放，而且中间间隔的单元数据还发生了改变，最终输出来的j和k都是0。

simple_va_fun (i=5) at newtest2.c:14
14	int j=0,k=0;
(gdb) p i
$1 = 5
(gdb) p &i
$2 = (int *) 0x7fffffffdcdc
(gdb) x/16uw 0x7fffffffdcdc
0x7fffffffdcdc:	5	0	0	0
0x7fffffffdcec:	0	0	0	7
0x7fffffffdcfc:	0	9	0	0
0x7fffffffdd0c:	0	4196000	0	4158552288
(gdb) p arg_ptr
$3 = 0x0
(gdb) s
15	va_start(arg_ptr,i);
(gdb) p arg_ptr
$4 = 0x0
(gdb) x/16uw 0x7fffffffdcdc
0x7fffffffdcdc:	5	0	0	0
0x7fffffffdcec:	0	0	0	7
0x7fffffffdcfc:	0	9	0	0
0x7fffffffdd0c:	0	4196000	0	4158552288
(gdb) i local
arg_ptr = 0x0
j = 0
k = 0
(gdb) s
16	j=va_arg(arg_ptr,int);
(gdb) i local
arg_ptr = 0x7fffffffdce0 ""
j = 0
k = 0
(gdb) x/16uw 0x7fffffffdce0
0x7fffffffdce0:	0	0	4294958304	32767
0x7fffffffdcf0:	0	0	7	0
0x7fffffffdd00:	9	0	0	0
0x7fffffffdd10:	4196000	0	4158552288	32767
(gdb) x/16uw 0x7fffffffdcdc
0x7fffffffdcdc:	5	0	0	4294958304
0x7fffffffdcec:	32767	0	0	7
0x7fffffffdcfc:	0	9	0	0
0x7fffffffdd0c:	0	4196000	0	4158552288