C语言函数之可变参数原理:va_start、va_arg及va_end

asmlinkage int printk(const char *fmt, ...)

/*

  * Use local definitions of C library macros and functions

  * NOTE: The function implementations may not be as efficient

  * as an inline or assembly code implementation provided by a

  * native C library.

  */

#ifndef va_arg

#ifndef _VALIST

#define _VALIST

typedef char *va_list;

#endif                /* _VALIST */

/*

 * Storage alignment properties

 */

#define _AUPBND (sizeof (acpi_native_int) - 1)

#define _ADNBND (sizeof (acpi_native_int) - 1)

/*

 * Variable argument list macro definitions

 */

#define _bnd(X, bnd) (((sizeof (X)) + (bnd)) & (~(bnd)))

#define va_arg(ap, T) (*(T *)(((ap) += (_bnd (T, _AUPBND))) - (_bnd (T,_ADNBND))))

#define va_end(ap) (void) 0

#define va_start(ap, A) (void) ((ap) = (((char *) &(A)) + (_bnd (A,_AUPBND))))

#endif                /* va_arg */

#include <stdio.h>

#include <stdarg.h>   /* 使用va_list、va_start等必须包含的头文件
 */

#include <string.h>

/* linux C没有itoa函数,所以要自己写 */

char *itoa(int i, char *str)

{

    int mod, div = fabs(i), index = 0;

    char *start, *end, temp;

    do

    {

        mod = div % 10;

        str[index++] = '0' + mod;

        div = div / 10;

    }while(div != 0);

    if (i < 0)

        str[index++] = '-';

    str[index] = '\0';

    for (start = str, end = str + strlen(str) - 1;

        start < end; start++, end--)

    {

        temp    = *start;

        *start    = *end;

        *end    = temp;

    }

    

    return str;

}

void print(const char *fmt, ...)

{

    char str[100];

    unsigned int len, i, index;

    int iTemp;

    char *strTemp;

    va_list args;

    va_start(args, fmt);

    len = strlen(fmt);

    for (i=0, index=0; i<len; i++)

    {

        if (fmt[i] != '%')    /*
 非格式化参数 */

        {

            str[index++] = fmt[i];

        }

        else                /* 格式化参数 */

        {

            switch(fmt[i+1])

            {

            case 'd':        /*
 整型 */

            case 'D':

                iTemp = va_arg(args, int);

                strTemp = itoa(iTemp, str+index);

                index += strlen(strTemp);

                i++;

                break;

            case 's':        /*
 字符串 */

            case 'S':

                strTemp = va_arg(args, char*);

                strcpy(str + index, strTemp);

                index += strlen(strTemp);

                i++;

                break;

            default:

                str[index++] = fmt[i];

            }

        }

    }

    str[index] = '\0';

    va_end(args);

    printf(str);        

}

int main()

{

    print("Version: %d; Modifier: %s\n", -958, "lingd");

    return 0;

}

void printascii(const char *); 

void printhex(unsigned long value, int nbytes); 

void printhex2(unsigned char value); 

void printhex4(unsigned short value); 

void printhex8(unsigned long value);

extern void printascii(const char *);

static void debug(const char *fmt, ...)

{

    va_list va;                

    char buff[256];

    va_start(va, fmt);

    

    /* 函数vsprintf:用于输出格式化字符串到缓冲区

     * 输出参数buff:用于保存结果的缓冲区

     * 输入参数fmt: 格式化字符串

     * 输入参数va:  可变参数列表

     * 返回值:      实际输出到缓冲区的字符数

     */

    vsprintf(buff, fmt, va);

    va_end(va);

    printascii(buff);

}