#pragma align

Purpose

Specifies the alignment of data objects in storage, which avoids performance problems with misaligned data.

Syntax

Parameters

packed: Bit field data is packed on a bitwise basis without respect to byte boundaries.
power: Uses the RISC System/6000 alignment rules. This is the default.
natural: Structure members are mapped to their natural boundaries. This has the same effect as the power suboption, except that it also applies alignment rules to double and long double members that are not the first member of a structure or union.
reset: Discards the current pragma setting and reverts to the setting specified by the previous pragma directive.

Usage

The power suboption is the default to ensure compatibility with existing objects. If compatibility with earlier versions is not necessary, you should consider using natural alignment to improve potential application performance.

The pragmas affect all aggregate definitions that appear after a given pragma directive; if a pragma is placed inside a nested aggregate, it applies only to the definitions that follow it, not to any containing definitions. Any aggregate variables that are declared use the alignment rule that applied at the point at which the aggregate was defined, regardless of pragmas that precede the declaration of the variables. See below for examples.

Examples

The following example shows how the pragma affects only an aggregate definition, not subsequent declarations of variables of that aggregate type.

/* file2.c  The default alignment rule in effect is power */ 

typedef struct A A2;  

#pragma align(packed) /*  The packed alignment rule is now in effect */ 
struct A {   
int a;   
char c; 
}; #pragma align(reset) /*  The default alignment rule is in effect again */  

struct A A1;  /*  A1 and A3 are aligned using the power alignment rule since   */ 
A2 A3;        /*  this rule applied when struct A was defined      */

The following example shows how a pragma embedded in a nested aggregate definition affects only the definitions that follow it.

/* file2.c  The default alignment rule in effect is power  */  
                                               
struct A {   
int a;
#pragma align(packed) /* Applies to B; A is unaffected  */ 
   struct B {     
   char c;     
   double d;       
            } BB;   /*  BB uses the packed alignment rule  */ 
} AA;               /*  AA uses the power alignment rule  /*