Using alignment modifiers

XL C also provides alignment modifiers, with which you can exercise even finer-grained control over alignment, at the level of declaration or definition of individual variables or aggregate members. Available modifiers are:

#pragma pack(...)

Valid application:

The entire aggregate (as a whole) immediately following the directive. Note: on AIX® #pragma pack does not apply to bit-field union members.

Effect:

Sets the maximum alignment of the members of the aggregate to which it applies, to a specific number of bytes. Also allows a bit-field to cross a container boundary. Used to reduce the effective alignment of the selected aggregate.

Valid values:

n: where n is 1, 2, 4, 8, or 16. That is, structure members are aligned on n-byte boundaries or on their natural alignment boundary, whichever is less. nopack: disables packing. pop: removes the previous value added with #pragma pack. Note: empty brackets has the same functionality as pop.

__attribute__((aligned(n)))

Valid application:

As a variable attribute, it applies to a single aggregate (as a whole), namely a structure, union, or class; or to an individual member of an aggregate.¹ As a type attribute, it applies to all aggregates declared of that type. If it is applied to a typedef declaration, it applies to all instances of that type.²

Effect:

Sets the minimum alignment of the specified variable (or variables), to a specific number of bytes. Typically used to increase the effective alignment of the selected variables.

Valid values:

n must be a positive power of 2, or NIL. NIL can be specified as either __attribute__((aligned())) or __attribute__((aligned)); this is the same as specifying the maximum system alignment (16 bytes on all UNIX platforms).

__attribute__((packed))

Valid application:

As a variable attribute, it applies to simple variables, or individual members of an aggregate, namely a structure. As a type attribute, it applies to all members of all aggregates declared of that type.

Effect:

Sets the maximum alignment of the selected variable, or variables, to which it applies, to the smallest possible alignment value, namely one byte for a variable and one bit for a bit field.

__align(n)

Effect:

Sets the minimum alignment of the variable or aggregate to which it applies to a specific number of bytes; also might effectively increase the amount of storage occupied by the variable. Used to increase the effective alignment of the selected variables.

Valid application:

Applies to simple static (or global) variables or to aggregates as a whole, rather than to individual members of aggregates, unless these are also aggregates.

Valid values:

n must be a positive power of 2. XL C also allows you to specify a value greater than the system maximum.