about pack and align for C/C++

http://msdn.microsoft.com/en-us/library/2e70t5y1(VS.80).aspx

 

#pragma pack(  n  )

n : Valid values are 1, 2, 4, 8, and 16.the alignment of a member will be on a boundary that is either a multiple of  n  or a multiple of the size of the member , whichever is smaller.

The default value for  n  is 8

成员变量对齐：选n 和该成员长度 的 最小值(的倍数)

结构ABCD中 d的对齐方式，选n和sizeof(d)的最小值8，然后按8字节对齐！！！

sizeof大小：选n 和最大成 员长度 的最小值，对齐

例子：

#pragma pack(8)

struct ABCD

{

bool b;

short s;

double d;

};

int main()

{

cout << offsetof(ABCD, b) << endl;

cout << offsetof(ABCD, s) << endl;

cout << offsetof(ABCD, d) << endl;

cout << sizeof(ABCD) << endl;

}

output is:

0 

2

8

16

http://msdn.microsoft.com/en-us/library/83ythb65(VS.71).aspx

__declspec( align( n ) )

__declspec( align() ) 和 #pragma pack 是一对兄弟，前者规定了对齐的最小值，后者规定了对齐的最大值，两者同时出现时，前者拥有更高的优先级。 

例子：

#define CACHE_ALIGN 64 

// Force each structure to be in a different cache line. 

struct __declspec(align(CACHE_ALIGN)) CUSTINFOAA { 

DWORD    dwCustomerID;     // Mostly read-only 

wchar_t  szName[100];      // Mostly read-only 

// Force the following members to be in a different cache line. 

__declspec(align(CACHE_ALIGN)) 

  int nBalanceDue;           // Read-write 

FILETIME ftLastOrderDate;  // Read-write 

}; 

int main()

{

cout << offsetof(CUSTINFOAA, dwCustomerID) << endl;

cout << offsetof(CUSTINFOAA, szName) << endl;

cout << offsetof(CUSTINFOAA, nBalanceDue) << endl;

cout << offsetof(CUSTINFOAA, ftLastOrderDate) << endl;

cout << sizeof(CUSTINFOAA) << endl;

}

Output:

0

4

256

260

320