va_start、va_arg、va_end分析

acenv.h中有：

#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 */

參數是存放在棧裏面的，所以得到的第一個參數，可以通過偏移等相關操作來得到第2個3個，直到NULL
網上找到的2個例子：

//vstart.c  
#include <stdio.h>  
#include <strings.h>  
#include <stdarg.h>  
int demo(char *fmt, ...);  
int main()   
{   
   demo("DEMO", "This", "is", "a", "demo!", "");   
   return 0;  
}   
int demo( char *fmt, ... )   
{   
   va_list argp;   
   int argno = 0;    
   char *para;   
   va_start(argp, fmt);   
   while (1)   
   {   
      para = va_arg(argp, char *);   
      if (strcmp( para, "") == 0)   
      break;   
      printf("Parameter #%d is: %s/n", argno, para);   
      argno++;   
   }   
   va_end( argp );   
   return 0;   
}   

#include <stdio.h>  
#include <stdarg.h>  
#include <string.h>  
int foo(char *fmt, ...);  
int main()  
{  
  char *a = "ast";  
  int b = 224;  
  char c = 'x';  
   
  foo("%s,%d,%c/n",a,b,c);  
  return 0;  
}  
int foo(char *fmt, ...)  
{  
  va_list ap;  
  int d;  
  char c, *s;  
  va_start(ap, fmt);  
  while (*fmt)  
  switch(*fmt++)  
  {  
     case 's':           /* string */  
        s = va_arg(ap, char *);  
        printf("string %s/n", s);  
        break;  
     case 'd':           /* int */  
        d = va_arg(ap, int);  
        printf("int %d/n", d);  
        break;  
     case 'c':           /* char */  
                             /* need a cast here since va_arg only 
                                takes fully promoted types */  
        c = (char) va_arg(ap, int);  
        printf("char %c/n", c);  
        break;  
  }  
  va_end(ap);  
  return 0;  
}  

跑一下試試吧！