Vulkan-Docs/doc/specs/vulkan/chapters/initialization.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[initialization]]
= Initialization

Before using {apiname}, an application must: initialize it by loading the
{apiname} commands, and creating a slink:VkInstance object.

[[initialization-functionpointers]]
== Command Function Pointers

{apiname} commands are not necessarily exposed statically on a platform.
Function pointers for all {apiname} commands can: be obtained with the
command:

include::../protos/vkGetInstanceProcAddr.txt[]

  * pname:instance is the instance that the function pointer will be
    compatible with.
  * pname:pName is the name of the command to obtain.

include::../validity/protos/vkGetInstanceProcAddr.txt[]

fname:vkGetInstanceProcAddr itself is obtained in a platform- and loader-
specific manner. Typically, the loader library will export this command as a
function symbol, so applications can: link against the loader library, or
load it dynamically and look up the symbol using platform-specific APIs.
Loaders are encouraged to export function symbols for all other core
{apiname} commands as well; if this is done, then applications that use only
the core {apiname} commands have no need to use fname:vkGetInstanceProcAddr.

Function pointers to commands that don't operate on a specific instance can:
be obtained by using this command with pname:instance equal to `NULL`. The
following commands can: be accessed this way:

  * fname:vkEnumerateInstanceExtensionProperties
  * fname:vkEnumerateInstanceLayerProperties
  * fname:vkCreateInstance

If pname:instance is a valid sname:VkInstance, function pointers to any
commands that operate on pname:instance or a child of pname:instance can: be
obtained. The returned function pointer must: only be called with a
dispatchable object (the first parameter) that is a child of pname:instance.

If pname:pName is not the name of a core {apiname} command, or is an
extension command for any extension not supported by any available layer or
implementation, then fname:vkGetInstanceProcAddr will return `NULL`.

ifdef::editing-notes[]
[NOTE]
.editing-note
====
(Jon, Bug 14886 / Gitlab issue 4) The WSI group tentatively agreed that the
WSI extensions were special, and should get static entry points in link
libraries and prototypes in +vulkan.h+, while future extensions would have
to be dynamically loaded. If this decision is upheld by the group as a
whole, it would probably be encoded in the previous paragraph, in the WSI
extensions branch of the Specification.

However, this decision hasn't been fully signed off on by the entire Vulkan
WG yet AFAIK. Note that implementations typically won't support many of the
WSI extensions, so ``static entry points'' doesn't relieve apps of the
neccessity of runtime enabling and testing of each extension before using
it.
====
endif::editing-notes[]

In order to support systems with multiple {apiname} implementations
comprising heterogenous collections of hardware and software, the function
pointers returned by fname:vkGetInstanceProcAddr may: point to dispatch
code, which calls a different real implementation for different
sname:VkDevice objects (and objects created from them). The overhead of this
internal dispatch can: be avoided by obtaining device-specific function
pointers for any commands that use a device or device-child object as their
dispatchable object. Such function pointers can: be obtained with the
command:

include::../protos/vkGetDeviceProcAddr.txt[]

  * pname:device is the logical device that provides the function pointer.
  * pname:pName is the name of any {apiname} command whose first parameter
    is one of
  ** sname:VkDevice
  ** sname:VkQueue
  ** sname:VkCommandBuffer

If pname:pName is not the name of one of these {apiname} commands, and is
not the name of an extension command belonging to an extension enabled for
pname:device, then fname:vkGetDeviceProcAddr will return `NULL`.

include::../validity/protos/vkGetDeviceProcAddr.txt[]

[[initialization-instances]]
== Instances

There is no global state in {apiname} and all per-application state is
stored in a sname:VkInstance object. Creating a sname:VkInstance object
initializes the {apiname} library and allows the application to pass
information about itself to the implementation.

To create an instance object, call:

include::../protos/vkCreateInstance.txt[]

  * pname:pCreateInfo points to an instance of slink:VkInstanceCreateInfo
    controlling creation of the instance.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pInstance points a sname:VkInstance handle in which the resulting
    instance is returned.

include::../validity/protos/vkCreateInstance.txt[]

The definition of sname:VkInstanceCreateInfo is:

include::../structs/VkInstanceCreateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.
  * pname:pApplicationInfo is `NULL` or a pointer to an instance of
    sname:VkApplicationInfo. If not `NULL`, this information helps
    implementations recognize behavior inherent to classes of applications.
    slink:VkApplicationInfo is defined in detail below.
  * pname:enabledLayerCount is the number of global layers to enable.
  * pname:ppEnabledLayerNames is a pointer to an array of
    pname:enabledLayerCount null-terminated UTF-8 strings containing the
    names of layers to enable.
  * pname:enabledExtensionCount is the number of global extensions to
    enable.
  * pname:ppEnabledExtensionNames is a pointer to an array of
    pname:enabledExtensionCount null-terminated UTF-8 strings containing the
    names of extensions to enable.

include::../validity/structs/VkInstanceCreateInfo.txt[]

fname:vkCreateInstance creates the instance, then enables and initializes
global layers and extensions requested by the application. If an extension
is provided by a layer, both the layer and extension must: be specified at
fname:vkCreateInstance time.

The pname:pApplicationInfo member of slink:VkInstanceCreateInfo can: point
to an instance of sname:VkApplicationInfo. This structure is defined as:

include::../structs/VkApplicationInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:pApplicationName is a pointer to a null-terminated UTF-8 string
    containing the name of the application.
  * pname:applicationVersion is an unsigned integer variable containing the
    developer-supplied version number of the application.
  * pname:pEngineName is a pointer to a null-terminated UTF-8 string
    containing the name of the engine (if any) used to create the
    application.
  * pname:engineVersion is an unsigned integer variable containing the
    developer-supplied version number of the engine used to create the
    application.
  * pname:apiVersion is the version of the {apiname} API against which the
    application expects to run, encoded as described in the
    <<fundamentals-versionnum,API Version Numbers and Semantics>> section.
    If pname:apiVersion is 0 the implementation must: ignore it, otherwise
    if the implementation does not support the requested pname:apiVersion
    it must: return VK_ERROR_INCOMPATIBLE_DRIVER.

include::../validity/structs/VkApplicationInfo.txt[]

To destroy an instance, call:

include::../protos/vkDestroyInstance.txt[]

  * pname:instance is the handle of the instance to destroy.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.

include::../validity/protos/vkDestroyInstance.txt[]