why does getting vulkan extensions from a method fail vulkan instance creation?

Question

I am following the vulkan-tutorial.com tutorial and im on the validation layers step. In the tutorial Alexander Overvoorde the author moved the gathering of available extensions for instance creation to its own function.

std::vector<const char*> getRequiredExtensions()

Previously i had gathered that info a little differently because i am using SDL2 instead of glfw, but my program ran with instance creation and no validation errors. Problem is when i moved the code to this function i can no longer get instance creation.

this worked fine:

unsigned int extensionCount = 0;
vkEnumerateInstanceExtensionProperties(NULL, &extensionCount, NULL);
std::vector<VkExtensionProperties> extensionProperties(extensionCount);
vkEnumerateInstanceExtensionProperties(NULL, &extensionCount, extensionProperties.data());
std::vector<const char*> extensionNames;
std::cout << "available extensions:" << std::endl;
int i = 0;
while (i < extensionCount) {
    extensionNames.push_back(extensionProperties[i].extensionName);
    std::cout << extensionNames[i] << std::endl;
    i++;
}
if (enableValidationLayers) {
    extensionNames.push_back(VK_EXT_DEBUG_REPORT_EXTENSION_NAME);
}*/
createInfo.enabledExtensionCount = extensionCount;
createInfo.ppEnabledExtensionNames = extensionNames.data();

but this failed even though the function uses the exact same code and returns extensionNames, i then use it like this:

std::vector<const char*> extensionNames = getRequiredExtensions();
createInfo.enabledExtensionCount = static_cast<uint32_t>(extensionNames.size());
createInfo.ppEnabledExtensionNames = extensionNames.data();

So why doesn't this work? I have a formally taught background of java but have coded in c++ for a year, so it is possibly like a syntax error or pointer that i am sending wrong. Additionally

reateInfo.enabledExtensionCount = static_cast<uint32_t>(getRequiredExtensions().size());

works just fine so the returned vector is of the correct size: 5 i believe because i have 4 extensions plus the debug one.

Quentin Quentin · Accepted Answer · 2017-05-24T00:46:52

From the documentation at Khronos.org, VkExtensionProperties looks like this:

typedef struct VkExtensionProperties {
    char        extensionName[VK_MAX_EXTENSION_NAME_SIZE];
    uint32_t    specVersion;
} VkExtensionProperties;

... so on this line:

extensionNames.push_back(extensionProperties[i].extensionName);

... what you're storing into extensionNames are pointers to arrays that live in extensionProperties, which is local to your function. When you return from the function, all of the arrays are destructed along with extensionProperties, and all of your pointers are now dangling.
What you get then is undefined behaviour when Vulkan tries to use these dead pointers.

why does getting vulkan extensions from a method fail vulkan instance creation?

1 Answers