Macintosh and Twobyte Alignment Rules

MacIntosh and Twobyte Alignment Rules

All unions and structures are halfword aligned regardless of their members. Within the aggregate, members are aligned according to their type. The size of types for the Macintosh system is the same as on the RISC System/6000 system. The table below summarizes alignment information for each type.

Type	Alignment	Size
Type, Size, and Alignment for the Macintosh System
char	byte aligned	byte
short	halfword aligned	halfword
(long) int	halfword aligned	word
long long int	halfword aligned	doubleword
pointer	halfword aligned	word
float	halfword aligned	word
double	halfword aligned	doubleword
long double	halfword aligned	doubleword
long double with -qlongdouble or -qldbl128 option.	halfword aligned	quadrupleword

Example
The following example uses these symbols to show padding and boundaries:

     p = padding
     | = halfword boundary
     : = byte boundary

For:

     #pragma options align=mac68k
     struct A {
       char a;
     }
 
     sizeof(A) == 2

The layout of A is:

     |a:p|

For:

     #pragma options align=mac68k
     struct B {
       char a;
       double b;
     }
 
     sizeof(B) == 10

The layout of B is:

     |a:p|b:b|b:b|b:b|b:b|

Bit fields for Macintosh Format
The following rules apply when you are laying out bit fields in structures.

An individual bit field can be at most 32 bits long.
Bit fields are packed into a word and are aligned on a 2-byte boundary.
Bit fields that would cross a word boundary are moved to the next halfword boundary even if they are already starting on a halfword boundary. (The bit field may still end up crossing a word boundary.)
A bit field of width zero forces the next member (even if it is not a bit field) to start at the next halfword boundary even if the zero-width bit field is currently at a halfword boundary.
A structure containing nothing but zero width bit fields is allowed and will have a length, in bytes, of two times the number of zero width bit fields.

For unions, there is one special case:

Unions whose largest element is a bit field of width 16 or less have a size of 2 bytes. If the width of the bit field is greater than 16, the size of the union is 4 bytes.

Example
The following example uses these symbols to show padding and boundaries:

     p = padding
     | = halfword boundary
     : = byte boundary

For:

     #pragma options align=mac68k
     struct A {
       char a;
       int : 0;
       int b : 4;
       int c : 17;
     }
 
     sizeof(A) == 8

The layout of A is:

     |a:p|b .. :p|c:c|c .. :p|

Type Compatibility between RISC System/6000 and Macintosh Systems
Different aggregate types with identical members are not compatible. Therefore such aggregates cannot be assigned to each other. Use the -qextchk option to assure type safe linkage for function declarations that may use different alignment rules.

align Compiler Option
RISC System/6000 Alignment Rules
Packed Alignment Rules
Alignment Rules for Nested Aggregates