hello i have a question about va_start

Question

0.00/5 (No votes)

See more:

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))))

Please tell me the feature of the _bnd,
thank you

Posted 27-Dec-10 19:03pm

lxlenovostar

Updated 27-Dec-10 20:10pm

Dalek Dave

v3

Add a Solution

Comments

Dalek Dave 28-Dec-10 2:10am

Edited for Grammar and Readability.

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

tanakahitori · Accepted Answer · 2010-12-28T01:06:00

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

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) 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

I hope I said right

Richard MacCutchan · Accepted Answer · 2010-12-27T23:15:00

Solution 1

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 ...).

Posted 27-Dec-10 23:15pm

Richard MacCutchan

Comments

lxlenovostar 28-Dec-10 22:22pm

thank you very much