Why does this C sizeof function show different values in different contexts?

Question

Some context: iOS app development. Objective-C code in Xcode.

This one has been confounding me for hours:

I have a little struct I created:

struct EIVertex {
    GLKVector3      P;
    GLKVector3      N;
    GLKVector3      barycentric;
    GLKVector2      st;
};

GLKVector3 and GLKVector2 are types found in the GLKit framework.

Here is what is weird:

sizeof(GLKVector2) = 8
sizeof(GLKVector3) = 12
sizeof(EIVertex) = 48

The size for EIVertex is 48 but should be 44 (12 + 12 + 12 + 8 = 44).

Can someone please explain why there is this size disparity?

There might be padding in the memory layout. The padding is used to get the member's address correctly aligned to some boundaries. In this case most likely 32-bit boundaries. — woodleg.as, Apr 10 '13 at 23:38

score 2 · Accepted Answer · edited May 23 '17 at 12:29

2

Structures in C tend to be padded for data alignment. Check this page for more information. Also refer to the answer in this question.

edited May 23 '17 at 12:29

Community

answered Apr 10 '13 at 23:38

1 Answers1