What does class_addIvar's alignment do in Objective-C?

Question

Someone has ask the same question before:Objective-C Runtime: What to put for size & alignment for class_addIvar? But it's not fully resolved.

The functions declaration is as follows:

BOOL class_addIvar(Class cls, const char *name, size_t size, uint8_t alignment, const char *types)

Which is used to add an instance variable to a dynamically created class in Objective-C.

The forth argument, uint8_t alignment, is described in Apple's documentation:

The instance variable's minimum alignment in bytes is 1<<align. The minimum alignment of an instance variable depends on the ivar's type and the machine architecture. For variables of any pointer type, pass log2(sizeof(pointer_type)).

In some tutorials, it's just claimed that if the ivar is pointer type, I should use log2(sizeof(pointer_type)); if the ivar is value type, I should use sizeof(value_type). But why? Can someone explain this in detail?

Answer 1

If you really want to learn where these values come from, you'll need to look at architecture specific ABI references, for OSX and iOS, they can be found here: OS X, iOS.

Each of those documents should have a section titled 'Data Types and Data Alignment', which helps to explain those values for the specific architecture.

In practice, since C11, you can use the _Alignof operator to have the compiler give you the correct value for a specific type (as it already needs to know this in order to generate proper machine code), so you can create a class_addIvar that looks something like this:

class_addIvar(myClass, "someIvar", sizeof(int), log2(_Alignof(int)), @encode(int))

Which should take care of all those gory details of the underlying type for you.