Question

#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 them?

Answer 1

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:

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