pNext和sType用法

刚接触 Vulkan 的你可能会注意到 Vulkan Spec 中无处不在的pNext和sType变量。 pNext主要用于扩展功能的衍生结构体。void* pNext在结构体之间创建链接，loader、layer和驱动使用VkStructureType sType来获知传入的pNext结构体类型。

两种基本结构体

Vulkan API 提供了两种基本结构：VkBaseInStructure和VkBaseOutStructure，用来遍历结构指针链表。

对loader、layer和驱动程序来说，VkBaseInStructure的In指pNext是const * 只读的，VkBaseOutStructure的Out指pNext是用来将数据返回给应用程序。

设置pNext示例

// An example with two simple structures, "a" and "b"
typedef struct VkA {
    VkStructureType sType;
    void* pNext;
    uint32_t value;
} VkA;

typedef struct VkB {
    VkStructureType sType;
    void* pNext;
    uint32_t value;
} VkB;

// A Vulkan Function that takes struct "a" as an argument
// This function is in charge of populating the values
void vkGetValue(VkA* pA);

// Define "a" and "b" and set their sType
struct VkB b = {};
b.sType = VK_STRUCTURE_TYPE_B;

struct VkA a = {};
a.sType = VK_STRUCTURE_TYPE_A;

// Set the pNext pointer from "a" to "b"
a.pNext = (void*)&b;

// Pass "a" to the function
vkGetValue(&a);

// Use the values which were both set from vkGetValue()
printf("VkA value = %u \n", a.value);
printf("VkB value = %u \n", b.value);

读取pNext示例

这个示例说明了loader、layer和驱动是如何找到pNext的链式结构的：

void vkGetValue(VkA* pA) {

    VkBaseOutStructure* next = reinterpret_cast<VkBaseOutStructure*>(pA->pNext);
    while (next != nullptr) {
        switch (next->sType) {

            case VK_STRUCTURE_TYPE_B:
                VkB* pB = reinterpret_cast<VkB*>(next);
                // This is where the "b.value" above got set
                pB->value = 42;
                break;

            case VK_STRUCTURE_TYPE_C:
                // Can chain as many structures as supported
                VkC* pC = reinterpret_cast<VkC*>(next);
                SomeFunction(pC);
                break;

            default:
                LOG("Unsupported sType %d", next->sType);
        }

        // This works because the first two values of all chainable Vulkan structs
        // are "sType" and "pNext" making the offsets the same for pNext
        next = reinterpret_cast<VkBaseOutStructure*>(next->pNext);
    }

    // ...
}