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align
Purpose
Specifies what aggregate alignment rules the compiler uses for file compilation. Use this option to specify the maximum alignment to be used when mapping a class-type object, either for the whole source program or for specific parts.
Syntax
where available alignment options are:
power The compiler uses a set of alignment rules to that provide binary compatibility with objects created by gcc V3.3 using the -malign option. This is the default.
natural The compiler uses a set of alignment rules to that provide binary compatibility with objects created by gcc V3.3 using the -malign option.
mac68k The compiler uses the Macintosh** alignment rules.
bit_packed The compiler uses the bit_packed alignment rules. Data, including bitfields, is packed as tightly as possible.
See also #pragma align and #pragma options.
Notes
If you use the -qalign option more than once on the command line, the last alignment rule specified applies to the file.
You can control the alignment of a subset of your code by using #pragma align=alignment_rule. Use #pragma align=reset to revert to a previous alignment rule. The compiler stacks alignment directives, so you can go back to using the previous alignment directive, without knowing what it is, by specifying the #pragma align=reset directive. For example, you can use this option if you have a class declaration within an include file and you do not want the alignment rule specified for the class to apply to the file in which the class is included.
You can code #pragma align=reset in a source file to change the alignment option to what it was before the last alignment option was specified. If no previous alignment rule appears in the file, the alignment rule specified in the invocation command is used.
Examples
Example 1 - Affecting Only Aggregate Definition
Using the compiler invocation:
xlc++ file2.C /* <-- default alignment rule for file is */
/* power since no alignment rule specified */
Where file2.C has:
extern struct A A1;
typedef struct A A2;
#pragma options align=bit_packed /* <-- use bit_packed alignment rules*/
struct A {
int a;
char c;
};
#pragma options align=reset /* <-- Go back to default alignment rules */
struct A A1; /* <-- aligned using bit_packed alignment rules since */
A2 A3; /* this rule applied when struct A was defined */
Example 2 - Imbedded #pragmas
Using the compiler invocation:
xlc -qalign=mac68k file.c /* <-- default alignment rule for file is */
/* Macintosh */
xlc -qalign=power file.c /* <-- default alignment rule for file */
/* is power */
Where file.c has:
struct A {
int a;
struct B {
char c;
double d;
#pragma options align=bit_packed /* <-- B will be unaffected by this */
/* #pragma, unlike previous behavior; */
/* power alignment rules still */
/* in effect */
} BB;
#pragma options align=reset /* <-- A is unaffected by this #pragma; */
} AA; /* power alignment rules still */
/* in effect */
Using the __align specifier
You can use the __align specifier to explicitly specify data alignment when declaring or defining a data item.
__align Specifier
Purpose
Use the __align specifier to explicitly specify alignment and padding when declaring or defining data items.
Syntax
declarator __align (int_const) identifier;
__align (int_const) struct_or_union_specifier [identifier] {struct_decln_list}
where:
int_const Specifies a byte-alignment boundary. int_const must be an integer greater than 0 and equal to a power of 2.
Notes
The __align specifier can only be used with declarations of first-level variables and aggregate definitions. It ignores parameters and automatics.
The __align specifier cannot be used on individual elements within an aggregate definition, but it can be used on an aggregate definition nested within another aggregate definition.
The __align specifier cannot be used in the following situations:
Individual elements within an aggregate definition.
Variables declared with incomplete type.
Aggregates declared without definition.
Individual elements of an array.
Other types of declarations or definitions, such as typedef, function, and enum.
Where the size of variable alignment is smaller than the size of type alignment.
Not all alignments may be representable in an object file.
Examples
Applying __align to first-level variables:
int __align(1024) varA; /* varA is aligned on a 1024-byte boundary
and padded with 1020 bytes */
static int __align(512) varB; /* varB is aligned on a 512-byte boundary
and padded with 508 bytes */
int __align(128) functionB( ); /* An error */
typedef int __align(128) T; /* An error */
__align enum C {a, b, c}; /* An error */
Applying __align to align and pad aggregate tags without affecting aggregate members:
__align(1024) struct structA {int i; int j;}; /* struct structA is aligned
on a 1024-byte boundary
with size including padding
of 1024 bytes */
__align(1024) union unionA {int i; int j;}; /* union unionA is aligned
on a 1024-byte boundary
with size including padding
of 1024 bytes */
Applying __align to a structure or union, where the size and alignment of the aggregate using the structure or union is affected:
__align(128) struct S {int i;}; /* sizeof(struct S) == 128 */
struct S sarray[10]; /* sarray is aligned on 128-byte boundary
with sizeof(sarray) == 1280 */
struct S __align(64) svar; /* error - alignment of variable is
smaller than alignment of type */
struct S2 {struct S s1; int a;} s2; /* s2 is aligned on 128-byte boundary
with sizeof(s2) == 256 bytes */
Applying __align to an array:
AnyType __align(64) arrayA[10]; /* Only arrayA is aligned on a 64-byte
boundary, and elements within that array
are aligned according to the alignment
of AnyType. Padding is applied after the
back of the array and does not affect
the size of the array member itself. */
Applying __align where size of variable alignment differs from size of type alignment:
__align(64) struct S {int i;};
struct S __align(32) s1; /* error, alignment of variable is smaller
than alignment of type */
struct S __align(128) s2; /* s2 is aligned on 128-byte boundary */
struct S __align(16) s3[10]; /* error */
int __align(1) s4; /* error */
__align(1) struct S {int i;}; /* error */
Related References
Compiler Command Line Options
#pragma align
#pragma pack
See also:
The Data Mapping and Storage section of the VisualAge C++ Programming Tasks manual.
__attribute__((aligned)).
__attribute__((packed)).