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"#pragma pack"和"#pragma pack"之间有什么区别?和"__attribute __((aligned))"

分类: 技术问答 • 2022-03-26 18:05:11

问题描述：

#pragma pack(L1_CACHE_LINE)
struct A {
  //...
};
#pragma pack()

A a;

和

struct A {
  //...
};

A a __attritube__((aligned(L1_CACHE_LINE)))

它们之间有什么区别?

答

#pragma pack(byte-alignment)会影响由byte-alignment输入指定的结构的每个成员，或者影响其自然对齐边界，以两者中的任意一个为准较少的.

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.

__attribute__((aligned(byte-alignment)))影响变量(如果在struct中指定，则为struct字段)的最小对齐方式

The __attribute__((aligned(byte-alignment))) affect the minimum alignment of the variable (or struct field if specified within the struct)

我相信以下是等价的

#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()

其中的a __attritube__((aligned(L1_CACHE_LINE)))将确保struct A内部的u_int32_t a将与2个字节对​​齐，但不会以相同的方式对齐其他变量.

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.

参考:

  1. http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Fcompiler%2Fref%2Frnpgpack.htm
  2. http://www. khronos.org/registry/cl/sdk/1.0/docs/man/xhtml/attributes-variables.html
  1. http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Fcompiler%2Fref%2Frnpgpack.htm
  2. http://www.khronos.org/registry/cl/sdk/1.0/docs/man/xhtml/attributes-variables.html

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