The Khronos® Vulkan® API Registry

SPDX-License-Identifier: CC-BY-4.0

1. Introduction

This document describes the Khronos Vulkan API Registry schema, and provides some additional information about using the registry and scripts to generate a variety of outputs, including C header files as well as several types of asciidoc include files used in the Vulkan API specification and reference pages. The underlying XML files and scripts are located on the Khronos public Github server at URL

https://github.com/KhronosGroup/Vulkan-Docs

The authoritative copy of the Registry is maintained in the default branch, currently main.

The registry uses an XML representation of the Vulkan API, together with a set of Python scripts to manipulate the registry once loaded. The scripts rely on the Python etree package to parse and operate on XML. An XML schema and validator target are included.

The schema is based on, but not identical to that used for the previously published OpenGL, OpenGL ES and EGL API registries. It was extended to represent additional types and concepts not needed for those APIs, such as structure and enumerant types, as well as additional types of registered information specific to Vulkan.

The Vulkan C header files generated from the registry are checked into a separate repository under

https://github.com/KhronosGroup/Vulkan-Headers/

1.1. Schema Choices

The XML schema is not pure XML all the way down. In particular, command return types/names and parameters, and structure members, are described in mixed-mode tag containing C declarations of the appropriate information, with some XML nodes annotating particular parts of the declaration such as its base type and name. This choice is based on prior experience with the SGI .spec file format used to describe OpenGL, and greatly eases human reading and writing the XML, and generating C-oriented output. The cost is that people writing output generators for other languages will have to include enough logic to parse the C declarations and extract the relevant information.

People who do not find the supplied Python scripts to suit their needs are likely to write their own parsers, interpreters, and/or converters operating on the registry XML. We hope that we have provided enough information in this document, the RNC schema (registry.rnc), and comments in the Registry (vk.xml) itself to enable such projects. If not and you need clarifications; if you have other problems using the registry; or if you have proposed changes and enhancements, then please file issues on Khronos' public Github project at

https://github.com/KhronosGroup/Vulkan-Docs/issues

Please tag your issues with [Registry] in the subject line to help us categorize them. We have considered separating the registry from the specification source into a separate repository, but since there is so much specification valid usage language imbedded in the registry XML, this is unlikely to happen.

2. Getting Started

See xml/README.adoc in the Vulkan-Docs repository for information on required toolchain components such as Python 3, g++, and GNU make.

Once you have the right tools installed, perform the following steps:

Check out the Vulkan-Docs repository linked above from Khronos Github (there are instructions at the link)
cd to the root directory in your checked-out repo
Switch to the default branch (main).
Invoke make clean ; make install ; make test

This should regenerate vulkan_core.h and a variety of platform-specific headers, install them in ../include/vulkan/, and verify that the headers build properly. If you build at the latest repository tag, the resulting headers should be identical to the latest versions in the Vulkan-Headers repository.

The install target also generates source code for a simple extension loader library in ../src/ext_loader/.

Other Makefile targets in xml/ include:

validate - validate vk.xml against the XML schema. Recommended if you are making nontrivial changes.
The asciidoc includes used by the Specification and Reference Pages are built using the 'make generated' target in the parent directory Makefile, although they use the scripts and XML in this directory. These files are generated dynamically when building the specs, since their contents depend on the exact set of extensions the Specification is being built to include.

If you just want to modify the API, changing vk.xml and running make should be all that is needed. See Examples / FAQ / How Do I? for some examples of modifying the XML.

If you want to use the registry for reasons other than generating the header file, extension loader, and asciidoc includes, or to generate headers for languages other than C, start with the Makefile rules and the files vk.xml and scripts genvk.py, reg.py, and generator.py. The scripts are described below and are all located in the scripts directory under the repository root.

2.1. Header Generation Script - `genvk.py`

When generating header files using the genvk.py script, an API name and profile name are required, as shown in the Makefile examples. Additionally, specific API versions and extensions can be required or excluded. Based on this information, the generator script extracts the relevant interfaces and creates a C-language header file for them. genvk.py contains predefined generator options for the current Vulkan Specification release.

The generator script is intended to be generalizable to other languages by writing new generator classes. Such generators would have to rewrite the C types and definitions in the XML to something appropriate to their language.

2.2. Registry Processing Script - `reg.py`

XML processing is done in reg.py, which contains several objects and methods for loading registries and extracting interfaces and extensions for use in header generation. There is some internal documentation in the form of comments, although nothing more extensive exists yet.

2.3. Output Generator Script - `generator.py`

Once the registry is loaded, the COutputGenerator class defined in generator.py is used to create a header file. The DocOutputGenerator class is used to create the asciidoc include files. Output generators for other purposes can be added as needed. There are a variety of output generators included:

cgenerator.py - generate C header file
docgenerator.py - generate asciidoc includes for APIs
hostsyncgenerator.py - generate host sync table includes for APIs
validitygenerator.py - generate validity language includes
pygenerator.py - generate a Python dictionary-based encoding of portions of the registry, used during spec generation
extensionStubSource.py - generate a simple C extension loader.

3. Vulkan Registry Schema

The format of the Vulkan registry is a top level registry tag containing types, enums, commands, feature, and extension tags describing the different elements of an API, as explained below. This description corresponds to a formal Relax NG schema file, registry.rnc, against which the XML registry files can be validated.

At present the only registry in this schema is the core Vulkan API registry, vk.xml.

3.1. Profiles

Types and enumerants can have different definitions depending on the API profile requested. This capability is not used in the current Vulkan API but may be in the future. Features and extensions can include some elements conditionally depending on the API profile requested.

3.2. API Names

Specific API versions features and extensions can be tagged as belonging to to classes of features with the use of API names. This is intended to allow multiple closely-related API specifications in the same family - such as desktop and mobile specifications - to share the same XML. An API name is an arbitrary alphanumeric string, although it should be chosen to match the corresponding API. For example, Vulkan and OpenXR use vulkan and openxr as their API names, respectively.

The api attribute of the feature tag and the supported attribute of the extensions tag must be comma-separated lists of one or more API names, all of which match that feature or extension. When generating headers and other artifacts from the XML, an API name may be specified to the processing scripts, causing the selection of only those features and extensions whose API names match the specified name.

Several other tags for defining types and groups of types also support api attributes. If present, the attribute value must be a comma-separated list of one or more API names. This allows specializing a definition for different, closely related APIs.

4. Registry Root (`registry` tag)

A registry contains the entire definition of one or more related APIs.

4.1. Attributes of `registry` tags

None.

4.2. Contents of `registry` tags

Zero or more of each of the following tags, normally in this order (although order should not be important):

comment - Contains arbitrary text, such as a copyright statement.
platforms - defines platform names corresponding to platform-specific API extensions.
tags - defines author IDs used for extensions and layers. Author IDs are described in detail in the “Layers & Extensions” section of the “Vulkan Documentation and Extensions: Procedures and Conventions” document.
types - defines API types. Usually only one tag is used.
enums - defines API token names and values. Usually multiple tags are used. Related groups may be tagged as an enumerated type corresponding to a type tag, and resulting in a C enum declaration. This ability is heavily used in the Vulkan API.
commands - defines API commands (functions). Usually only one tag is used.
feature - defines API feature interfaces (API versions, more or less). One tag per feature set.
extensions - defines API extension interfaces. Usually only one tag is used, wrapping many extensions.
formats - defines properties of image formats. Only one tag is used, wrapping all the formats.
spirvextensions - defines relationship between SPIR-V extensions and API interfaces which enable each extension. Only one tag is used, wrapping all the SPIR-V extensions.
spirvcapabilities - defines relationship between SPIR-V capabilities and API interfaces which enable each capability. Only one tag is used, wrapping all the SPIR-V capabilities.

4.2.1. Comment Tags (`comment` tag)

A comment tag contains an arbitrary string, and is unused. Comment tags may appear in multiple places in the schema, as described below. Comment tags are removed by output generators if they would otherwise appear in generated headers, asciidoc include files, etc.

5. Platform Name Blocks (`platforms` tag)

A platforms contains descriptions of platform IDs for platforms supported by window system-specific extensions to Vulkan.

5.1. Attributes of `platforms` tags

comment - optional. Arbitrary string (unused).

5.2. Contents of `platforms` tags

Zero or more platform tags, in arbitrary order (though they are typically ordered by sorting on the platform name).

6. Platform Names (`platform` tag)

A platform tag describes a single platform name.

6.1. Attributes of `platform` tags

name - required. The platform name. This must be a short alphanumeric string corresponding to the platform name, valid as part of a C99 identifier. Lower-case is preferred. In some cases, it may be desirable to distinguish a subset of platform functionality from the entire platform. In these cases, the platform name should begin with the entire platform name, followed by _ and the subset name.

Note

For example,

name="xlib"

is used for the X Window System, Xlib client library platform.

name="xlib_xrandr"

is used for the XRandR functionality within the xlib platform.
protect - required. This must be a C99 preprocessor token beginning with VK_USE_PLATFORM_ followed by the platform name, converted to upper case, followed by _ and the extension suffix of the corresponding window system-specific extension supporting the platform.

Note

For example,

protect="VK_USE_PLATFORM_XLIB_XRANDR_EXT"

is used for the xlib_xrandr platform name.
comment - optional. Arbitrary string (unused).

6.2. Contents of `platform` tags

No contents are allowed. All information is contained in the attributes.

7. Author ID Blocks (`tags` tag)

A tags tag contains authorid tags describing reserved author IDs used by extension and layer authors.

7.1. Attributes of `tags` tags

comment - optional. Arbitrary string (unused).

7.2. Contents of `tags` tags

Zero or more tag tags, in arbitrary order (though they are typically ordered by sorting on the author ID).

8. Author IDs (`tag` tag)

A tag tag contains information defining a single author ID.

8.1. Attributes of `tag` tags

name - required. The author ID, as registered with Khronos. A short, upper-case string, usually an abbreviation of an author, project or company name.
author - required. The author name, such as a full company or project name.
contact - required. The contact who registered or is currently responsible for extensions and layers using the ID, including sufficient contact information to reach the contact such as individual name together with email address, Github username, or other contact information.

8.2. Contents of `tag` tags

No contents are allowed. All information is contained in the attributes.

9. API Type Blocks (`types` tag)

A types tag contains definitions of derived types used in the API.

9.1. Attributes of `types` tags

comment - optional. Arbitrary string (unused).

9.2. Contents of `types` tags

Zero or more type and comment tags, in arbitrary order (though they are typically ordered by putting dependencies of other types earlier in the list). The comment tags are used mostly to indicate grouping of related types.

10. API Type (`type` tag)

A type tag contains information which can be used to generate C code corresponding to the type. In many cases, this is simply legal C code, with attributes or embedded tags denoting the type name and other types used in defining this type. In some cases, additional attribute and embedded type information is used to generate more complicated C types.

10.1. Attributes of `type` tags

requires - optional. Another type name this type requires to complete its definition.
name - optional. Name of this type (if not defined in the tag body).
alias - optional. Another type name which this type is an alias of. Must match the name of another type element. This is typically used when promoting a type defined by an extension to a new core version of the API. The old extension type is still defined, but as an alias of the new type.
api - optional API names for which this definition is specialized, so that different APIs may have different definitions for the same type. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.
category - optional. A string which indicates that this type contains a more complex structured definition. At present the only accepted categories are basetype, bitmask, define, enum, funcpointer, group, handle, include, struct, and union, as described below.
comment - optional. Arbitrary string (unused).
parent - only applicable if "category" is handle. Notes another type with the handle category that acts as a parent object for this type.
returnedonly - only applicable if "category" is struct or union. Notes that this struct/union is going to be filled in by the API, rather than an application filling it out and passing it to the API.
structextends only applicable if category is struct or union. This is a comma-separated list of structures whose pNext can include this type. This should usually only list the top-level structure that is extended, for all possible extending structures. This will generate a validity statement on the top level structure that validates the entire chain in one go, rather than each extending structure repeating the list of valid structs. There is no need to set the noautovalidity attribute on the pNext members of extending structures.
allowduplicate - only applicable if category is "struct". If "true", then structures whose pNext chains include this structure may include more than one instance of it.
objtypeenum - only applicable at present if category is "handle". Specifies the name of a VkObjectType enumerant which corresponds to this type. The enumerant must be defined.

10.2. Contents of `type` tags

The valid contents depend on the category attribute.

10.2.1. Enumerated types - `category` `"enum"`

If the category tag has the value enum, the type is a C enumeration. The body of the tag is ignored in this case. The value of the name attribute must be provided and must match the name attribute of a enums tag. The enumerant values defined within the enums tag are used to generate a C enum type declaration.

10.2.2. Structure types - `category` `"struct"` or `"union"`

If the category tag has the values struct or union, the type is a C structure or union, respectively. In this case, the name attribute must be provided, and the contents of the type tag are a series of member tags defining the members of the aggregate type, in order, interleaved with any number of comment tags.

Structure member (`member`) tags

The member tag defines the type and name of a structure or union member.

Attributes of `member` tags

api - optional API names for which this definition is specialized, so that different APIs may have different definitions for the same type. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.
values - only valid on the sType member of a struct. This is a comma-separated list of enumerant values that are valid for the structure type; usually there is only a single value.
len - if the member is an array, len may be one or more of the following things, separated by commas (one for each array indirection): another member of that struct; "null-terminated" for a string; "1" to indicate it is just a pointer (used for nested pointers); or an equation in math markup for incorporation in the specification (a LaTeX math expression delimited by $‘ a n d ‘$ . The only variables in the equation should be the names of members of the structure.
altlen - if the len attribute is specified, and contains a latexmath: equation, this attribute should be specified with an equivalent equation using only C builtin operators, C math library function names, and variables as allowed for len. It must be a valid C99 expression whose result is equal to len for all possible inputs. It is a comma separated list that has size equal to only the latexmath item count in len list. This attribute is intended to support consumers of the XML who need to generate validation code from the allowed length.
externsync - denotes that the member should be externally synchronized when accessed by Vulkan

optional - optional. A value of "true" or "false" determines whether this member can be omitted by providing NULL (for pointers), VK_NULL_HANDLE (for handles), or 0 (for other scalar types). If the member is a pointer to one of those types, multiple values may be provided, separated by commas - one for each pointer indirection. If not present, the value is assumed to be "false" (the member must not be omitted). Structure members with name pNext must always be specified with optional="true", since there is no requirement that any member of a pNext chain have a following member in the chain.

Note

While the optional attribute can be used for scalar types such as integers, it does not affect the output generators included with the Vulkan Specification. In this case, the attribute serves only as an indicator to human readers of the XML.

selector - optional. If the member is a union, selector identifies another member of the struct that is used to select which of that union’s members are valid.
selection - optional. For a member of a union, selection identifies a value of the selector that indicates this member is valid.
noautovalidity - prevents automatic validity language being generated for the tagged item. Only suppresses item-specific validity - parenting issues etc. are still captured. It must also be used for structures that have no implicit validity when such structure has explicit validity.
limittype - only applicable for members of VkPhysicalDeviceProperties and VkPhysicalDeviceProperties2, their substrucutres, and extensions. Specifies the type of a device limit. This type describes how a value might be compared with the value of a member in order to check whether it fits the limit. Valid values:
- "min" and "max" denote minimum and maximum limits. They may also apply to arrays and VkExtent*D.
- "bitmask" corresponds to bitmasks and VkBool32, where set bits indicate the presence of a capability
- "range" specifies a [min, max] range
- "struct" means that the member’s fields should be compared.
- "noauto" limits cannot be trivially compared. This is the default value, if unspecified.
objecttype - only applicable for members which are uint64_t values representing a Vulkan obejct handle. Specifies the name of another member which must be a VkObjectType or VkDebugReportObjectTypeEXT value specifying the type of object the handle refers to.

Contents of `member` tags

The text elements of a member tag, with all other tags removed, is a legal C declaration of a struct or union member. In addition it may contain several semantic tags:

The type tag is optional. It contains text which is a valid type name found in another type tag, and indicates that this type must be previously defined for the definition of the command to succeed. Builtin C types should not be wrapped in type tags.
The name tag is required, and contains the struct/union member name being described.
The enum tag is optional. It contains text which is a valid enumerant name found in another type tag, and indicates that this enumerant must be previously defined for the definition of the command to succeed. Typically this is used to semantically tag static array lengths.
The comment tag is optional. It contains an arbitrary string (unused).

10.2.3. All other types

If the category attribute is one of basetype, bitmask, define, funcpointer, group, handle or include, or is not specified, type contains text which is legal C code for a type declaration. It may also contain embedded tags:

type - nested type tags contain other type names which are required by the definition of this type.
apientry/ - insert a platform calling convention macro here during header generation, used mostly for function pointer types.
name - contains the name of this type (if not defined in the tag attributes).
bitvalues - contains the name of the enumeration defining flag values for a bitmask type. Ignored for other types.

There is no restriction on which sorts of definitions may be made in a given category, although the contents of tags with category enum, struct or union are interpreted specially as described above.

However, when generating the header, types within each category are grouped together, and categories are generated in the order given by the following list. Therefore, types in a category should correspond to the intended purpose given for that category. If this recommendation is not followed, it is possible that the resulting header file will not compile due to out-of-order type dependencies. The intended purpose of each category is:

include (#include) directives)
define (macro #define directives)
basetype (built-in C language types; scalar API typedefs, such as the definition of VkFlags; and types defined by external APIs, such as an underlying OS or window system
handle (invocations of macros defining scalar types such as VkInstance)
enum (enumeration types and #define for constant values)
group (currently unused)
bitmask (enumeration types whose members are bitmasks)
funcpointer (function pointer typedefs)
struct and union together (struct and union types)

10.3. Example of a `types` tag

<types>
    <type name="stddef">#include &lt;stddef.h&gt;</type>
    <type requires="stddef">typedef ptrdiff_t <name>VKlongint</name>;</type>
    <type name="VkEnum" category="enum"/>
    <type category="struct" name="VkStruct">
        <member><type>VkEnum</type> <name>srcEnum</name></member>
        <member><type>VkEnum</type> <name>dstEnum</name></member>
    </type>
</types>

<enums name="VkEnum" type="enum">
    <enum value="0" name="VK_ENUM_ZERO"/>
    <enum value="42" name="VK_ENUM_FORTY_TWO"/>
</enums>

The VkStruct type is defined to require the types VkEnum and VKlongint as well. If VkStruct is in turn required by a command or another type during header generation, it will result in the following declarations:

#include <stddef.h>
typedef ptrdiff_t VKlongint.

typedef enum {
    VK_ENUM_ZERO = 0,
    VK_ENUM_FORTY_TWO = 42
} VkEnum;

typedef struct {
    VkEnum    dstEnum;
    VkLongint dstVal;
} VkStruct;

Note that the angle brackets around stddef.h are represented as XML entities in the registry. This could also be done using a CDATA block but unless there are many characters requiring special representation in XML, using entities is preferred.

11. Enumerant Blocks (`enums` tag)

The enums tags contain individual enum tags describing each of the token names used in the API. In some cases these correspond to a C enum, and in some cases they are simply compile time constants (e.g. #define).

Note

It would make more sense to call these const or define tags. This is a historical hangover from the OpenGL XML format which this schema was based on.

11.1. Attributes of `enums` tags

name - optional. String naming the C enum type whose members are defined by this enum group. If present, this attribute should match the name attribute of a corresponding type tag.
type - optional. String describing the data type of the values of this group of enums. At present the only accepted categories are enum and bitmask, as described below.
start, end - optional. Integers defining the start and end of a reserved range of enumerants for a particular vendor or purpose. start must be less than or equal to end. These fields define formal enumerant allocations, and are made by the Khronos Registrar on request from implementors following the enum allocation policy.
vendor - optional. String describing the vendor or purpose to whom a reserved range of enumerants is allocated.
comment - optional. Arbitrary string (unused).
bitwidth - optional. Bit width required for the generated enum value type. If omitted, a default value of 32 is used.

11.2. Contents of `enums` tags

Each enums block contains zero or more enum, unused, and comment tags, in arbitrary order (although they are typically ordered by sorting on enumerant values, to improve human readability).

11.3. Example of `enums` tags

An example showing a tag with attribute type`="enum"` is given above. The following example is for non-enumerated tokens.

<enums>
    <enum value="256" name="VK_MAX_EXTENSION_NAME"/>
    <enum value="MAX_FLOAT"  name="VK_LOD_CLAMP_NONE"/>
</enums>

When processed into a C header, and assuming all these tokens were required, this results in

#define VK_MAX_EXTENSION_NAME   256
#define VK_LOD_CLAMP_NONE       MAX_FLOAT

12. Enumerants (`enum` tag)

Each enum tag defines a single Vulkan (or other API) token.

12.1. Attributes of `enum` tags

value is a numeric value in the form of a legal C expression when evaluated at compile time in the generated header files. This is usually either a literal integer value or the name of an alias for a previously defined value, though more complex expressions are sometimes employed for compile time constants.
bitpos is a literal integer bit position in a bitmask. The bit position must be in the range [0,30] when used as a flag bit in a Vk*FlagBits data type. Bit positions 31 and up may be used for values that are not flag bits, or for flag bits used with 64-bit flag types. Exactly one of value and bitpos must be present in an enum tag.
name - required. Enumerant name, a legal C preprocessor token name.
api - optional API names for which this definition is specialized, so that different APIs may have different values for the same token. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.
type - may be used only when value is specified. In this case, type is optional except when defining a compile time constant, in which case it is required when using some output generator paths. If present the attribute must be a C scalar type corresponding to the type of value, although only uint32_t, uint64_t, and float are currently meaningful. type is used by some output generators to generate constant declarations, although the default behavior is to use C #define for compile-time constants.
alias - optional. Name of another enumerant this is an alias of, used where token names have been changed as a result of profile changes or for consistency purposes. An enumerant alias is simply a different name for the exact same value or bitpos.
protect - optional. An additional preprocessor token used to protect an enum definition.

Note

In older versions of the schema, type was described as allowing only the C integer suffix types u and ull, which is inconsistent with the current definition. However, type was not actually used in the registry processing scripts or vk.xml at the time the current definition was introduced, so this is expected to be a benign change.

12.2. Contents of `enum` tags

enum tags have no allowed contents. All information is contained in the attributes.

13. Unused Enumerants (`unused` tag)

Each unused tag defines a range of enumerants which is allocated, but not yet assigned to specific enums. This just tracks the unused values for the Registrar’s use, and is not used for header generation.

Note

unused tags could be generated and inserted automatically, which would be a good way to avoid the attributes becoming out of date. However, they are rarely used in the Vulkan XML schema, unlike the OpenGL XML schema it was based on.

13.1. Attributes of `unused` tags

start - required, end - optional. Integers defining the start and end of an unused range of enumerants. start must be ≤ end. If end is not present, then start defines a single unused enumerant. This range should not exceed the range reserved by the surrounding enums tag.
vendor - optional. String describing the vendor or purposes to whom a reserved range of enumerants is allocated. Usually identical to the vendor attribute of the surrounding enums block.
comment - optional. Arbitrary string (unused).

13.2. Contents of `unused` tags

None.

14. Command Blocks (`commands` tag)

The commands tag contains definitions of each of the functions (commands) used in the API.

14.1. Attributes of `commands` tags

comment - optional. Arbitrary string (unused).

14.2. Contents of `commands` tags

Each commands block contains zero or more command tags, in arbitrary order (although they are typically ordered by sorting on the command name, to improve human readability).

15. Commands (`command` tag)

The command tag contains a structured definition of a single API command (function).

15.1. Attributes of `command` tags

There are two ways to define a command. The first uses a set of attributes to the command tag defining properties of the command used for constructing automatic validation rules, and the contents of the command tag define the name, signature, and parameters of the command. In this case the allowed attributes include:

queues - optional. A string identifying the command queues this command can be placed on. The format of the string is one or more of the terms "compute", "transfer", and "graphics", with multiple terms separated by commas (",").
successcodes - optional. A string describing possible successful return codes from the command, as a comma-separated list of Vulkan result code names.
errorcodes - optional. A string describing possible error return codes from the command, as a comma-separated list of Vulkan result code names.
renderpass - optional. A string identifying whether the command can be issued only inside a render pass ("inside"), only outside a render pass ("outside"), or both ("both").
cmdbufferlevel - optional. A string identifying the command buffer levels that this command can be called by. The format of the string is one or more of the terms "primary" and "secondary", with multiple terms separated by commas (",").
comment - optional. Arbitrary string (unused).

The second way of defining a command is as an alias of another command. For example when an extension is promoted from extension to core status, the commands defined by that extensions become aliases of the corresponding new core commands. In this case, only two attributes are allowed:

name - required. A string naming the command defined by the tag.
alias - required. A string naming the command that name is an alias of. The string must be the same as the name value of another command defining another command.

Both forms of command support these options:

api - optional API names for which this definition is specialized, so that different APIs may have different values for the same token. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.

15.2. Contents of `command` tags

proto is required and must be the first element. It is a tag defining the C function prototype of a command as described below, up to the function name and return type but not including function parameters.
param elements for each command parameter follow, defining its name and type, as described below. If a command takes no arguments, it has no param tags.

Following these elements, the remaining elements in a command tag are optional and may be in any order:

alias - optional. Has no attributes and contains a string which is the name of another command this command is an alias of, used when promoting a function from vendor to Khronos extension or Khronos extension to core API status. A command alias describes the case where there are two function names which implement the same behavior.
description - optional. Unused text.
implicitexternsyncparams - optional. Contains a list of param tags, each containing asciidoc source text describing an object which is not a parameter of the command but is related to one, and which also requires external synchronization. The text is intended to be incorporated into the API specification.

Note

Versions of the registry documentation prior to 1.1.93 asserted that command aliases “resolve to the same entry point in the underlying layer stack.” Whilst this may be true on many implementations, it is not required - each command alias must be queried separately through vkGetInstanceProcAddr or vkGetDeviceProcAddr.

15.3. Command prototype (`proto` tags)

The proto tag defines the return type and name of a command.

15.3.1. Attributes of `proto` tags

None.

15.3.2. Contents of `proto` tags

The text elements of a proto tag, with all other tags removed, is legal C code describing the return type and name of a command. In addition to text, it may contain two semantic tags:

The type tag is optional, and contains text which is a valid type name found in a type tag. It indicates that this type must be previously defined for the definition of the command to succeed. Builtin C types, and any derived types which are expected to be found in other header files, should not be wrapped in type tags.
The name tag is required, and contains the command name being described.

15.4. Command parameter (`param` tags)

The param tag defines the type and name of a parameter. Its contents are very similar to the member tag used to define struct and union members.

15.4.1. Attributes of `param` tags

api - optional API names for which this definition is specialized, so that different APIs may have different definitions for the same type. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.
len - if the param is an array, len may be one or more of the following things, separated by commas (one for each array indirection): another param of that command; "null-terminated" for a string; "1" to indicate it is just a pointer (used for nested pointers); or an equation in math markup for incorporation in the specification (a LaTeX math expression delimited by $‘ a n d ‘$ . The only variables in the equation should be the names of this or other parameters.
altlen - if the len attribute is specified, and contains a latexmath: equation, this attribute should be specified with an equivalent equation using only C builtin operators, C math library function names, and variables as allowed for len. It must be a valid C99 expression whose result is equal to len for all possible inputs. It is a comma separated list that has size equal to only the latexmath item count in len list. This attribute is intended to support consumers of the XML who need to generate validation code from the allowed length.

optional - optional. A value of "true" or "false" determines whether this parameter can be omitted by providing NULL (for pointers), VK_NULL_HANDLE (for handles), or 0 (for other scalar types). If the parameter is a pointer to one of those types, multiple values may be provided, separated by commas - one for each pointer indirection. If not present, the value is assumed to be "false" (the parameter must not be omitted).

Note

selector - optional. If the parameter is a union, selector identifies another parameter of the command that is used to select which of that union’s members are valid.
noautovalidity - prevents automatic validity language being generated for the tagged item. Only suppresses item-specific validity - parenting issues etc. are still captured.
externsync - optional. A value of "true" indicates that this parameter (e.g. the object a handle refers to, or the contents of an array a pointer refers to) is modified by the command, and is not protected against modification in multiple app threads. If only certain members of an object or elements of an array are modified, multiple strings may be provided, separated by commas. Each string describes a member which is modified. For example, the vkQueueSubmit command includes externsync attributes for the pSubmits array indicating that only specific members of each element of the array are modified:
<param len="submitCount" externsync="pSubmits[].pWaitSemaphores[],pSubmits[].pSignalSemaphores[]">const <type>VkSubmitInfo</type>* <name>pSubmits</name></param>
Parameters which do not have an externsync attribute are assumed to not require external synchronization.
objecttype - only applicable for parameters which are uint64_t values representing a Vulkan obejct handle. Specifies the name of another parameter which must be a VkObjectType or VkDebugReportObjectTypeEXT value specifying the type of object the handle refers to.

15.4.2. Contents of `param` tags

The text elements of a param tag, with all other tags removed, is legal C code describing the type and name of a function parameter. In addition it may contain two semantic tags:

The type tag is optional, and contains text which is a valid type name found in type tag, and indicates that this type must be previously defined for the definition of the command to succeed. Builtin C types, and any derived types which are expected to be found in other header files, should not be wrapped in type tags.
The name tag is required, and contains the parameter name being described.

15.5. Example of a `commands` tag

<commands>
    <command>
        <proto><type>VkResult</type> <name>vkCreateInstance</name></proto>
        <param>const <type>VkInstanceCreateInfo</type>* <name>pCreateInfo</name></param>
        <param><type>VkInstance</type>* <name>pInstance</name></param>
    </command>
</commands>

When processed into a C header, this results in

VkResult vkCreateInstance(
    const VkInstanceCreateInfo* pCreateInfo,
    VkInstance* pInstance);

16. API Features and Versions (`feature` tag)

API features are described in individual feature tags. A feature is the set of interfaces (enumerants and commands) defined by a particular API and version, such as Vulkan 1.0, and includes all profiles of that API and version.

16.1. Attributes of `feature` tags

api - required API names this feature is defined for, such as vulkan.
name - required. Version name, used as the C preprocessor token under which the version’s interfaces are protected against multiple inclusion. Example: "VK_VERSION_1_0".
number - required. Feature version number, usually a string interpreted as majorNumber.minorNumber. Example: 4.2.
sortorder - optional. A decimal number which specifies an order relative to other feature tags when calling output generators. Defaults to 0. Rarely used, for when ordering by name is insufficient.
protect - optional. An additional preprocessor token used to protect a feature definition. Usually another feature or extension name. Rarely used, for odd circumstances where the definition of a feature or extension requires another to be defined first.
comment - optional. Arbitrary string (unused).

Note

The name attribute used for Vulkan core versions, such as "VK_VERSION_1_0", is not an API construct. It is used only as a preprocessor guard in the headers, and an asciidoctor conditional in the specification sources. The similar "VK_API_VERSION_1_0" symbols are part of the API and their values are packed integers containing Vulkan core version numbers.

16.2. Contents of `feature` tags

Zero or more require and remove tags, in arbitrary order. Each tag describes a set of interfaces that is respectively required for, or removed from, this feature, as described below.

16.3. Example of a `feature` tag

<feature api="vulkan" name="VK_VERSION_1_0" number="1.0">
    <require comment="Header boilerplate">
        <type name="vk_platform"/>
    </require>
    <require comment="API constants">
        <enum name="VK_MAX_PHYSICAL_DEVICE_NAME"/>
        <enum name="VK_LOD_CLAMP_NONE"/>
    </require>
    <require comment="Device initialization">
        <command name="vkCreateInstance"/>
    </require>
</feature>

When processed into a C header for Vulkan, this results in:

#ifndef VK_VERSION_1_0
#define VK_VERSION_1_0 1
#define VK_MAX_EXTENSION_NAME   256
#define VK_LOD_CLAMP_NONE       MAX_FLOAT
typedef VkResult (VKAPI_PTR *PFN_vkCreateInstance)(const VkInstanceCreateInfo* pCreateInfo, VkInstance* pInstance);
#ifndef VK_NO_PROTOTYPES
VKAPI_ATTR VkResult VKAPI_CALL vkCreateInstance(
    const VkInstanceCreateInfo*                 pCreateInfo,
    VkInstance*                                 pInstance);
#endif
#endif /* VK_VERSION_1_0 */

17. Extension Blocks (`extensions` tag)

The extensions tag contains definitions of each of the extensions which are defined for the API.

17.1. Attributes of `extensions` tags

comment - optional. Arbitrary string (unused).

17.2. Contents of `extensions` tags

Each extensions block contains zero or more extension tags, each describing an API extension, in arbitrary order (although they are typically ordered by sorting on the extension name, to improve human readability).

18. API Extensions (`extension` tag)

API extensions are described in individual extension tags. An extension is the set of interfaces defined by a particular API extension specification, such as ARB_multitexture. extension is similar to feature, but instead of having an number attribute, it instead has a supported attribute, which describes the set of API names which the extension can potentially be implemented against.

18.1. Attributes of `extension` tags

name - required. Extension name, following the conventions in the Vulkan Specification. Example: name="VK_VERSION_1_0".
number - required. A decimal number which is the registered, unique extension number for name.
sortorder - optional. A decimal number which specifies an order relative to other extension tags when calling output generators. Defaults to 0. Rarely used, for when ordering by number is insufficient.
author - optional. The author name, such as a full company name. If not present, this can be taken from the corresponding tag attribute. However, EXT and other multi-vendor extensions may not have a well-defined author or contact in the tag. This attribute is not used in processing the XML. It is just metadata, mostly used to track the original author of an extension (which may have since been promoted to use a different author ID).
contact - optional. The contact who registered or is currently responsible for extensions and layers using the tag, including sufficient contact information to reach the contact such as individual name together with Github username (@username), Khronos internal Gitlab username (gitlab:@username) if no public Github contact is available, or other contact information. If not present, this can be taken from the corresponding tag attribute just like author.
type - required if the supported attribute is not 'disabled'. Must be either 'device' or 'instance', if present.
requires - optional. Comma-separated list of extension names this extension requires to be supported. Extensions whose type is 'instance' must not require extensions whose type is 'device'.
requiresCore - optional. Core version of Vulkan required by the extension, e.g. "1.1". Defaults to "1.0".
protect - optional. An additional preprocessor token used to protect an extension definition. Usually another feature or extension name. Rarely used, for odd circumstances where the definition of an extension requires another extension or a header file to be defined first.
platform - optional. Indicates that the extension is specific to the platform identified by the attribute value, and should be emitted conditional on that platform being available, in a platform-specific header, etc. The attribute value must be the same as one of the platform name attribute values.
supported - required API names this extension is defined for. When the extension tag is just reserving an extension number, use supported="disabled" to indicate this extension should never be processed. Interfaces defined in a disabled extension block are tentative at best and must not be generated or otherwise used by scripts processing the XML. The only exception to this rule is for scripts used solely for reserving, or checking for reserved bitflag values.
promotedto - optional. A Vulkan version or a name of an extension that this extension was promoted to. E.g. "VK_VERSION_1_1", or "VK_KHR_draw_indirect_count".
deprecatedby - optional. A Vulkan version or a name of an extension that deprecates this extension. It may be an empty string. E.g. "VK_VERSION_1_1", or "VK_EXT_debug_utils", or "".
obsoletedby - optional. A Vulkan version or a name of an extension that obsoletes this extension. It may be an empty string. E.g. "VK_VERSION_1_1", or "VK_KHR_maintenance1", or "".
provisional - optional. 'true' if this extension is released provisionally.
specialuse - optional. If present, must contain one or more tokens separated by commas, indicating a special purpose of the extension. Tokens may include:
- 'cadsupport' - for support of CAD software.
- 'd3demulation' - for support of Direct3D emulation layers or libraries, or applications porting from Direct3D.
- 'debugging' - for debugging an application.
- 'devtools' - for support of developer tools, such as capture-replay libraries.
- 'glemulation' - for support of OpenGL and/or OpenGL ES emulation layers or libraries, or applications porting from those APIs.
comment - optional. Arbitrary string (unused).

Note

The requires attribute is used to specify other extensions that must be enabled for an extension to be enabled.

In some cases, an extension may include functionality which is only defined if another extension is enabled. Such functionality should be specified within a require, using the extension attribute to specify that extension.

18.2. Contents of `extension` tags

Zero or more require and remove tags, in arbitrary order. Each tag describes a set of interfaces that is respectively required for, or removed from, this extension, as described below.

18.3. Example of an `extensions` tag

<extension name="VK_KHR_display_swapchain" number="4" supported="vulkan">
    <require>
        <enum value="9" name="VK_KHR_DISPLAY_SWAPCHAIN_SPEC_VERSION"/>
        <enum value="4" name="VK_KHR_DISPLAY_SWAPCHAIN_EXTENSION_NUMBER"/>
        <enum value="&quot;VK_KHR_display_swapchain&quot;"
              name="VK_KHR_DISPLAY_SWAPCHAIN_EXTENSION_NAME"/>
        <type name="VkDisplayPresentInfoKHR"/>
        <command name="vkCreateSharedSwapchainsKHR"/>
    </require>
</extension>

The supported attribute says that the extension is defined for the default profile (vulkan). When processed into a C header for the vulkan profile, this results in header contents something like (assuming corresponding definitions of the specified type and command elsewhere in the XML):

#define VK_KHR_display_swapchain 1
#define VK_KHR_DISPLAY_SWAPCHAIN_SPEC_VERSION 9
#define VK_KHR_DISPLAY_SWAPCHAIN_EXTENSION_NUMBER 4
#define VK_KHR_DISPLAY_SWAPCHAIN_EXTENSION_NAME "VK_KHR_display_swapchain"

typedef struct VkDisplayPresentInfoKHR {
    VkStructureType                             sType;
    const void*                                 pNext;
    VkRect2D                                    srcRect;
    VkRect2D                                    dstRect;
    VkBool32                                    persistent;
} VkDisplayPresentInfoKHR;

typedef VkResult (VKAPI_PTR *PFN_vkCreateSharedSwapchainsKHR)(
    VkDevice device, uint32_t swapchainCount,
    const VkSwapchainCreateInfoKHR* pCreateInfos,
    const VkAllocationCallbacks* pAllocator,
    VkSwapchainKHR* pSwapchains);

#ifndef VK_NO_PROTOTYPES
VKAPI_ATTR VkResult VKAPI_CALL vkCreateSharedSwapchainsKHR(
    VkDevice                                    device,
    uint32_t                                    swapchainCount,
    const VkSwapchainCreateInfoKHR*             pCreateInfos,
    const VkAllocationCallbacks*                pAllocator,
    VkSwapchainKHR*                             pSwapchains);
#endif

19. Required and Removed Interfaces (`require` and `remove` tags)

A require block defines a set of interfaces (types, enumerants and commands) 'required' by a feature or extension. A remove block defines a set of interfaces 'removed' by a feature. This is primarily for future profiles of an API which may choose to deprecate and/or remove some interfaces. Extensions should never remove interfaces, although this usage is allowed by the schema). Except for the tag name and behavior, the contents of require and remove tags are identical.

19.1. Attributes of `require` and `remove` tags

profile - optional. String name of an API profile. Interfaces in the tag are only required (or removed) if the specified profile is being generated. If not specified, interfaces are required (or removed) for all API profiles.
comment - optional. Arbitrary string (unused).
api - optional API names requiring or removing these interfaces. Interfaces in the tag are only required (or removed) if the requested API name matches the attribute. If not specified, interfaces are required (or removed) for all APIs.

Note

The api attribute is only supported inside extension tags, since feature tags already define a specific API.

19.2. Attributes of `require` tags

These attribues are allowed only for a require tag.

extension - optional, and only for require tags. String containing an API extension name. Interfaces in the tag are only required if the string matches the name of an extension tag, and that extension is enabled.
feature - optional, and only for require tags. String containing an API feature name. Interfaces in the tag are only required if the string matches the name of a feature tag, and that feature is enabled.

Note

The extension attribute currently does not affect output generators in any way, and is simply metadata. This will be addressed as we better define different types of dependencies between extensions.

19.3. Contents of `require` and `remove` tags

Zero or more of the following tags, in any order:

19.3.1. Comment Tags

comment (as described above).

19.3.2. Command Tags

command specifies an required (or removed) command defined in a commands block. The tag has no content, but contains attributes:

name - required. Name of the command.
comment - optional. Arbitrary string (unused).

19.3.3. Enum tags

enum specifies an required (or removed) enumerant defined in a enums block. All forms of this tag support the following attributes:

name - required. Name of the enumerant.
comment - optional. Arbitrary string (unused).
api - optional API names for which this definition is specialized, so that different APIs may have different values for the same token. This definition is only used if the requested API name matches the attribute. May be used to address subtle incompatibilities.

There are two forms of enum tags:

Reference enums simply pull in the definition of an enumerant given in a separate enums block. No attributes other than name and comment are supported for them. enum tags appearing inside remove tags should always be reference enums. Reference enums may also be used inside require tags, if the corresponding value is defined in a enums block. This is typically used for constants not part of an enumerated type.

Extension enums define the value of an enumerant inline in an feature or extensions block. Typically these are used to add additional values specified by an extension or core feature to an existing enumerated type. There are a variety of attributes which are used to specify the value of the enumerant:

value and type - define a constant value in the same fashion as an enum tag in an enums block.
bitpos - define a constant bitmask value in the same fashion as an enum tag in an enums block. bitpos is a literal integer bit position in a bitmask. The same value and usage constraints apply to this bit position as are applied to the <<tag-enum, bitpos attribute of an enum tag.
extends - the name of a separately defined enumerated type (e.g. a type tag with category="enum") to which the extension enumerant is added. The enumerated type is required to complete the definition of the enumerant, in the same fashion as the requires attribute of a type tag. If not present, the enumerant is treated as a global constant value.
extnumber - an extension number. The extension number in turn specifies the starting value of a block (range) of values reserved for enumerants defined by or associated with the corresponding extension tag with the same number. This is used when an extension or core feature needs to extend an enumerated type in a block defined by a different extension.
Attribute offset - the offset within an extension block. If extnumber is not present, the extension number defining that block is given by the number attribute of the surrounding extension tag. The actual numeric value of the enumerant is computed as defined in the “Layers and Extensions” appendix of the Vulkan Specification.
Attribute dir - if present, the calculated enumerant value will be negative, instead of positive. Negative enumerant values are normally used only for Vulkan error codes. The attribute value must be specified as dir="-".
alias - the name of another enumerant this is an alias of. An enumerant alias is simply a different name for the same enumerant value. This is typically used when promoting an enumerant defined by an extension to a new core version of the API. The old extension enumerant is still defined, but as an alias of the new core enumerant. It may also be used when token names have been changed as a result of profile changes, or for consistency purposes.
protect - define a preprocessor protection symbol for the enum in the same fashion as a enum tag in an enums block.

Not all combinations of attributes are either meaningful or supported. The protect attribute may always be present. For other attributes, the allowed combinations are:

Table 1. Valid Combinations of `enum` Attributes for Extension Enums
`value`	`bitpos`	`alias`	`offset`	`extnumber`	`dir`	`extends`	Description
{yes}	{no}	{no}	{no}	{no}	{no}	{opt}²	Numeric value
{no}	{yes}	{no}	{no}	{no}	{no}	{opt}²	Bitmask value
{no}	{no}	{yes}	{no}	{no}	{no}	{opt}²	Alias of another enumerant
{no}	{no}	{no}	{yes}	{opt}¹	{opt}	{yes}	Value added to an enumeration
{no}	{no}	{no}	{yes}	{opt}¹	{opt}	{yes}	Value added to an enumeration

[1]: If extnumber is not present, the enum tag may only be within a extension. Otherwise, the enum tag may also be within a feature.

[2]: If extends is not present, the enumerant value is a global constant. Otherwise, the value is added to the specified enumeration.

Examples of various types of extension enumerants are given below.

19.3.4. Type tags

type specifies a required (or removed) type defined in a types block. Most types are picked up implicitly by using the type tags of commands, but in a few cases, additional types need to be specified explicitly. It is unlikely that a type would ever be removed, although this usage is allowed by the schema. The tag has no content, but contains elements:

name - required. Name of the type.
comment - optional. Arbitrary string (unused).

19.4. Examples of Extension Enumerants

Examples of some of the supported extension enumerant enum tags are given below.

<extensions>
    <extension name="VK_KHR_test_extension" number="1" supported="vulkan">
        <require>
            <enum value="42" name="VK_KHR_TEST_ANSWER"/>
            <enum bitpos="29" name="VK_KHR_TEST_BITMASK"/>
            <enum offset="0" dir="-" extends="VkResult"
                  name="VK_ERROR_SURFACE_LOST_KHR"/>
            <enum offset="1" extends="VkResult"
                  name="VK_SUBOPTIMAL_KHR"/>
            <enum bitpos="30" extends="VkCullModeFlagBits"
                  name="VK_KHR_TEST_CULL_MODE_BIT"/>
        </require>
    </extension>
</extensions>

The corresponding header file will include definitions like this:

typedef enum VkResult {
    <previously defined VkResult enumerant values},
    VK_ERROR_SURFACE_LOST_KHR = -1000000000,
    VK_SUBOPTIMAL_KHR = 1000000001,
    VK_KHR_EXTENSION_BIT = 0x80000000,
};

#define VK_KHR_test_extension 1
#define VK_KHR_theanswer 42
#define VK_KHR_bitmask 0x20000000

20. Formats (`formats` tag)

The formats tag contains definitions of each of the image formats which are defined for the API.

20.1. Attributes of `formats` tags

None.

20.2. Contents of `formats` tags

A formats block contains zero or more format tags, each describing an image format, in arbitrary order.

21. Image Format (`format` tag)

Image formats are described in individual format tags. An image format corresponds to a Vulkan VkFormat enumerant. This tag contains information specifying the structure and meaning of different parts of the format. The meaning of different parts of the format information is described in more detail in the “Format Definition” section of the Vulkan API Specification.

21.1. Attributes of `format` tags

name - required. Format name, matching a VkFormat enum name. Example: name="VK_FORMAT_R8_UNORM".
class - required. Format class. A string whose value is shared by a group of formats which may be compatible, and is a textual description of something important that group has in common. Example: class="8-bit".
blockSize - required. A decimal integer which is the texel block size, in bytes, of the format.
texelsPerBlock - required A decimal integer which is the number of texels in a texel block of the format.
blockExtent - optional. Three-dimensional extent of a texel block. A comma-separated list of three decimal integers. If not present, blockExtent="1,1,1" is assumed.
packed - optional. A decimal integer which is the number of bits into which the format is packed. If defined, a single image element in this format can be stored in the same space as a scalar type of this bit width.
compressed - optional. A string whose value is shared by a group of formats which use the same general texture compression scheme, and is a textual description of that scheme. Example: compressed="ASTC LDR".
chroma - optional. A string used to mark if Y′C_BC_R samplers are required by default when using this format. Must be one of the three values "420", "422", or "444" corresponding to different Y′C_BC_R encodings.

21.2. Contents of `format` tags

One or more component tags. The order of component tags corresponds to the memory order of components of the format. Each tag describes the size and format of that component.

Zero or more plane tags, in arbitrary order. Each tag describes the scale of a specific image plane of the format relative to the overall format.

Zero or more spirvimageformat tags, in arbitrary order. Each tag describes a SPIR-V format name corresponding to the format.

21.3. Example of a `format` tag

<format name="VK_FORMAT_G16_B16R16_2PLANE_420_UNORM" class="16-bit 2-plane 420" blockSize="6" texelsPerBlock="1" chroma="420">
    <component name="G" bits="16" numericFormat="UNORM" planeIndex="0"/>
    <component name="B" bits="16" numericFormat="UNORM" planeIndex="1"/>
    <component name="R" bits="16" numericFormat="UNORM" planeIndex="1"/>
    <plane index="0" widthDivisor="1" heightDivisor="1" compatible="VK_FORMAT_R16_UNORM"/>
    <plane index="1" widthDivisor="2" heightDivisor="2" compatible="VK_FORMAT_R16G16_UNORM"/>
</format>

22. Format Components (`component` tag)

The component tag contains definitions of each of the components which are part of an image format.

22.1. Attributes of `component` tags

name - required. A string specifying the name of this component. Must be one of the values "R", "G", "B", "A", "S", or "D" corresponding to red, green, blue, alpha, depth, and stencil components, respectively.
bits - required. Must be either a decimal integer which is the number of bits in this component, or "compressed", corresponding to a specific compression scheme.
numericFormat - required. A string specifying the scalar data type of the component. Must be one of the following values:
- "SFLOAT" - signed floating-point numbers
- "SINT" - signed integer values in the range [-2^n-1,2^n-1-1]
- "SNORM" - signed normalized values in the range [-1,1]
- "SRGB" - R, G, and B components are unsigned normalized values that represent values using sRGB nonlinear encoding, while the A component (if one exists) is a regular unsigned normalized value
- "SSCALED" - signed integer values that get converted to floating-point in the range [-2^n-1,2^n-1-1]
- "UFLOAT" - unsigned floating-point numbers (used by packed, shared exponent, and some compressed formats)
- "UINT" - unsigned integer values in the range [0,2ⁿ-1]
- "UNORM" - unsigned normalized values in the range [0,1]
- "USCALED" - unsigned integer values that get converted to floating-point in the range [0,2ⁿ-1]
planeIndex - optional. A decimal integer specifying which plane this component lies in. If present, must correspond to the index attribute value of a plane tag for the same component.

22.2. Contents of `component` tags

None.

23. Format Planes (`plane` tag)

The plane tag contains definitions of each of the image planes which are part of an image format.

23.1. Attributes of `plane` tags

index - required. An integer specifying the image plane being defined. Image planes are in the range [0,p-1] where p is the number of planes in the format.
widthDivisor - required. An integer specifying the relative width of this plane. A value of k means that this plane is 1/k the width of the overall format.
heightDivisor - required. An integer specifying the relative height of this plane. A value of k means that this plane is 1/k the height of the overall format.
compatible - required. A string naming another, single-plane format that this plane is compatible with. Must match the name of another format.

23.2. Contents of `plane` tags

None.

24. SPIR-V Image Formats (`spirvimageformat` tag)

The spirvimageformat tag specifies the name of a SPIR-V image format equivalent to this format.

24.1. Attributes of `spirvimageformat` tags

name - required. The name of the SPIR-V image format. Example: name="R11fG11fB10f".

24.2. Contents of `spirvimageformat` tags

None.

25. SPIR-V Extensions (`spirvextensions` tag)

The spirvextensions tag contains definitions of each of the SPIR-V extensions which are defined for the API.

25.1. Attributes of `spirvextensions` tags

comment - optional. Arbitrary string (unused).

25.2. Contents of `spirvextensions` tags

Each spirvextensions block contains zero or more spirvextension tags, each describing an single SPIR-V extension, in arbitrary order.

26. SPIR-V Extension (`spirvextension` tag)

SPIR-V extensions are described in individual spirvextension tags. A SPIR-V extension is enabled by API versions or extensions.

26.1. Attributes of `spirvextension` tags

name - required. SPIR-V extension name. Example: name="SPV_KHR_variable_pointers"

26.2. Contents of `spirvextension` tags

One or more enable tags, in arbitrary order. Each tag describes a single enabling mechanism for the extension.

26.3. Example of a `spirvextensions` tag

<spirvextension name="SPV_KHR_multiview">
    <enable version="VK_VERSION_1_1"/>
    <enable extension="VK_KHR_multiview"/>
</spirvextension>

27. SPIR-V Capabilities (`spirvcapabilities` tag)

The spirvcapabilities tag contains definitions of each of the SPIR-V capabilities which are defined for the API.

27.1. Attributes of `spirvcapabilities` tags

comment - optional. Arbitrary string (unused).

27.2. Contents of `spirvcapabilities` tags

Each spirvcapabilities block contains zero or more spirvcapability tags, each describing an single SPIR-V capability, in arbitrary order.

28. SPIR-V Capability (`spirvcapability` tag)

SPIR-V capabilities are described in individual spirvcapability tags. A SPIR-V capability is enabled by API versions, extensions, features, or properties.

28.1. Attributes of `spirvcapability` tags

name - required. SPIR-V capability name. Example: name="SPV_KHR_variable_pointers"

28.2. Contents of `spirvcapability` tags

One or more enable tags, in arbitrary order. Each tag describes a single enabling mechanism for the capability.

28.3. Example of a `spirvcapabilities` tag

<spirvcapability name="ImageCubeArray">
    <enable struct="VkPhysicalDeviceFeatures" feature="imageCubeArray" requires="VK_VERSION_1_0"/>
</spirvcapability>
<spirvcapability name="GroupNonUniform">
    <enable property="VkPhysicalDeviceVulkan11Properties" member="subgroupSupportedOperations" value="VK_SUBGROUP_FEATURE_BASIC_BIT" requires="VK_VERSION_1_1"/>
</spirvcapability>

29. SPIR-V Enables (`enable` tag)

The enable tag describes a single mechanism in the API which enables a spirvextension or spirvcapability. There are four forms of enable tags corresponding to different ways the API may advertise enablement, though not all forms may be used with spirvextension tags. Each form is described separately below.

29.1. Attributes for API Version Number Enables

version - required. An API feature name, matching a feature name attribute value.

If the API version is supported, the SPIR-V extension or capability is enabled.

29.2. Attributes for API Extension Enables

extension - required. An API extension name, matching an extension name attribute value.

If the API extension is supported and enabled, the SPIR-V extension or capability is enabled.

29.3. Attributes for API Feature Enables

struct - required. An API feature structure name, matching a struct name attribute value.
feature - required. An API feature name, matching a member name value of the feature structure.
requires - required. A comma-separated list of API feature version numbers and/or extension names.
alias - optional. Another API feature name which is an alias of feature. Needed when the same feature is provided by two different API versions or extensions.

If one of the API feature version numbers or extensions in the requires list is supported or enabled, respectively; and if the feature name is enabled in the feature structure, the SPIR-V capability is enabled.

API feature enables are not supported for spirvextension tags.

29.4. Attributes for API Property Enables

property - required. An API property structure name, matching a struct name attribute value.
member - required. An API property name, matching a member name value of the property structure.
value - required. A value, matching an API enum name value. If the property is a bitfield, value must be a bitmask value belonging to the member bitfield type. Otherwise, value must be an enum name defined for the member enumeration type.
requires - optional. A comma-separated list of API feature version numbers and/or extension names.

If one of the API feature version numbers or extensions in the requires list is supported or enabled, respectively; and if the member property contains the value bit, or matches the value, the SPIR-V capability is enabled.

API property enables are not supported for spirvextension tags.

29.5. Contents of `enable` tags

None.

30. Examples / FAQ / How Do I?

For people new to the Registry, it will not be immediately obvious how to make changes. This section includes some tips and examples that will help you make changes to the Vulkan headers by changing the Registry XML description.

First, follow the steps described to get the Vulkan Github repository containing the registry and assemble the tools necessary to work with the XML registry. Once you are able to regenerate the Vulkan headers from vk.xml, you can start making changes.

30.1. General Strategy

If you are adding to the API, perform the following steps to create the description of that API element:

For each type, enum group, compile time constant, and command being added, create appropriate new type, enums, enum, or command tags defining the interface in question.
Make sure that all added types and commands appropriately tag their dependencies on other types by adding nested type tags.
Make sure that each new tag defines the name of the corresponding type, enum group, constant, or command, and that structure/union types and commands tag the types and names of all their members and parameters. This is essential for the automatic dependency process to work.

If you are modifying existing APIs, just make appropriate changes in the existing tags.

Once the definition is added, proceed to the next section to create dependencies on the changed feature.

30.2. API Feature Dependencies

When you add new API elements, they will not result in corresponding changes in the generated header unless they are required by the interface being generated. This makes it possible to include different API versions and extensions in a single registry and pull them out as needed. So you must introduce a dependency on new features in the corresponding feature tag.

Initially, the only API feature is Vulkan 1.0, so there is only one feature tag in vk.xml. You can find it by searching for the following block of vk.xml:

<comment>
<feature api="vulkan" name="VK_VERSION_1_0" number="1.0"
         comment="Vulkan core API interface definitions">

Inside the feature tag are nested multiple require tags. These are just being used as a logical grouping mechanism for related parts of Vulkan 1.0 at present, though they may have more meaningful roles in the future if different API profiles are defined.

30.2.1. API Feature Walkthrough

This section walks through the first few required API features in the vk.xml feature tag, showing how each requirement pulls in type, token, and command definitions and turns those into definitions in the C header file vulkan_core.h.

Consider the first few lines of the feature:

<require comment="Header boilerplate">
    <type name="vk_platform"/>
</require>
<require comment="API constants">
    <enum name="VK_MAX_PHYSICAL_DEVICE_NAME"/>
    <enum name="VK_MAX_EXTENSION_NAME"/>
    ...
</require>
<require comment="Device initialization">
    <command name="vkCreateInstance"/>
    ...

The first require block says to require a type named vk_platform. If you look at the beginning of the types section, there is a corresponding definition section:

<type name="vk_platform">#include "vk_platform.h"
#define VK_MAKE_VERSION(major, minor, patch) \
    ((major &lt;&lt; 22) | (minor &lt;&lt; 12) | patch)
    ...

This section is invoked by the requirement and emits a bunch of boilerplate C code. The explicit dependency is not strictly required since vk_platform will be required by many other types, but placing it first causes this to appear first in the output file.

Note that vk_platform does not correspond to an actual C type, but instead to a collection of freeform preprocessor includes and macros and comments. Most other type tags do define a specific type and are much simpler, but this approach can be used to inject arbitrary C into the Vulkan headers when there is no other way. In general inserting arbitrary C is strongly discouraged outside of specific special cases like this.

The next require block pulls in some compile time constants. These correspond to the definitions found in the first enums section of vk.xml:

<enums name="API Constants"
       comment="Vulkan hardcoded constants - not an enumerated type, part of the header boilerplate">
    <enum value="256"        name="VK_MAX_PHYSICAL_DEVICE_NAME"/>
    <enum value="256"        name="VK_MAX_EXTENSION_NAME"/>
    ...

The third require block starts pulling in some Vulkan commands. The first command corresponds to the following definition found in the commands section of vk.xml:

<commands>
    <command>
        <proto><type>VkResult</type> <name>vkCreateInstance</name></proto>
        <param>const <type>VkInstanceCreateInfo</type>* <name>pCreateInfo</name></param>
        <param><type>VkInstance</type>* <name>pInstance</name></param>
    </command>
    ...

In turn, the command tag requires the types VkResult, VkInstanceCreateInfo, and VkInstance as part of its definition. The definitions of these types are determined as follows:

For VkResult, the corresponding required type is:

<type name="VkResult" category="enum"/>

Since this is an enumeration type, it simply links to an enums tag with the same name:

<enums name="VkResult" type="enum" comment="API result codes">
        <comment>Return codes (positive values)</comment>
    <enum value="0"     name="VK_SUCCESS"/>
    <enum value="1"     name="VK_UNSUPPORTED"/>
    <enum value="2"     name="VK_NOT_READY"/>
    ...
        <comment>Error codes (negative values)</comment>
    <enum value="-1"    name="VK_ERROR_OUT_OF_HOST_MEMORY" comment="A host memory allocation has failed"/>
    ...

For VkInstanceCreateInfo, the required type is:

<type category="struct" name="VkInstanceCreateInfo">
    <member values="VK_STRUCTURE_TYPE_INSTANCE_CREATE_INFO"><type>VkStructureType</type> <name>sType</name></member>
    <member>const void*                         <name>pNext</name></member>
    <member>const <type>VkApplicationInfo</type>* <name>pAppInfo</name></member>
    <member>const <type>VkAllocCallbacks</type>* <name>pAllocCb</name></member>
    <member><type>uint32_t</type>               <name>extensionCount</name></member>
    <member>const <type>char</type>*const*      <name>ppEnabledExtensionNames</name></member>
</type>

This is a structure type, defining a C struct with all the members defined in each member tag in order. In addition, it requires some other types, whose definitions are located by name in exactly the same fashion.

For the final direct dependency of the command, VkInstance, the required type is:

    <comment>Types which can be void pointers or class pointers, selected at compile time</comment>
<type>VK_DEFINE_BASE_HANDLE(<name>VkObject</name>)</type>
<type>VK_DEFINE_DISP_SUBCLASS_HANDLE(<name>VkInstance</name>, <type>VkObject</type>)</type>

In this case, the type VkInstance is defined by a special compile time macro which defines it as a derived class of VkObject (for C``) or a less typesafe definition (for C). This macro is not part of the type dependency analysis, just the boilerplate used in the header.

If these are the only feature dependencies in vk.xml, the resulting vulkan_core.h header will look like this:

#ifndef VULKAN_H_
#define VULKAN_H_ 1

#ifdef __cplusplus
extern "C" {
#endif

/*
** Copyright 2015-2021 The Khronos Group Inc.
    ...
*/

/*
** This header is generated from the Khronos Vulkan XML API Registry.
**
** Generated on date 20170208
*/


#define VK_VERSION_1_0 1
#include "vk_platform.h"
#define VK_MAKE_VERSION(major, minor, patch) \
    ((major << 22) | (minor << 12) | patch)

// Vulkan API version supported by this file
#define VK_API_VERSION VK_MAKE_VERSION(0, 104, 0)

#if defined (__cplusplus) && (VK_UINTPTRLEAST64_MAX == UINTPTR_MAX)
    #define VK_TYPE_SAFE_COMPATIBLE_HANDLES 1
#endif

#if defined(VK_TYPE_SAFE_COMPATIBLE_HANDLES) && !defined(VK_DISABLE_TYPE_SAFE_HANDLES)
    #define VK_DEFINE_PTR_HANDLE(_obj) struct _obj##_T { char _placeholder; }; typedef _obj##_T* _obj;
    #define VK_DEFINE_PTR_SUBCLASS_HANDLE(_obj, _base) struct _obj##_T : public _base##_T {}; typedef _obj##_T* _obj;

    #define VK_DEFINE_BASE_HANDLE(_obj) VK_DEFINE_PTR_HANDLE(_obj)
    #define VK_DEFINE_DISP_SUBCLASS_HANDLE(_obj, _base) VK_DEFINE_PTR_SUBCLASS_HANDLE(_obj, _base)
    #define VK_DEFINE_NONDISP_SUBCLASS_HANDLE(_obj, _base) VK_DEFINE_PTR_SUBCLASS_HANDLE(_obj, _base)
#else
    #define VK_DEFINE_BASE_HANDLE(_obj) typedef VkUintPtrLeast64 _obj;
    #define VK_DEFINE_DISP_SUBCLASS_HANDLE(_obj, _base) typedef uintptr_t _obj;
    #define VK_DEFINE_NONDISP_SUBCLASS_HANDLE(_obj, _base) typedef VkUintPtrLeast64 _obj;
#endif

typedef enum {
    VK_SUCCESS = 0,
    VK_UNSUPPORTED = 1,
    VK_NOT_READY = 2,
    ...
} VkResult;
typedef enum {
    VK_STRUCTURE_TYPE_APPLICATION_INFO = 0,
    ...
} VKStructureType;
typedef struct {
    VkStructureType                             sType;
    const void*                                 pNext;
    const char*                                 pAppName;
    uint32_t                                    appVersion;
    const char*                                 pEngineName;
    uint32_t                                    engineVersion;
    uint32_t                                    apiVersion;
} VkApplicationInfo;
typedef enum {
    VK_SYSTEM_ALLOC_TYPE_API_OBJECT = 0,
    ...
} VkSystemAllocType;
typedef void* (VKAPI_PTR *PFN_vkAllocFunction)(
    void*                           pUserData,
    size_t                          size,
    size_t                          alignment,
    VkSystemAllocType               allocType);
typedef void (VKAPI_PTR *PFN_vkFreeFunction)(
    void*                           pUserData,
    void*                           pMem);
typedef struct {
    void*                                       pUserData;
    PFN_vkAllocFunction                         pfnAlloc;
    PFN_vkFreeFunction                          pfnFree;
} VkAllocCallbacks;
typedef struct {
    VkStructureType                             sType;
    const void*                                 pNext;
    const VkApplicationInfo*                    pAppInfo;
    const VkAllocCallbacks*                     pAllocCb;
    uint32_t                                    extensionCount;
    const char*const*                           ppEnabledExtensionNames;
} VkInstanceCreateInfo;
VK_DEFINE_BASE_HANDLE(VkObject)
VK_DEFINE_DISP_SUBCLASS_HANDLE(VkInstance, VkObject)
#define VK_MAX_PHYSICAL_DEVICE_NAME       256
#define VK_MAX_EXTENSION_NAME             256
typedef VkResult (VKAPI_PTR *PFN_vkCreateInstance)(const VkInstanceCreateInfo* pCreateInfo, VkInstance* pInstance);
#ifndef VK_NO_PROTOTYPES
VKAPI_ATTR VkResult VKAPI_CALL vkCreateInstance(
    const VkInstanceCreateInfo*                 pCreateInfo,
    VkInstance*                                 pInstance);
#endif

#ifdef __cplusplus
}
#endif

#endif

Note that several additional types are pulled in by the type dependency analysis, but only those types, commands, and tokens required by the specified features are generated.

30.3. How To Add A Compile Time Constant

Go to the desired feature or extension tag. Add (if not present) a nested require block labelled

<require comment="API constants">

In this block, add an (appropriately indented) tag like

    <enum name="VK_THE_ANSWER"/>

Then go to the enums block labelled

<enums comment="Misc. hardcoded constants - not an enumerated type">

In this block, add a tag whose name attribute matches the name you defined above and whose value attribute is the value to give the constant:

    <enum value="42" type="uint32_t" name="VK_THE_ANSWER"/>

The type attribute must be present, and must have one of the allowed values uint32_t, uint64_t, or float.

30.4. Allowed Format of Compile Time Constants

The value attribute must be a legal C99 constant scalar expression when evaluated at compilation time. Allowed expressions are additionally restricted to the following syntax:

a single C decimal integer or floating-point value
optionally prefixed with ~
optionally suffixed with U, UL, ULL, or F
and the entire expression optionally surrounded by paired ( and ).

30.5. How To Add A Struct or Union Type

For this example, assume we want to define a type corresponding to a C struct defined as follows:

typedef struct {
    VkStructureType          sType;
    const void*              pNext;
    const VkApplicationInfo* pAppInfo;
    const VkAllocCallbacks*  pAllocCb;
    uint32_t                 extensionCount;
    const char*const*        ppEnabledExtensionNames;
} VkInstanceCreateInfo;

If VkInstanceCreateInfo is the type of a parameter of a command in the API, make sure that command’s definition (see below for how to add a command) puts VkInstanceCreateInfo in nested type tags where it is used.

Otherwise, if the struct type is not used directly by a command in the API, nor required by a chain of type dependencies for other commands, an explicit type dependency should be added to the feature tag. Go to the types tag and search for the nested block labelled

<require comment="Types not directly used by the API. Include e.g. structs that are not parameter types of commands, but still defined by the API.">
    ...

In this block, add a tag whose name attribute matches the name of the struct type being defined:

<require comment="API types not used by commands">
    <type name="VkInstanceCreateInfo"/>
    ...

Then go to the types tag and add a new type tag defining the struct names and members, somewhere below the corresponding comment, like this:

<types>
    ...
        <comment>Struct types</comment>
    <type category="struct" name="VkInstanceCreateInfo">
        <member><type>VkStructureType</type>
                <name>sType</name></member>
        <member>const void*
                <name>pNext</name></member>
        <member>const <type>VkApplicationInfo</type>*
                <name>pAppInfo</name></member>
        <member>const <type>VkAllocCallbacks</type>*
                <name>pAllocCb</name></member>
        <member><type>uint32_t</type>
                <name>extensionCount</name></member>
        <member>const <type>char</type>*const*
                <name>ppEnabledExtensionNames</name></member>
    </type>
    ...

If any of the member types are types also defined in the header, make sure to enclose those type names in nested type tags, as shown above. Basic C types should not be tagged.

If the type is a C union, rather than a struct, then set the value of the category attribute to "union" instead of "struct".

30.6. How To Add An Enumerated Type

For this example, assume we want to define a type corresponding to a C enum defined as follows:

typedef enum {
    VK_DEVICE_CREATE_VALIDATION_BIT = 0x00000001,
    VK_DEVICE_CREATE_MULTI_DEVICE_IQ_MATCH_BIT = 0x00000002;
} VkDeviceCreateFlagBits.

If VkDeviceCreateFlagBits is the type of a parameter to a command in the API, or of a member in a structure or union, make sure that command parameter or struct member’s definition puts VkDeviceCreateFlagBits in nested type tags where it is used.

Otherwise, if the enumerated type is not used directly by a command in the API, nor required by a chain of type dependencies for commands and structs, an explicit type dependency should be added to the feature tag in exactly the same fashion as described above for struct types.

Next, go to the line labelled

<comment>Vulkan enumerant (token) definitions</comment>

At an appropriate point below this line, add an enums tag whose name attribute matches the type name VkDeviceCreateFlagBits, and whose contents correspond to the individual fields of the enumerated type:

<enums name="VkDeviceCreateFlagBits" type="bitmask">
    <enum bitpos="0" name="VK_DEVICE_CREATE_VALIDATION_BIT"/>
    <enum bitpos="1" name="VK_DEVICE_CREATE_MULTI_DEVICE_IQ_MATCH_BIT"/>
</enums>

Several other attributes of the enums tag can be set. In this case, the type attribute is set to "bitmask", indicating that the individual enumerants represent elements of a bitmask.

The individual enum tags define the enumerants, just like the definition for compile time constants described above. In this case, because the enumerants are bits in a bitmask, their values are specified using the bitpos attribute. The value of this attribute must be an integer in the range [0,30] specifying a single bit number, and the resulting value is printed as a hexadecimal constant corresponding to that bit.

It is also possible to specify enumerant values using the value attribute, in which case the specified numeric value is passed through to the C header unchanged.

30.7. How To Add Bit Flags

Bit masks are defined by two types in the xml - the type of the mask itself, and the type of the valid flags.

For this example, assume we want to define bit flags that can handle up to 64 independent values as follows:

// Flag bits for VkExampleFlagBits
typedef VkFlags64 VkExampleFlagBits;
static const VkExampleFlagBits VK_EXAMPLE_NONE = 0;
static const VkExampleFlagBits VK_EXAMPLE_FIRST_BIT = 0x00000001;
static const VkExampleFlagBits VK_EXAMPLE_SECOND_BIT = 0x00000002;

typedef VkFlags64 VkExampleFlags;

An explicit type dependency should be added to the feature tag in exactly the same fashion as described above for struct types.

Firstly, a definition is needed for the flags type used as a parameter to commands or member of functions. Go to the line labelled:

<comment>Bitmask types</comment>

At the end of the list of VkFlags and VkFlags64 types, add a definition of the flags type like so:

<type bitvalues="VkExampleFlagBits" category="bitmask">typedef <type>VkFlags64</type> <name>VkExampleFlags</name>;</type>

The category defines this as a "bitmask" type. The bitvalues attribute identifies the *FlagBits entry defining the flag bits associated with this type.

Next, go to the line labelled:

<comment>Types generated from corresponding enums tags below</comment>

At an appropriate point in the list of enum types after this comment, add the following line:

<type name="VkExampleFlagBits" category="enum"/>

This defines a type for the flag bits for generators that need it. The category attribute of "enum" identifies that this is an enumerated type.

Finally, go to the line labelled:

<comment>Vulkan enumerant (token) definitions</comment>

At the end of the list of enum definitions below this line, add an enums tag whose name attribute matches the type name VkExampleFlagBits, and whose contents correspond to the individual fields of the enumerated type:

<enums name="VkExampleFlagBits" type="bitmask" bitwidth="64">
    <enum value="0" name="VK_EXAMPLE_NONE"/>
    <enum bitpos="0" name="VK_EXAMPLE_FIRST_BIT"/>
    <enum bitpos="1" name="VK_EXAMPLE_SECOND_BIT"/>
</enums>

The type attribute is set to "bitmask", indicating that the individual enumerants represent elements of a bitmask. The bitwidth attribute is set to "64" indicating that this is a 64-bit flag type.

The individual enum tags define the enumerants, just like the definition for compile time constants described above. In this case, a "no flags" type is defined in VK_EXAMPLE_NONE with the value attribute defining it to have a hard value of 0. The other types have their values are specified using the bitpos attribute, as these are actual bit flag values. The value of this attribute must be an integer in the range [0,63] specifying a single bit number, and the resulting value is printed as a hexadecimal constant corresponding to that bit.

30.7.1. 32-bit Flags

Bit flags can also be defined using 32-bit C enum types. Doing so is broadly similar to 64-bit bit flags, but with a few key differences. For this example, assume we want to define the same type as above, but corresponding to a C enum and flags type defined as follows:

typedef enum VkExampleFlagBits {
    VK_EXAMPLE_NONE
    VK_DEVICE_CREATE_VALIDATION_BIT = 0x00000001,
    VK_DEVICE_CREATE_MULTI_DEVICE_IQ_MATCH_BIT = 0x00000002;
} VkExampleFlagBits;

typedef VkFlags VkExampleFlags;

To add this to the xml, entries need to be added to the XML in the same way as above, but with slightly different attributes:

For the flag type definition, the entry should use VkFlags instead of VkFlags64, and require the flag bits type, instead of specifying the bitvalues attribute:

<type requires="VkExampleFlagBits" category="bitmask">typedef <type>VkFlags</type> <name>VkExampleFlags</name>;</type>

For the definition of the enumerated flag values themselves, the bitwidth needs to either be changed to "32", or omitted entirely (which defaults to a bitwidth of 32) as follows:

<enums name="VkExampleFlagBits" type="bitmask">

Note that 32-bit bitmasks must use an integer in the range [0,30] - C enums are only guaranteed to support signed 32-bit integer values, and defining an unsigned value for the 31st bit could change the size of the enum type. The generator scripts will warn about values exceeding this range.

30.8. How to Add A Command

For this example, assume we want to define the command:

VKAPI_ATTR VkResult VKAPI_CALL vkCreateInstance(
    const VkInstanceCreateInfo*                 pCreateInfo,
    VkInstance*                                 pInstance);

Commands must always be explicitly required in the feature tag. In that tag, you can use an existing require block including API features which the new command should be grouped with, or define a new block. For this example, add a new block, and require the command by using the command tag inside that block:

<feature api="vulkan" name="VK_VERSION_1_0" number="1.0" comment="Vulkan core API interface definitions">
    ...
    <require comment="Device initialization">
        <command name="vkCreateInstance"/>
    </require>
    ...
</feature>

The require block may include a comment attribute whose value is a descriptive comment of the contents required within that block. The comment is not currently used in header generation, but might be in the future, so use comments which are polite and meaningful to users of the generated header files.

Then go to the commands tag and add a new command tag defining the command, preferably sorted into alphabetic order with other commands for ease of reading, as follows:

<commands comment="Vulkan command definitions">
    ...
    <command>
        <proto><type>VkResult</type>
               <name>vkCreateInstance</name></proto>
        <param>const <type>VkInstanceCreateInfo</type>*
               <name>pCreateInfo</name></param>
        <param><type>VkInstance</type>*
               <name>pInstance</name></param>
    </command>
    ...
</commands>

The proto tag defines the return type and function name of the command. The param tags define the command’s parameters in the order in which they are passed, including the parameter type and name. The contents are laid out in the same way as the structure member tags described previously.

30.9. More Complicated API Representations

The registry schema can represent a good deal of additional information, for example by creating multiple feature tags defining different API versions and extensions. This capability is not yet relevant to Vulkan. Those capabilities will be documented as they are needed.

30.10. More Complicated Output Formats And Other Languages

The registry schema is oriented towards C-language APIs. Types and commands are defined using syntax which is a subset of C, especially for structure members and command parameters. It would be possible to use a language-independent syntax for representing such information, but since we are writing a C API, any such representation would have to be converted into C anyway at some stage.

The vulkan.h header is written using an output generator object in the Python scripts. This output generator is specialized for C, but the design of the scripts is intended to support writing output generators for other languages as well as purposes such as documentation (e.g. generating asciidoc fragments corresponding to types and commands for use in the API specification and reference pages). When targeting other languages, the amount of parsing required to convert type declarations into other languages is small. However, it will probably be necessary to modify some of the boilerplate C text, or specialize the tags by language, to support such generators.

30.11. Additional Semantic Tagging

The schema is being extended to support semantic tags describing various properties of API features, such as:

constraints on allowed scalar values to function parameters (non-NULL, normalized floating-point, etc.)
length of arrays corresponding to function pointer parameters
miscellaneous properties of commands such as whether the application or system is responsible for threadsafe use; which queues they may be issued on; whether they are aliases or otherwise related to other commands; etc.

These tags will be used by other tools for purposes such as helping create validation layers, generating serialization code, and so on. We would like to eventually represent everything about the API that is amenable to automatic processing within the registry schema. Please make suggestions on the Github issue tracker.

30.12. Stability of the XML Database and Schema

The Vulkan XML schema is evolving in response to corresponding changes in the Vulkan API and ecosystem. Most such change will probably be confined to adding attributes to existing tags and properly expressing the relationships to them, and making API changes corresponding to accepted feature requests. Changes to the schema should be described in the change log of this document. Changes to the .xml files and Python scripts are logged in Github history.

31. Change Log

2021-12-13 - Add api attribute to the member and param tags.
2021-11-29 - Add api attribute to the command tag.
2021-10-11 - Add description of the formats tag for describing Vulkan VkFormat image formats, and of the spirvextensions and spirvcapabilities tags for describing enabling mechanisms for SPIR-V extensions (internal issue 2484).
2021-08-22 - Update introductory descriptions of toolchain and scripts.
2021-08-15 - Add an explicit description of the enum extends attribute as introducing a requirement for the enumerated type being extended.
2021-07-12 - Note that extension tags describing instance extensions must not have dependencies on device extensions (internal issue 2387).
2021-06-14 - Add an objecttype attribute which specifies the relationship between a Vulkan handle and another member or parameter specifying the type of object that handle refers to (public issue 1536).
2021-06-06 - Update description of the supported attribute of extension tags to mandate that disabled extensions are not processed (public issue 1549).
2021-04-21 - Add the limittype attribute to structure member tags, to describe how queried limits are interpreted (internal issue 2427).
2021-03-30 - Add a description of the allowed format of compile time constants (internal merge request 4451).
2021-03-22 - Update allowed values for the type attribute of enum tags and make it mandatory (internal issue 2564).
2021-01-11 - Expand the scope of the optional attribute member and param tags to specify that the member or parameter may be 0 for all scalar types, not just bitmasks and array sizes (internal issue 2435).
2020-11-23 - Add objtypeenum attribute to <<type, type>> tags to link the object name to the corresponding VK_OBJECT_TYPE_* enumerant, if any (internal issue 2393).
2020-11-22 - Add requirement that pNext members have the optional="true" attribute set (internal issue 2428).
2020-10-14 - Remove advice to set the noautovalidity attribute on the pNext member of extending structures in the <<type, type tag>>, since the validity generator scripts now take care of this (internal issue 2335).
2020-06-02 - Add description of how to switch between 64- and 32-bit flags.
2020-05-07 - Update description of API Names to current usage, including allowing specifying multiple API names for a given feature or extension.
2020-04-29 - Expand use of category basetype in type tags to include external API types.
2020-02-20 - Clarify that enum tags inside remove tags must be reference enums, not containing attributes defining values.
2020-01-13 - Restrict bitpos to [0,30] to avoid poorly defined compiler behavior.
2019-08-25 - Add sortorder attribute to feature and extension tags.
2018-12-06 - Specify that command aliases are not guaranteed to resolve to the same entry point in the underlying layer stack, matching a related clarification in the Vulkan Specification.
2018-10-01 - Add description of the default value of optional member and parameter attributes, if not specified.
2018-08-28 - Add optional provisional attribute to extension tags.
2018-07-07 - Add optional promotedto, deprecatedby, and obsoletedby attributes to extension tags.
2018-06-25 - Remove vendorids tags for Khronos vendor IDs.
2018-05-08 - Add driverids and driverid tags for describing Vulkan driver implementation identification information.
2018-04-15 - Add requiresCore.
2018-03-07 - Updated for Vulkan 1.1 release.
2018-02-21 - Add descriptions of the extnumber and alias attributes used for defining enum attributes, the alias attribute used for defining type aliases, the name and alias attributes used for defining command aliases, the platform attribute of extension tags, and the feature attribute of require tags; and update the document to the header naming and grouping scheme used starting in Vulkan 1.1.
2018-01-07 - Add platforms and platform tags for describing Vulkan platform names and preprocessor symbols.
2017-09-10 - Define syntax of member and parameter altlen attributes, for use by code generators.
2017-09-01 - Define syntax of member and parameter len attributes consistently and correctly for current uses of latexmath:
2017-08-24 - Note that the extension attribute type must be specified if the extension is not disabled.
2017-07-27 - Finish removing validextensionstructs attribute and replacing it with structextends.
2017-07-14 - Add comment attributes or tags as valid content in several places, replacing XML comments which could not be preserved by XML transformation tools.
2017-02-20 - Change to asciidoctor markup and move into the specification source directory for ease of building.
2016-09-27 - Remove validity and usage tags, since these explicit usage statements have been moved to the specification source.
2016-08-26 - Update for the single-branch model.
2016-07-28 - Add type and requires attributes to extension tags.
2016-02-22 - Change math markup in len attributes to use asciidoc $‘ a n d ‘$ delimiters.
2016-02-19 - Add successcodes and errorcodes attributes of command tags. Add a subsection to the introduction describing the schema choices and how to file issues against the registry.
2016-02-07 - Add vendorids tags for Khronos vendor IDs.
2015-12-10 - Add author and contact attributes for extension tags.
2015-12-07 - Move vulkan/vulkan.h to a subdirectory.
2015-12-01 - Add tags tags for author tags.
2015-11-18 - Bring documentation and schema up to date for extension enumerants.
2015-11-02 - Bring documentation and schema up to date with several recent merges, including validity tags. Still out of date WRT extension enumerants, but that will change soon.
2015-09-08 - Rename threadsafe attribute to externsync, and implicitunsafeparams tag to implicitexternsync.
2015-09-07 - Update command tag description to remove the threadsafe attribute and replace it with a combination of threadunsafe attributes on individual parameters, and implicitunsafeparams tags describing additional unsafe objects for the command.
2015-08-04 - Add basetype and funcpointer category values for type tags, and explain the intended use and order in which types in each category are emitted.
2015-07-02 - Update description of Makefile targets. Add descriptions of threadsafe, queues, and renderpass attributes of command tags, and of modified attributes of param tags.
2015-06-17 - add descriptions of allowed category attribute values of type tags, used to group and sort related categories of declarations together in the generated header.
2015-06-04 - Add examples of making changes and additions to the registry.
2015-06-03 - Move location to new vulkan Git repository. Add definition of type tags for C struct/unions. Start adding examples of making changes.
2015-06-02 - Branch from OpenGL specfile documentation and bring up to date with current Vulkan schema.
2015-07-10 - Remove contractions to match the style guide.
2015-07-19 - Move this document from LaTeX to asciidoc source format and make minor changes to markup.

The Khronos® Vulkan® API Registry

1. Introduction

1.1. Schema Choices

2. Getting Started

2.1. Header Generation Script - genvk.py

2.2. Registry Processing Script - reg.py

2.3. Output Generator Script - generator.py

3. Vulkan Registry Schema

3.1. Profiles

3.2. API Names

4. Registry Root (registry tag)

4.1. Attributes of registry tags

4.2. Contents of registry tags

4.2.1. Comment Tags (comment tag)

5. Platform Name Blocks (platforms tag)

5.1. Attributes of platforms tags

5.2. Contents of platforms tags

6. Platform Names (platform tag)

6.1. Attributes of platform tags

6.2. Contents of platform tags

7. Author ID Blocks (tags tag)

7.1. Attributes of tags tags

7.2. Contents of tags tags

8. Author IDs (tag tag)

8.1. Attributes of tag tags

8.2. Contents of tag tags

9. API Type Blocks (types tag)

9.1. Attributes of types tags

9.2. Contents of types tags

10. API Type (type tag)

10.1. Attributes of type tags

10.2. Contents of type tags

10.2.1. Enumerated types - category "enum"

10.2.2. Structure types - category "struct" or "union"

Structure member (member) tags

Attributes of member tags

Contents of member tags

10.2.3. All other types

10.3. Example of a types tag

11. Enumerant Blocks (enums tag)

11.1. Attributes of enums tags

11.2. Contents of enums tags

11.3. Example of enums tags

12. Enumerants (enum tag)

12.1. Attributes of enum tags

12.2. Contents of enum tags

13. Unused Enumerants (unused tag)

13.1. Attributes of unused tags

13.2. Contents of unused tags

14. Command Blocks (commands tag)

14.1. Attributes of commands tags

14.2. Contents of commands tags

15. Commands (command tag)

15.1. Attributes of command tags

15.2. Contents of command tags

15.3. Command prototype (proto tags)

15.3.1. Attributes of proto tags

15.3.2. Contents of proto tags

15.4. Command parameter (param tags)

15.4.1. Attributes of param tags

15.4.2. Contents of param tags

15.5. Example of a commands tag

16. API Features and Versions (feature tag)

16.1. Attributes of feature tags

16.2. Contents of feature tags

16.3. Example of a feature tag

17. Extension Blocks (extensions tag)

17.1. Attributes of extensions tags

17.2. Contents of extensions tags

18. API Extensions (extension tag)

18.1. Attributes of extension tags

18.2. Contents of extension tags

18.3. Example of an extensions tag

19. Required and Removed Interfaces (require and remove tags)

19.1. Attributes of require and remove tags

19.2. Attributes of require tags

19.3. Contents of require and remove tags

19.3.1. Comment Tags

19.3.2. Command Tags

19.3.3. Enum tags

The Khronos^® Vulkan^® API Registry

2.1. Header Generation Script - `genvk.py`

2.2. Registry Processing Script - `reg.py`

2.3. Output Generator Script - `generator.py`

4. Registry Root (`registry` tag)

4.1. Attributes of `registry` tags

4.2. Contents of `registry` tags

4.2.1. Comment Tags (`comment` tag)

5. Platform Name Blocks (`platforms` tag)

5.1. Attributes of `platforms` tags

5.2. Contents of `platforms` tags

6. Platform Names (`platform` tag)

6.1. Attributes of `platform` tags

6.2. Contents of `platform` tags

7. Author ID Blocks (`tags` tag)

7.1. Attributes of `tags` tags

7.2. Contents of `tags` tags

8. Author IDs (`tag` tag)

8.1. Attributes of `tag` tags

8.2. Contents of `tag` tags

9. API Type Blocks (`types` tag)

9.1. Attributes of `types` tags

9.2. Contents of `types` tags

10. API Type (`type` tag)

10.1. Attributes of `type` tags

10.2. Contents of `type` tags

10.2.1. Enumerated types - `category` `"enum"`

10.2.2. Structure types - `category` `"struct"` or `"union"`

Structure member (`member`) tags

Attributes of `member` tags

Contents of `member` tags

10.3. Example of a `types` tag

11. Enumerant Blocks (`enums` tag)

11.1. Attributes of `enums` tags

11.2. Contents of `enums` tags

11.3. Example of `enums` tags

12. Enumerants (`enum` tag)

12.1. Attributes of `enum` tags

12.2. Contents of `enum` tags

13. Unused Enumerants (`unused` tag)

13.1. Attributes of `unused` tags

13.2. Contents of `unused` tags

14. Command Blocks (`commands` tag)

14.1. Attributes of `commands` tags

14.2. Contents of `commands` tags

15. Commands (`command` tag)

15.1. Attributes of `command` tags

15.2. Contents of `command` tags

15.3. Command prototype (`proto` tags)

15.3.1. Attributes of `proto` tags

15.3.2. Contents of `proto` tags

15.4. Command parameter (`param` tags)

15.4.1. Attributes of `param` tags

15.4.2. Contents of `param` tags

15.5. Example of a `commands` tag

16. API Features and Versions (`feature` tag)

16.1. Attributes of `feature` tags

16.2. Contents of `feature` tags

16.3. Example of a `feature` tag

17. Extension Blocks (`extensions` tag)

17.1. Attributes of `extensions` tags

17.2. Contents of `extensions` tags

18. API Extensions (`extension` tag)

18.1. Attributes of `extension` tags

18.2. Contents of `extension` tags

18.3. Example of an `extensions` tag

19. Required and Removed Interfaces (`require` and `remove` tags)

19.1. Attributes of `require` and `remove` tags

19.2. Attributes of `require` tags

19.3. Contents of `require` and `remove` tags

20. Formats (`formats` tag)

20.1. Attributes of `formats` tags

20.2. Contents of `formats` tags

21. Image Format (`format` tag)

21.1. Attributes of `format` tags

21.2. Contents of `format` tags

21.3. Example of a `format` tag

22. Format Components (`component` tag)

22.1. Attributes of `component` tags

22.2. Contents of `component` tags