rhynodegreat rhynodegreat - 3 months ago 9
C Question

What's the reason for this struct's alignment?

In the Vulkan header

vulkan.h
there is a struct defined as

typedef struct VkSwapchainCreateInfoKHR {
VkStructureType sType;
const void* pNext;
VkSwapchainCreateFlagsKHR flags;
VkSurfaceKHR surface;
uint32_t minImageCount;
VkFormat imageFormat;
VkColorSpaceKHR imageColorSpace;
VkExtent2D imageExtent;
uint32_t imageArrayLayers;
VkImageUsageFlags imageUsage;
VkSharingMode imageSharingMode;
uint32_t queueFamilyIndexCount;
const uint32_t* pQueueFamilyIndices;
VkSurfaceTransformFlagBitsKHR preTransform;
VkCompositeAlphaFlagBitsKHR compositeAlpha;
VkPresentModeKHR presentMode;
VkBool32 clipped;
VkSwapchainKHR oldSwapchain;
} VkSwapchainCreateInfoKHR;


I've used the following code to see each field's alignment (Visual Studio 2015)

std::cout <<
"sType: " << offsetof(VkSwapchainCreateInfoKHR, sType) << std::endl <<
"pNext: " << offsetof(VkSwapchainCreateInfoKHR, pNext) << std::endl <<
"flags: " << offsetof(VkSwapchainCreateInfoKHR, flags) << std::endl <<
"surface: " << offsetof(VkSwapchainCreateInfoKHR, surface) << std::endl <<
"minImageCount: " << offsetof(VkSwapchainCreateInfoKHR, minImageCount) << std::endl <<
"imageFormat: " << offsetof(VkSwapchainCreateInfoKHR, imageFormat) << std::endl <<
"imageColorSpace: " << offsetof(VkSwapchainCreateInfoKHR, imageColorSpace) << std::endl <<
"imageExtent: " << offsetof(VkSwapchainCreateInfoKHR, imageExtent) << std::endl <<
"imageArrayLayers: " << offsetof(VkSwapchainCreateInfoKHR, imageArrayLayers) << std::endl <<
"imageUsage: " << offsetof(VkSwapchainCreateInfoKHR, imageUsage) << std::endl <<
"imageSharingMode: " << offsetof(VkSwapchainCreateInfoKHR, imageSharingMode) << std::endl <<
"queueFamilyIndexCount: " << offsetof(VkSwapchainCreateInfoKHR, queueFamilyIndexCount) << std::endl <<
"pQueueFamilyIndices: " << offsetof(VkSwapchainCreateInfoKHR, pQueueFamilyIndices) << std::endl <<
"preTransform: " << offsetof(VkSwapchainCreateInfoKHR, preTransform) << std::endl <<
"compositeAlpha: " << offsetof(VkSwapchainCreateInfoKHR, compositeAlpha) << std::endl <<
"presentMode: " << offsetof(VkSwapchainCreateInfoKHR, presentMode) << std::endl <<
"clipped: " << offsetof(VkSwapchainCreateInfoKHR, clipped) << std::endl <<
"oldSwapchain: " << offsetof(VkSwapchainCreateInfoKHR, oldSwapchain) << std::endl <<
std::endl;


And got these results

sType: 0
pNext: 8
flags: 16
surface: 24
minImageCount: 32
imageFormat: 36
imageColorSpace: 40
imageExtent: 44
imageArrayLayers: 52
imageUsageFlags: 56
imageSharingMode: 60
queueFamilyIndexCount: 64
pQueueFamilyIndices: 72
preTransform: 80
compositeAlpha: 84
presentMode: 88
clipped: 92
oldSwapchain: 96


Between the fields
flags
and
surface
there is an 8 byte gap, even though
flags
has an underlying type of
uint32_t
. The same is true with the fields
queueFamilyIndexCount
and
pQueueFamilyIndices
. Why do
flags
and
queueFamilyIndexCount
take up 8 bytes when they are only 4 bytes wide and every other field of type
uint32_t
takes up only 4 bytes? Is there something special about the memory alignment requirements at those offsets?

Answer

VkSurfaceKHR is a non-dispatchable handle. Which, by Vulkan's definitions, is a 64-bit integer. And therefore it must have 8-byte alignment.

When structures are being laid out, the compiler will ensure that each member gets the alignment that the type requires. If surface had come immediately after flags, it would not have 8-byte alignment. Thus, the compiler inserts 4 bytes of padding between the two.