在VC6.0下测试可变参数函数

　　在linux内核中经常看到可变参数函数，如我们熟悉的printf、printk、scanf、sscanf.....这些函数都使用了va_start，va_arg，va_end。

　　我们首先得知道调用一个含参数的函数时，输入参数少的利用通用寄存器存储，输入参数多的一部分存入通用寄存器，一部分存入堆栈。在VC6.0中，在定义函数时可以在前面加入_cdecl（其实默认就是），使得输入参数全部存入堆栈，这样每个参数在内存中都是相邻的。

　　在看va_start，va_arg，va_end的作用，在VC6.0这3个宏定义在stdarg.h头文件中：

/***
*stdarg.h - defines ANSI-style macros for variable argument functions
*
*       Copyright (c) 1985-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
*       This file defines ANSI-style macros for accessing arguments
*       of functions which take a variable number of arguments.
*       [ANSI]
*
*       [Public]
*
****/

#if     _MSC_VER > 1000
#pragma once
#endif

#ifndef _INC_STDARG
#define _INC_STDARG

#if     !defined(_WIN32) && !defined(_MAC)
#error ERROR: Only Mac or Win32 targets supported!
#endif


#ifdef  _MSC_VER
/*
 * Currently, all MS C compilers for Win32 platforms default to 8 byte
 * alignment.
 */
#pragma pack(push,8)
#endif  /* _MSC_VER */

#ifdef  __cplusplus
extern "C" {
#endif



#ifndef _VA_LIST_DEFINED
#ifdef  _M_ALPHA
typedef struct {
        char *a0;       /* pointer to first homed integer argument */
        int offset;     /* byte offset of next parameter */
} va_list;
#else
typedef char *  va_list;
#endif
#define _VA_LIST_DEFINED
#endif

#ifdef  _M_IX86


#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )

#define va_start(ap,v)  ( ap = (va_list)&v + _INTSIZEOF(v) )
#define va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define va_end(ap)      ( ap = (va_list)0 )

#elif   defined(_M_MRX000)


/* Use these types and definitions if generating code for MIPS */

#define va_start(ap,v) ap  = (va_list)&v + sizeof(v)
#define va_end(list)
#define va_arg(list, mode) ((mode *)(list =
 (char *) ((((int)list + (__builtin_alignof(mode)<=4?3:7)) &
 (__builtin_alignof(mode)<=4?-4:-8))+sizeof(mode))))[-1]

/*  +++++++++++++++++++++++++++++++++++++++++++
    Because of parameter passing conventions in C:
    use mode=int for char, and short types
    use mode=double for float types
    use a pointer for array types
    +++++++++++++++++++++++++++++++++++++++++++ */


#elif   defined(_M_ALPHA)


/* Use these types and definitions if generating code for ALPHA */

/*
 * The Alpha compiler supports two builtin functions that are used to
 * implement stdarg/varargs.  The __builtin_va_start function is used
 * by va_start to initialize the data structure that locates the next
 * argument.  The __builtin_isfloat function is used by va_arg to pick
 * which part of the home area a given register argument is stored in.
 * The home area is where up to six integer and/or six floating point
 * register arguments are stored down (so they can also be referenced
 * by a pointer like any arguments passed on the stack).
 */

extern void * __builtin_va_start(va_list, ...);

#ifdef  _CFRONT
#define __builtin_isfloat(a) __builtin_alignof(a)
#endif

#define va_start(list, v) __builtin_va_start(list, v, 1)
#define va_end(list)
#define va_arg(list, mode) 
    ( *(        ((list).offset += ((int)sizeof(mode) + 7) & -8) , 
        (mode *)((list).a0 + (list).offset - 
                    ((__builtin_isfloat(mode) && (list).offset <= (6 * 8)) ? 
                        (6 * 8) + 8 : ((int)sizeof(mode) + 7) & -8) 
                ) 
       ) 
    )

#elif   defined(_M_PPC)

/* Microsoft C8 front end (used in Motorola Merged compiler) */
/* bytes that a type occupies in the argument list */
#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )
/* return 'ap' adjusted for type 't' in arglist */
#define _ALIGNIT(ap,t) 
        ((((int)(ap))+(sizeof(t)<8?3:7)) & (sizeof(t)<8?~3:~7))

#define va_start(ap,v)  ( ap = (va_list)&v + _INTSIZEOF(v) )
#define va_arg(ap,t)    ( *(t *)((ap = (char *) (_ALIGNIT(ap, t) + _INTSIZEOF(t))) - _INTSIZEOF(t)) )
#define va_end(ap)      ( ap = (va_list)0 )

#elif   defined(_M_M68K)
#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )

#define va_start(ap,v)  ( ap = (va_list)&v + (sizeof(v) < sizeof(int) ? sizeof(v) : _INTSIZEOF(v)) )
#define va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define va_end(ap)      ( ap = (va_list)0 )

#elif   defined(_M_MPPC)
#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )

#define va_start(ap,v)  ( ap = (va_list)&v + _INTSIZEOF(v) )
#define va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define va_end(ap)      ( ap = (va_list)0 )

#else

/* A guess at the proper definitions for other platforms */

#define _INTSIZEOF(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )

#define va_start(ap,v)  ( ap = (va_list)&v + _INTSIZEOF(v) )
#define va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define va_end(ap)      ( ap = (va_list)0 )


#endif


#ifdef  __cplusplus
}
#endif

#ifdef  _MSC_VER
#pragma pack(pop)
#endif  /* _MSC_VER */

#endif  /* _INC_STDARG */

　　里面有不同架构的定义，其实都是一个意思：

　　va_start(ap,v) : 将v变量下一个变量的地址赋给ap;

　　m = va_arg(ap,t) : t一般是变量类型，就是在ap地址处获取类型为t的变量值赋值给m, 然后ap = ap + sizeof(t)(忽略对齐)，也就是指向下一个变量地址。

　　va_end(ap) : 参数提取完毕，ap清空为0，防止再让ap指向非法内存。

　　大概就是这样的意思吧，下面我把我的测试代码放上来：

#include <stdio.h>
#include <stdarg.h>

int _cdecl square_sum(int count, ... )
{
    va_list args;
    int sum = 0, tmp;
    
    va_start(args, count);
    while(count)
    {
        tmp = va_arg(args, int);
        sum += (tmp * tmp);
        count--;
    }
    va_end(args);

    return sum;
}

void main(void) 
{
    printf("sum = %d
", square_sum(1,1));
    printf("sum = %d
", square_sum(2,1,2));
    printf("sum = %d
", square_sum(3,1,2,3));
    printf("sum = %d
", square_sum(4,1,2,3,4));
}

　　这个代码是求多个数的平方和。

　　结果：