“#pragma pack”的区别是什么？和“__属性__（（对齐））”

问题描述

例如：

  
 $ b $  __属性__（（aligned（byte-alignment）））会影响变量（或结构体字段，如果在struct中指定的话）
 
 
 
我相信以下内容是相同的 
 
 
  #define L1_CACHE_LINE 2 
 $ b $ struct A 
 {
 u_int32_t a __attribute__（（aligned（L1_CACHE_LINE）））; 
 u_int32_t b __attribute__（（aligned（L1_CACHE_LINE）））; 
 u_int16_t c __attribute__（（aligned（L1_CACHE_LINE）））; 
 u_int16_t d __attribute__（（aligned（L1_CACHE_LINE）））; 
 u_int32_t e __attribute__（（aligned（L1_CACHE_LINE）））; 
}; 
 
 
 #pragma pack（L1_CACHE_LINE）
 struct A 
 {
 u_int32_t a; 
 u_int32_t b; 
 u_int16_t c; 
 u_int16_t d; 
 u_int32_t e; 
}; 
 #pragma pack（）
  
其中A  a __attritube __（ （对齐的（L1_CACHE_LINE）））将保证结构A 内的 u_int32_t a  2字节，但不会以相同的方式对齐其他变量。
 
 
 
参考： 
 
 
 
 
  
 
 
  
 
 
 
Eg:
#pragma pack(L1_CACHE_LINE)
struct A {
  //...
};
#pragma pack()

A a;
and
struct A {
  //...
};

A a __attritube__((aligned(L1_CACHE_LINE)))
What's difference between thwm?
Thanks advance!
 解决方案 
The #pragma pack(byte-alignment) effect each member of the struct as specified by the byte-alignment input, or on their natural alignment boundary, whichever is less. 
The __attribute__((aligned(byte-alignment))) affect the minimum alignment of the variable (or struct field if specified within the struct)
I believe the following are equivalent 
#define L1_CACHE_LINE 2

struct A
{
    u_int32_t   a   __attribute__ ( (aligned(L1_CACHE_LINE)) );
    u_int32_t   b   __attribute__ ( (aligned(L1_CACHE_LINE)) );
    u_int16_t   c   __attribute__ ( (aligned(L1_CACHE_LINE)) );
    u_int16_t   d   __attribute__ ( (aligned(L1_CACHE_LINE)) );
    u_int32_t   e   __attribute__ ( (aligned(L1_CACHE_LINE)) );
};


#pragma pack(L1_CACHE_LINE)
struct A
{
    u_int32_t   a;
    u_int32_t   b;
    u_int16_t   c;
    u_int16_t   d;
    u_int32_t   e;
};
#pragma pack()
where is A a __attritube__((aligned(L1_CACHE_LINE))) will insure u_int32_t a inside struct A will aligned with 2 byte but will not align the other variable in the same manner.
Reference:
http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Fcompiler%2Fref%2Frnpgpack.htm
http://www.khronos.org/registry/cl/sdk/1.0/docs/man/xhtml/attributes-variables.html
                        
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