Using alignment modifiers

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XL C/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 as follows:

#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
  • number: where number is 1, 2, 4, 8, or 16. That is, structure members are aligned on number-byte boundaries or on their natural alignment boundary, whichever is less.
  • nopack: disables packing.
  • pop: removes the previous value added with #pragma pack.
  • 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 it applies 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 two, 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 or class1. 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/C++ also allows you to specify a value greater than the system maximum.
Notes:
  1. In a comma-separated list of variables in a declaration, if the modifier is placed at the beginning of the declaration, it applies to all the variables in the declaration. Otherwise, it applies only to the variable immediately preceding it.
  2. Depending on the placement of the modifier in the declaration of a struct, it can apply to the definition of the type, and hence applies to all instances of that type; or it can apply to only a single instance of the type. For details, see the information about type attributes in the XL C/C++ Language Reference and the C C++ onlyand C++C++ only language standards.