9.2 __align

The__alignkeyword instructs the compiler to align a variable on ann-byte boundary.

__alignis a storage class modifier. It does not affect the type of the function.

Syntax

__align(n) Where: n is the alignment boundary. For local variables,ncan take the values 1, 2, 4, or 8. For global variables,ncan take any value up to 0x80000000 in powers of 2.

Usage

__align(n)is useful when the normal alignment of the variable being declared is less thann. Eight-byte alignment can give a significant performance advantage with VFP instructions. __aligncan be used in conjunction withexternandstatic.

Restrictions

Because__alignis a storage class modifier, it cannot be used on:

• Types, includingtypedefs and structure definitions.
• Function parameters.

You can only overalign. That is, you can make a two-byte object four-byte aligned but you cannot align a four-byte object at 2 bytes.

Examples

__align(8) char buffer[128];  // buffer starts on eight-byte boundary
[/mw_shl_code]
void foo(void)
{
    ...
    __align(16) int i; // this alignment value is not permitted for
                       // a local variable
    ...
}
__align(16) int i; // permitted as a global variable.
[/mw_shl_code]
			
		
	
	
		

			Related reference
		
		
			7.104 --min_array_alignment=opt
		
		
			9.60 __attribute__((aligned)) 
variable attribute
		
	
                                                                    

                                                        


            

__attribute__((aligned)) variable attribute

Non-Confidential   ARM DUI0375E ARM^® Compiler v5.04 for ?Vision armcc User Guide Version 5

#pragma pack(n)

Non-Confidential   ARM DUI0375E ARM^® Compiler v5.04 for ?Vision armcc User Guide Version 5

回复【17楼】roarghy:
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受教了，感谢提供资料，而且得到一种新的思路，编译器的help文件真的很少去看。

MARK了