Aligning data

XL C/C++ provides many mechanisms for specifying data alignment at the levels of individual variables, members of aggregates, entire aggregates, and entire compilation units. If you are porting applications between different platforms, or between 32-bit and 64-bit modes, you need to take into account the differences between alignment settings available in different environments, to prevent possible data corruption and deterioration in performance. In particular, vector types have special alignment requirements which, if not followed, can produce incorrect results. For more information, see the AltiVec Technology Programming Interface Manual.

XL C/C++ provides alignment modes and alignment modifiers for specifying data alignment. Using alignment modes, you can set alignment defaults for all data types for a compilation unit (or subsection of a compilation unit) by specifying a predefined suboption.

Using alignment modifiers, you can set the alignment for specific variables or data types within a compilation unit by specifying the exact number of bytes that should be used for the alignment.

Using alignment modes discusses the default alignment modes for all data types on different platforms and addressing models, the suboptions and pragmas that you can use to change or override the defaults, and rules for the alignment modes for simple variables, aggregates, and bit fields.

Using alignment modifiers discusses the different specifiers, pragmas, and attributes you can use in your source code to override the alignment mode currently in effect, for specific variable declarations. It also provides the rules that govern the precedence of alignment modes and modifiers during compilation.