(gcc.info) Alignment

 
 5.35 Inquiring on Alignment of Types or Variables
 =================================================
 
 The keyword `__alignof__' allows you to inquire about how an object is
 aligned, or the minimum alignment usually required by a type.  Its
 syntax is just like `sizeof'.
 
  For example, if the target machine requires a `double' value to be
 aligned on an 8-byte boundary, then `__alignof__ (double)' is 8.  This
 is true on many RISC machines.  On more traditional machine designs,
 `__alignof__ (double)' is 4 or even 2.
 
  Some machines never actually require alignment; they allow reference
 to any data type even at an odd address.  For these machines,
 `__alignof__' reports the smallest alignment that GCC will give the
 data type, usually as mandated by the target ABI.
 
  If the operand of `__alignof__' is an lvalue rather than a type, its
 value is the required alignment for its type, taking into account any
 minimum alignment specified with GCC's `__attribute__' extension (
 Variable Attributes).  For example, after this declaration:
 
      struct foo { int x; char y; } foo1;
 
 the value of `__alignof__ (foo1.y)' is 1, even though its actual
 alignment is probably 2 or 4, the same as `__alignof__ (int)'.
 
  It is an error to ask for the alignment of an incomplete type.

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