你好，我有一个关于va_start的问题 [英] hello i have a question about va_start

问题描述

你好，

我了解va_start，但无法理解此代码:

Hello,

I''m understanding the va_start, but I can''t understand this code:

typedef int s32;
typedef s32 acpi_native_int;
#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))))

请告诉我_bnd的功能，
谢谢您.

Please tell me the feature of the _bnd,
thank you

推荐答案

我认为_bnd(X，bnd)的功能是将X(缓冲区的长度)设置为int <的倍数. br/>
举个例子
1

I think the feature of _bnd(X, bnd) is set length of X(buffer or you could it array) to multiple of int

take a exmple
1

struct 
{
char c1 ;
char c2 ;
char c3 ;
} A ; //length of A is 3 Bytes = 24 bit

(_bnd (A,_AUPBND)

2

2

typedef int s32; //assume int is 2 Bytes
typedef s32 acpi_native_int;
 
//value of _AUPBND should be 1. it is "0000 0001" in memory 
#define  _AUPBND   (sizeof (acpi_native_int) - 1)

3(_bnd(A，_AUPBND)应该是(_bnd(A，1))

3 (_bnd (A,_AUPBND) should be (_bnd (A,1))

#define _bnd(X, bnd) (((sizeof (X)) + (bnd)) &(~(bnd)))
//_bnd (A,1)
//sizeof(A) = 3  "0000 0011" in memory
//3 + 1 = 4      "0000 0100" in memory
//(~(1)) =       "1111 1110" in memory
 
//final step
//4 & (~(1)) = 4 "0000 0100" in memory
//and you will found the lowest-1bits always has been clean to zero.
//because this will insure result of _bnd is multiple of int

//first time A is 3 Bytes , now _bnd return 4 (Bytes) is multiple of int

我希望我说对了

I hope I said right

据我所见，这是一个宏，用于确定arg数组中参数的大小，以便计算从va_start到下一个的偏移量争论.请记住，有些参数可能是指针(32或64位)，而有些则可能是值(8、16、32、64 ...).

As far as I can see this is a macro to determine the size of the parameters in the arg array in order to calculate the offset from va_start to the next argument. Remember that some arguments may be pointers (32 or 64 bits) while some may be values (8, 16, 32, 64 ...).

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