Each data type supported by XL C is
aligned along byte boundaries according to platform-specific default
alignment modes. On AIX®, the
default alignment mode is power or full,
which are equivalent.
You can change the default alignment mode, by using any of the
following mechanisms:
- Set the alignment mode for all variables in a single file or multiple
files during compilation
- To use this approach, you specify the -qalign compiler
option during compilation, with one of the suboptions listed in Table 1.
- Set the alignment mode for all variables in a section of source
code
- To use this approach, you specify the #pragma align or #pragma
options align directives in the source files, with one of
the suboptions listed in Table 1.
Each directive changes the alignment mode in effect for all variables
that follow the directive until another directive is encountered,
or until the end of the compilation unit.
Each of the valid alignment modes is defined in Table 1, which provides the alignment
value, in bytes, for scalar variables, for all data types. Where there
are differences between 32-bit and 64-bit modes, these are indicated.
Also, where there are differences between the first (scalar) member
of an aggregate and subsequent members of the aggregate, these are
indicated.
Table 1. Alignment
settings (values given in bytes)| Data type |
Storage |
Alignment setting |
|---|
| natural |
power, full |
mac68k, twobyte3 |
bit_packed2 |
packed2 |
|---|
| _Bool (32-bit
mode) |
1 |
1 |
1 |
1 |
1 |
1 |
| _Bool (64-bit mode) |
1 |
1 |
1 |
not supported |
1 |
1 |
| char, signed char, unsigned char |
1 |
1 |
1 |
1 |
1 |
1 |
| wchar_t (32-bit mode) |
2 |
2 |
2 |
2 |
1 |
1 |
| wchar_t (64-bit mode) |
4 |
4 |
4 |
not supported |
1 |
1 |
| int, unsigned int |
4 |
4 |
4 |
2 |
1 |
1 |
| short int, unsigned short int |
2 |
2 |
2 |
2 |
1 |
1 |
| long int, unsigned long int (32-bit mode) |
4 |
4 |
4 |
2 |
1 |
1 |
| long int, unsigned long int (64-bit mode) |
8 |
8 |
8 |
not supported |
1 |
1 |
| _Decimal32 |
4 |
4 |
4 |
2 |
1 |
1 |
| _Decimal64 |
8 |
8 |
8 |
2 |
1 |
1 |
| _Decimal128 |
16 |
16 |
16 |
2 |
1 |
1 |
| long long |
8 |
8 |
8 |
2 |
1 |
1 |
| float |
4 |
4 |
4 |
2 |
1 |
1 |
| double |
8 |
8 |
see note1 |
2 |
1 |
1 |
| long double |
8 |
8 |
see note1 |
2 |
1 |
1 |
| long double with -qldbl128 |
16 |
16 |
see note1 |
2 |
1 |
1 |
| pointer (32-bit mode) |
4 |
4 |
4 |
2 |
1 |
1 |
| pointer (64-bit mode) |
8 |
8 |
8 |
not supported |
1 |
1 |
| vector types |
16 |
16 |
16 |
16 |
1 |
1 |
Notes: - In aggregates, the first member of this data type is aligned according
to its natural alignment value; subsequent members of the aggregate
are aligned on 4-byte boundaries.
- The packed alignment will not pack bit-field
members at the bit level; use the bit_packed alignment
if you want to pack bit fields at the bit level.
- For mac68k alignment, if the aggregate
does not contain a vector member, the alignment is 2 bytes. If an
aggregate contains a vector member, then the alignment is the largest
alignment of all of its members.
|
If you are working with aggregates containing double, long
long, or long double data types, use the natural mode
for highest performance, as each member of the aggregate is aligned
according to its natural alignment value. If you generate data
with an application on one platform and read the data with an application
on another platform, it is recommended that you use the
bit_packed mode,
which
results in equivalent data alignment on all platforms. Note: Vectors
in a bit-packed structure may not be correctly aligned unless you
take extra action to ensure their alignment.
Alignment of aggregates discusses
the rules for the alignment of entire aggregates and provide examples
of aggregate layouts. Alignment of bit fields discusses
additional rules and considerations for the use and alignment of bit
fields, and provides an example of bit-packed alignment.