Alignment examples

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Note: This topic does not apply to the Clang-based front end of IBM® XL C/C++ for AIX®, which is invoked by xlclang/xlclang++.

The following examples use these symbols to show padding and boundaries:

p = padding

| = halfword (2-byte) boundary

: = byte boundary

Mac68K example

#pragma options align=mac68k
struct B {
    char a;
    double b;
     };
#pragma options align=reset

The size of B is 10 bytes. The alignment of B is 2 bytes. The layout of B is as follows:

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

Packed example

#pragma options align=bit_packed
struct {
   char a;
   double b;
     } B;
#pragma options align=reset

The size of B is 9 bytes. The layout of B is as follows:

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

Nested aggregate example

#pragma options align=mac68k
struct A {
  char a;
  #pragma options align=power
  struct B {
     int b;
     char c;
     } B1;    // <-- B1 laid out using power alignment rules
  #pragma options align=reset    // <-- has no effect on A or B, 
                                       but on subsequent structs
  char d;
};
#pragma options align=reset

The size of A is 12 bytes. The alignment of A is 2 bytes. The layout of A is as follows:

|a:p|b:b|b:b|c:p|p:p|d:p|

C++ derived class example

In 32-bit mode:

#pragma options align=natural

class A {    
double _a;
} sa;

class C : public A {
public:
   virtual void f() {}
private:
   char* name;
} sc;

The size of sc is 24 bytes. The alignment of sc is 8 bytes. The layout of sc is as follows:

|a:a|a:a|a:a|a:a|f:f|f:f|p:p|p:p|n:n|n:n|p:p|p:p|