// Copyright 2013-2021 The Khronos Group Inc.
//
// SPDX-License-Identifier: CC-BY-4.0

= The Khronos^®^ Vulkan^®^ API Registry
Jon Leech
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[[introduction]]
= 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/


== 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.


[[starting]]
= Getting Started

See
https://gitlab.khronos.org/vulkan/vulkan/blob/main/xml/README.adoc[`xml/README.adoc`]
in the `Vulkan-Docs` repository for information on required toolchain
components such as Python 3, pass:[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
link:https://github.com/KhronosGroup/Vulkan-Headers/[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>> 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.

== 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.


== 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.


== 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.


[[schema]]
= Vulkan Registry Schema

The format of the Vulkan registry is a top level tag:registry tag containing
tag:types, tag:enums, tag:commands, tag:feature, and tag: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`.


[[schema:profile]]
== 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.


[[schema:apiname]]
== 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 attr:api attribute of the tag:feature tag and the attr:supported
attribute of the tag: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
attr: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.


[[schema:root]]
= Registry Root (tag:registry tag)

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

== Attributes of tag:registry tags

None.

== Contents of tag:registry tags

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

  * tag:comment - Contains arbitrary text, such as a copyright statement.
  * <<tag-platforms,tag:platforms>> - defines platform names corresponding
    to platform-specific <<tag-extension,API extensions>>.
  * <<tag-tags,tag: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.
  * <<tag-types,tag:types>> - defines API types.
    Usually only one tag is used.
  * <<tag-enums,tag:enums>> - defines API token names and values.
    Usually multiple tags are used.
    Related groups may be tagged as an enumerated type corresponding to a
    tag:type tag, and resulting in a C `enum` declaration.
    This ability is heavily used in the Vulkan API.
  * <<tag-commands,tag:commands>> - defines API commands (functions).
    Usually only one tag is used.
  * <<tag-feature,tag:feature>> - defines API feature interfaces (API
    versions, more or less).
    One tag per feature set.
  * <<tag-extensions,tag:extensions>> - defines API extension interfaces.
    Usually only one tag is used, wrapping many extensions.


[[tag-comment]]
=== Comment Tags (tag:comment tag)

A tag: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.


[[tag-platforms]]
= Platform Name Blocks (tag:platforms tag)

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

== Attributes of tag:platforms tags

  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:platforms tags

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


[[tag-platform]]
= Platform Names (tag:platform tag)

A tag:platform tag describes a single platform name.

== Attributes of tag:platform tags

  * attr: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]
.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.
====
--
  * attr: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]
.Note
====
For example,

`protect="VK_USE_PLATFORM_XLIB_XRANDR_EXT"`

is used for the `xlib_xrandr` platform name.
====
--
  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:platform tags

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


[[tag-tags]]
= Author ID Blocks (tag:tags tag)

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

== Attributes of tag:tags tags

  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:tags tags

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


[[tag-tag]]
= Author IDs (tag:tag tag)

A tag:tag tag contains information defining a single author ID.

== Attributes of tag:tag tags

  * attr:name - required.
    The author ID, as registered with Khronos.
    A short, upper-case string, usually an abbreviation of an author,
    project or company name.
  * attr:author - required.
    The author name, such as a full company or project name.
  * attr: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.

== Contents of tag:tag tags

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


[[tag-types]]
= API Type Blocks (tag:types tag)

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

== Attributes of tag:types tags

  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:types tags

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


[[tag-type]]
= API Type (tag:type tag)

A tag: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.

== Attributes of tag:type tags

  * attr:requires - optional.
    Another type name this type requires to complete its definition.
  * attr:name - optional.
    Name of this type (if not defined in the tag body).
  * attr:alias - optional.
    Another type name which this type is an alias of.
    Must match the name of another tag: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.
  * attr:api - optional <<schema:apiname, 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.
  * attr: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.
  * attr:comment - optional.
    Arbitrary string (unused).
  * attr:parent - only applicable if `"category"` is `handle`.
    Notes another type with the `handle` category that acts as a parent
    object for this type.
  * attr: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.
  * attr: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 attr:noautovalidity attribute on the `pNext`
    members of extending structures.
  * attr:allowduplicate - only applicable if attr:category is `"struct"`.
    If `"true"`, then structures whose `pNext` chains include this structure
    may include more than one instance of it.
  * attr:objtypeenum - only applicable at present if attr:category is
    `"handle"`.
    Specifies the name of a `VkObjectType` enumerant which corresponds to
    this type.
    The enumerant must be defined.

== Contents of tag:type tags

The valid contents depend on the attr:category attribute.

=== Enumerated types - attr:category `"enum"`

If the attr: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 attr:name attribute must be provided and must match the
attr:name attribute of a <<tag-enums,tag:enums>> tag.
The enumerant values defined within the tag:enums tag are used to generate a
C `enum` type declaration.

=== Structure types - attr:category `"struct"` or `"union"`

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

==== Structure member (tag:member) tags

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

==== Attributes of tag:member tags

  * attr: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.
  * attr: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 `latexmath:[` and `]`.
    The only variables in the equation should be the names of members of the
    structure.
  * attr:altlen - if the attr: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 attr:len.
    It must be a valid C99 expression whose result is equal to attr:len for
    all possible inputs.
    It is a comma separated list that has size equal to only the `latexmath`
    item count in attr:len list.
    This attribute is intended to support consumers of the XML who need to
    generate validation code from the allowed length.
  * attr:externsync - denotes that the member should be externally
    synchronized when accessed by Vulkan
  * attr: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]
.Note
====
While the attr: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.
====
--
  * attr:selector - optional.
    If the member is a union, attr:selector identifies another member of the
    struct that is used to select which of that union's members are valid.
  * attr:selection - optional.
    For a member of a union, attr:selection identifies a value of the
    attr:selector that indicates this member is valid.
  * attr: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.
  * attr: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.
  * attr: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 tag:member tags

The text elements of a tag: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 tag:type tag is optional.
    It contains text which is a valid type name found in another tag: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 tag:type tags.
  * The tag:name tag is required, and contains the struct/union member name
    being described.
  * The tag:enum tag is optional.
    It contains text which is a valid enumerant name found in another
    tag: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 tag:comment tag is optional.
    It contains an arbitrary string (unused).


=== All other types

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

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


[[tag-types:example]]
== Example of a tag:types tag

[source,xml]
--------------------------------------
<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:

[source,c]
--------------------------------------
#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.


[[tag-enums]]
= Enumerant Blocks (tag:enums tag)

The tag:enums tags contain individual tag: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]
.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.
====


== Attributes of tag:enums tags

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

== Contents of tag:enums tags

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

== Example of tag:enums tags

<<tag-types:example,An example>> showing a tag with attribute
attr:type`="enum"` is given above.
The following example is for non-enumerated tokens.

[source,xml]
--------------------------------------
<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

[source,c]
--------------------------------------
#define VK_MAX_EXTENSION_NAME   256
#define VK_LOD_CLAMP_NONE       MAX_FLOAT
--------------------------------------


[[tag-enum]]
= Enumerants (tag:enum tag)

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

== Attributes of tag:enum tags

  * attr: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, compile time
    constants>>.
  * attr: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 <<adding-bitflags, flag bits used with 64-bit flag types>>.
    Exactly one of attr:value and attr:bitpos must be present in an tag:enum
    tag.
  * attr:name - required.
    Enumerant name, a legal C preprocessor token name.
  * attr:api - optional <<schema:apiname, 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.
  * attr:type - may be used only when attr:value is specified.
    In this case, attr:type is optional except when defining a
    <<compile-time-constants, 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 attr:value, although only `uint32_t`, `uint64_t`, and `float`
    are currently meaningful.
    attr:type is used by some output generators to generate constant
    declarations, although the default behavior is to use C `#define` for
    compile-time constants.
  * attr: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 attr:name for the exact same
    attr:value or attr:bitpos.
  * attr:protect - optional.
    An additional preprocessor token used to protect an enum definition.

[NOTE]
.Note
====
In older versions of the schema, attr:type was described as allowing only
the C integer suffix types `u` and `ull`, which is inconsistent with the
current definition.
However, attr: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.
====


== Contents of tag:enum tags

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


[[tag-unused]]
= Unused Enumerants (tag:unused tag)

Each tag: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]
.Note
====
tag: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.
====

== Attributes of tag:unused tags

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

== Contents of tag:unused tags

None.


[[tag-commands]]
= Command Blocks (tag:commands tag)

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

== Attributes of tag:commands tags

  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:commands tags

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


[[tag-command]]
= Commands (tag:command tag)

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

== Attributes of tag:command tags

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

  * attr: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
    (`","`).
  * attr:successcodes - optional.
    A string describing possible successful return codes from the command,
    as a comma-separated list of Vulkan result code names.
  * attr:errorcodes - optional.
    A string describing possible error return codes from the command, as a
    comma-separated list of Vulkan result code names.
  * attr: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"`).
  * attr: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 (`","`).
  * attr: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:

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

== Contents of tag:command tags

  * tag: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.
  * tag:param elements for each command parameter follow, defining its name
    and type, as described below.
    If a command takes no arguments, it has no tag:param tags.

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

  * tag: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.
  * tag:description - optional.
    Unused text.
  * tag:implicitexternsyncparams - optional.
    Contains a list of tag: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 <<tag-command:param:attr,requires
    external synchronization>>.
    The text is intended to be incorporated into the API specification.

[NOTE]
.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 flink:vkGetInstanceProcAddr
or flink:vkGetDeviceProcAddr.
====


[[tag-command:proto]]
== Command prototype (tag:proto tags)

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

=== Attributes of tag:proto tags

None.

// attr:group - group name, an arbitrary string.
//
// If the group name is defined, it may be interpreted as described in
// <<tag-group:meaning>>.

=== Contents of tag:proto tags

The text elements of a tag: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 tag:type tag is optional, and contains text which is a valid type
    name found in a tag: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 tag:type tags.
  * The tag:name tag is required, and contains the command name being
    described.


[[tag-command:param]]
== Command parameter (tag:param tags)

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


[[tag-command:param:attr]]
=== Attributes of tag:param tags

  * attr: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 `latexmath:[` and `]`.
    The only variables in the equation should be the names of this or other
    parameters.
  * attr:altlen - if the attr: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 attr:len.
    It must be a valid C99 expression whose result is equal to attr:len for
    all possible inputs.
    It is a comma separated list that has size equal to only the `latexmath`
    item count in attr:len list.
    This attribute is intended to support consumers of the XML who need to
    generate validation code from the allowed length.
  * attr: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]
.Note
====
While the attr: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.
====
--
  * attr:selector - optional.
    If the parameter is a union, attr:selector identifies another parameter
    of the command that is used to select which of that union's members are
    valid.
  * attr:noautovalidity - prevents automatic validity language being
    generated for the tagged item.
    Only suppresses item-specific validity - parenting issues etc.
    are still captured.
  * attr: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 attr:externsync
    attributes for the `pSubmits` array indicating that only specific
    members of each element of the array are modified:
+
--
[source,xml]
--------------------------------------
<param len="submitCount" externsync="pSubmits[].pWaitSemaphores[],pSubmits[].pSignalSemaphores[]">const <type>VkSubmitInfo</type>* <name>pSubmits</name></param>
--------------------------------------

Parameters which do not have an attr:externsync attribute are assumed to not
require external synchronization.
--
  * attr: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.

=== Contents of tag:param tags

The text elements of a tag: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 tag:type tag is optional, and contains text which is a valid type
    name found in tag: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 tag:type tags.
  * The tag:name tag is required, and contains the parameter name being
    described.

== Example of a tag:commands tag

[source,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>
</commands>
--------------------------------------

When processed into a C header, this results in

[source,c]
--------------------------------------
VkResult vkCreateInstance(
    const VkInstanceCreateInfo* pCreateInfo,
    VkInstance* pInstance);
--------------------------------------


[[tag-feature]]
= API Features and Versions (tag:feature tag)

API features are described in individual tag: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.

== Attributes of tag:feature tags

  * attr:api - required <<schema:apiname, API names>> this feature is
    defined for, such as `vulkan`.
  * attr: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"`.
  * attr:number - required.
    Feature version number, usually a string interpreted as
    `majorNumber.minorNumber`.
    Example: `4.2`.
  * attr:sortorder - optional.
    A decimal number which specifies an order relative to other tag:feature
    tags when calling output generators.
    Defaults to `0`.
    Rarely used, for when ordering by attr:name is insufficient.
  * attr:protect - optional.
    An additional preprocessor token used to protect a feature definition.
    Usually another feature or extension attr:name.
    Rarely used, for odd circumstances where the definition of a feature or
    extension requires another to be defined first.
  * attr:comment - optional.
    Arbitrary string (unused).

[NOTE]
.Note
====
The attr: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.
====

== Contents of tag:feature tags

Zero or more <<tag-required,tag:require and tag: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.

== Example of a tag:feature tag

[source,xml]
--------------------------------------
<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:

[source,c]
--------------------------------------
#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 */
--------------------------------------


[[tag-extensions]]
= Extension Blocks (tag:extensions tag)

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

== Attributes of tag:extensions tags

  * attr:comment - optional.
    Arbitrary string (unused).

== Contents of tag:extensions tags

Each tag:extensions block contains zero or more tag: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).


[[tag-extension]]
= API Extensions (tag:extension tag)

API extensions are described in individual tag:extension tags.
An extension is the set of interfaces defined by a particular API extension
specification, such as `ARB_multitexture`.
tag:extension is similar to tag:feature, but instead of having attr:version
and attr:profile attributes, instead has a attr:supported attribute, which
describes the set of API names which the extension can potentially be
implemented against.

== Attributes of tag:extension tags

  * attr:name - required.
    Extension name, following the conventions in the Vulkan Specification.
    Example: `name="VK_VERSION_1_0"`.
  * attr:number - required.
    A decimal number which is the registered, unique extension number for
    attr:name.
  * attr:sortorder - optional.
    A decimal number which specifies an order relative to other
    tag:extension tags when calling output generators.
    Defaults to `0`.
    Rarely used, for when ordering by attr:number is insufficient.
  * attr:author - optional.
    The author name, such as a full company name.
    If not present, this can be taken from the corresponding tag: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).
  * attr: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:tag
    attribute just like attr:author.
  * attr:type - required if the attr:supported attribute is not
    `'disabled'`.
    Must be either `'device'` or `'instance'`, if present.
  * attr:requires - optional.
    Comma-separated list of extension names this extension requires to be
    supported.
    Extensions whose attr:type is `'instance'` must not require extensions
    whose attr:type is `'device'`.
  * attr:requiresCore - optional.
    Core version of Vulkan required by the extension, e.g. "1.1".
    Defaults to "1.0".
  * attr:protect - optional.
    An additional preprocessor token used to protect an extension
    definition.
    Usually another feature or extension attr:name.
    Rarely used, for odd circumstances where the definition of an extension
    requires another extension or a header file to be defined first.
  * attr: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 tag:platform tag:name
    attribute values.
  * attr:supported - required <<schema:apiname, 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.
  * attr: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"`.
  * attr: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 `""`.
  * attr: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 `""`.
  * attr:provisional - optional.
    'true' if this extension is released provisionally.
  * attr: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.
  * attr:comment - optional.
    Arbitrary string (unused).


[NOTE]
.Note
====
The attr: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 tag:require, using the
attr:extension attribute to specify that extension.
====

== Contents of tag:extension tags

Zero or more <<tag-required,tag:require and tag: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.

== Example of an tag:extensions tag

[source,xml]
--------------------------------------
<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 attr: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 tag:type and tag:command elsewhere in the XML):

[source,c]
--------------------------------------
#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
--------------------------------------


[[tag-required]]
= Required and Removed Interfaces (tag:require and tag:remove tags)

A tag:require block defines a set of interfaces (types, enumerants and
commands) 'required' by a tag:feature or tag:extension.
A tag:remove block defines a set of interfaces 'removed' by a tag: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 tag:require and
tag:remove tags are identical.

== Attributes of tag:require and tag:remove tags

  * attr: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.
  * attr:comment - optional.
    Arbitrary string (unused).
  * attr:api - optional <<schema:apiname, 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]
.Note
====
The attr:api attribute is only supported inside tag:extension tags, since
tag:feature tags already define a specific API.
====

== Attributes of tag:require tags

These attribues are allowed only for a tag:require tag.

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

[NOTE]
.Note
====
The attr: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.
====

== Contents of tag:require and tag:remove tags

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

=== Comment Tags

<<tag-comment, tag:comment>> (as described above).

=== Command Tags

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

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

=== Enum tags

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

  * attr:name - required.
    Name of the enumerant.
  * attr:comment - optional.
    Arbitrary string (unused).
  * attr:api - optional <<schema:apiname, 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 tag:enum tags:

_Reference enums_ simply pull in the definition of an enumerant given in a
separate tag:enums block.
No attributes other than attr:name and attr:comment are supported for them.
tag:enum tags appearing inside tag:remove tags should always be reference
enums.
Reference enums may also be used inside tag:require tags, if the
corresponding value is defined in a tag: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 tag:feature
or tag: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:

  * attr:value and attr:type - define a constant value in the same fashion
    as an tag:enum tag in an <<tag-enum,tag:enums>> block.
  * attr:bitpos - define a constant bitmask value in the same fashion as an
    <<tag-enum,tag:enum>> tag in an tag:enums block.
    attr: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, attr:bitpos attribute of an tag:enum tag.
  * attr:extends - the name of a separately defined enumerated type (e.g. a
    tag:type tag with attr: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 attr:requires attribute of a
    tag:type tag.
    If not present, the enumerant is treated as a global constant value.
  * attr: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 tag:extension tag with the same attr: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 attr:offset - the offset within an extension block.
    If attr:extnumber is not present, the extension number defining that
    block is given by the attr:number attribute of the surrounding
    tag:extension tag.
    The actual numeric value of the enumerant is computed as defined in the
    "`Layers and Extensions`" appendix of the Vulkan Specification.
  * Attribute attr: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="-"`.
  * attr: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.
  * attr:protect - define a preprocessor protection symbol for the enum in
    the same fashion as a tag:enum tag in an <<tag-enum,tag:enums>> block.

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

.Valid Combinations of attr:enum Attributes for Extension Enums
|====
| attr:value | attr:bitpos | attr:alias | attr:offset | attr:extnumber | attr:dir | attr: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 attr:extnumber is not present, the tag:enum tag may only be within a
tag:extension.
Otherwise, the tag:enum tag may also be within a tag:feature.

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

Examples of <<tag-required-examples,various types of extension enumerants>>
are given below.


=== Type tags

tag:type specifies a required (or removed) type defined in a tag:types
block.
Most types are picked up implicitly by using the tag: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:

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


[[tag-required-examples]]
== Examples of Extension Enumerants

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

[source,xml]
--------------------------------------
<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:

[source,c]
--------------------------------------
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
--------------------------------------


[[examples]]
= 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 <<starting,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.


== 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 tag:type, tag:enums, tag:enum, or
    tag: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 tag: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.


== 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
tag:feature tag.

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

[source,xml]
--------------------------------------
<comment>
<feature api="vulkan" name="VK_VERSION_1_0" number="1.0"
         comment="Vulkan core API interface definitions">
--------------------------------------

Inside the tag:feature tag are nested multiple tag: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.


=== API Feature Walkthrough

This section walks through the first few required API features in the
`vk.xml` tag: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 tag:feature:

[source,xml]
--------------------------------------
<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 tag:require block says to require a type named `vk_platform`.
If you look at the beginning of the tag:types section, there is a
corresponding definition section:

[source,xml]
--------------------------------------
<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 tag: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 tag:require block pulls in some compile time constants.
These correspond to the definitions found in the first tag:enums section of
`vk.xml`:

[source,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 tag:require block starts pulling in some Vulkan commands.
The first command corresponds to the following definition found in the
tag:commands section of `vk.xml`:

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

For `VkResult`, the corresponding required tag:type is:

[source,xml]
--------------------------------------
<type name="VkResult" category="enum"/>
--------------------------------------

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

[source,xml]
--------------------------------------
<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 tag:type is:

[source,xml]
--------------------------------------
<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 tag: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
tag:type is:

[source,xml]
--------------------------------------
    <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 tag:feature dependencies in `vk.xml`, the resulting
`vulkan_core.h` header will look like this:

[source,c]
--------------------------------------
#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.


[[compile-time-constants]]
== How To Add A Compile Time Constant

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

[source,xml]
--------------------------------------
<require comment="API constants">
--------------------------------------

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

[source,xml]
--------------------------------------
    <enum name="VK_THE_ANSWER"/>
--------------------------------------

Then go to the tag:enums block labelled

[source,xml]
--------------------------------------
<enums comment="Misc. hardcoded constants - not an enumerated type">
--------------------------------------

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

[source,xml]
--------------------------------------
    <enum value="42" type="uint32_t" name="VK_THE_ANSWER"/>
--------------------------------------

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


[[compile-time-constants-format]]
== Allowed Format of Compile Time Constants

The attr: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 `)`.


== 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:

[source,c]
--------------------------------------
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 tag: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
tag:type dependency should be added to the tag:feature tag.
Go to the tag:types tag and search for the nested block labelled

[source,xml]
--------------------------------------
<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 attr:name attribute matches the attr:name of
the struct type being defined:

[source,xml]
--------------------------------------
<require comment="API types not used by commands">
    <type name="VkInstanceCreateInfo"/>
    ...
--------------------------------------

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

[source,xml]
--------------------------------------
<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 tag: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 attr:category attribute to `"union"` instead of `"struct"`.


== How To Add An Enumerated Type

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

[source,c]
--------------------------------------
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 tag: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 tag:type dependency should be added to the tag:feature tag in
exactly the same fashion as described above for `struct` types.

Next, go to the line labelled

[source,xml]
--------------------------------------
<comment>Vulkan enumerant (token) definitions</comment>
--------------------------------------

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

[source,xml]
--------------------------------------
<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 tag:enums tag can be set.
In this case, the attr:type attribute is set to `"bitmask"`, indicating that
the individual enumerants represent elements of a bitmask.

The individual tag: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 attr: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 attr:value
attribute, in which case the specified numeric value is passed through to
the C header unchanged.


[[adding-bitflags]]
== 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:

[source,c]
--------------------------------------
// 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 tag:type dependency should be added to the tag: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:

[source,xml]
--------------------------------------
<comment>Bitmask types</comment>
--------------------------------------

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

[source,xml]
--------------------------------------
<type bitvalues="VkExampleFlagBits" category="bitmask">typedef <type>VkFlags64</type> <name>VkExampleFlags</name>;</type>
--------------------------------------

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

Next, go to the line labelled:

[source,xml]
--------------------------------------
<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:

[source,xml]
--------------------------------------
<type name="VkExampleFlagBits" category="enum"/>
--------------------------------------

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

Finally, go to the line labelled:

[source,xml]
--------------------------------------
<comment>Vulkan enumerant (token) definitions</comment>
--------------------------------------

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

[source,xml]
--------------------------------------
<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 attr:type attribute is set to `"bitmask"`, indicating that the
individual enumerants represent elements of a bitmask.
The attr:bitwidth attribute is set to `"64"` indicating that this is a
64-bit flag type.

The individual tag: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
attr:value attribute defining it to have a hard value of 0.
The other types have their values are specified using the attr: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.


=== 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:

[source,c]
--------------------------------------
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
attr:bitvalues attribute:

[source,xml]
--------------------------------------
<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:

[source,xml]
--------------------------------------
<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.


== How to Add A Command

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

[source,c]
--------------------------------------
VKAPI_ATTR VkResult VKAPI_CALL vkCreateInstance(
    const VkInstanceCreateInfo*                 pCreateInfo,
    VkInstance*                                 pInstance);
--------------------------------------

Commands must always be explicitly required in the tag:feature tag.
In that tag, you can use an existing tag: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
tag:command tag inside that block:

[source,xml]
--------------------------------------
<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 tag:require block may include a attr: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 tag:commands tag and add a new tag:command tag defining the
command, preferably sorted into alphabetic order with other commands for
ease of reading, as follows:

[source,xml]
--------------------------------------
<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 tag:proto tag defines the return type and function name of the command.
The tag: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 tag:member tags
described previously.


== More Complicated API Representations

The registry schema can represent a good deal of additional information, for
example by creating multiple tag: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.


== 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.


== 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.


[[general:stability]]
== 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 <<changelog,change log>> of
this document.
Changes to the `.xml` files and Python scripts are logged in Github history.


[[changelog]]
= Change Log

  * 2021-08-22 - Update introductory descriptions of toolchain and scripts.
  * 2021-08-15 - Add an explicit description of the tag:enum attr:extends
    attribute as introducing a requirement for the enumerated type being
    extended.
  * 2021-07-12 - Note that tag: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 attr:supported attribute of
    <<tag-extension, tag:extension tags>> to mandate that `disabled`
    extensions are not processed (public issue 1549).
  * 2021-04-21 - Add the attr:limittype attribute to <<tag-type, structure
    tag:member tags>>, to describe how queried limits are interpreted
    (internal issue 2427).
  * 2021-03-30 - Add a description of the <<compile-time-constants-format,
    allowed format of compile time constants>> (internal merge request
    4451).
  * 2021-03-22 - Update allowed values for the attr:type attribute of
    tag:enum tags and make it mandatory (internal issue 2564).
  * 2021-01-11 - Expand the scope of the attr:optional attribute tag:member
    and tag: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 <<tag:type, tag: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 attr:noautovalidity attribute on
    the `pNext` member of extending structures in the <<tag:type, tag: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 <<schema:apiname, API Names>> to
    current usage, including allowing specifying multiple API names for a
    given feature or extension.
  * 2020-04-29 - Expand use of attr:category `basetype` in tag:type tags to
    include external API types.
  * 2020-02-20 - Clarify that tag:enum tags inside tag:remove tags must be
    reference enums, not containing attributes defining values.
  * 2020-01-13 - Restrict attr:bitpos to [0,30] to avoid poorly defined
    compiler behavior.
  * 2019-08-25 - Add attr:sortorder attribute to tag:feature and
    tag: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 attr:optional
    member and parameter attributes, if not specified.
  * 2018-08-28 - Add optional attr:provisional attribute to tag:extension
    tags.
  * 2018-07-07 - Add optional attr:promotedto, attr:deprecatedby, and
    attr:obsoletedby attributes to tag:extension tags.
  * 2018-06-25 - Remove attr:vendorids tags for Khronos vendor IDs.
  * 2018-05-08 - Add tag:driverids and tag:driverid tags for describing
    Vulkan driver implementation identification information.
  * 2018-04-15 - Add attr:requiresCore.
  * 2018-03-07 - Updated for Vulkan 1.1 release.
  * 2018-02-21 - Add descriptions of the attr:extnumber and attr:alias
    attributes used for defining tag:enum attributes, the attr:alias
    attribute used for defining tag:type aliases, the attr:name and
    attr:alias attributes used for defining tag:command aliases, the
    attr:platform attribute of tag:extension tags, and the attr:feature
    attribute of tag:require tags; and update the document to the header
    naming and grouping scheme used starting in Vulkan 1.1.
  * 2018-01-07 - Add tag:platforms and tag:platform tags for describing
    Vulkan platform names and preprocessor symbols.
  * 2017-09-10 - Define syntax of member and parameter attr:altlen
    attributes, for use by code generators.
  * 2017-09-01 - Define syntax of member and parameter attr:len attributes
    consistently and correctly for current uses of latexmath:
  * 2017-08-24 - Note that the tag:extension attribute attr: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 tag:validity and tag: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 attr:type and attr:requires attributes to tag:extension
    tags.
  * 2016-02-22 - Change math markup in attr:len attributes to use asciidoc
    `latexmath:[$` and `$]` delimiters.
  * 2016-02-19 - Add attr:successcodes and attr:errorcodes attributes of
    tag:command tags.
    Add a subsection to the introduction describing the schema choices and
    how to file issues against the registry.
  * 2016-02-07 - Add attr:vendorids tags for Khronos vendor IDs.
  * 2015-12-10 - Add attr:author and attr:contact attributes for
    tag:extension tags.
  * 2015-12-07 - Move `vulkan/vulkan.h` to a subdirectory.
  * 2015-12-01 - Add tag: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 tag:validity tags.
    Still out of date WRT extension enumerants, but that will change soon.
  * 2015-09-08 - Rename `threadsafe` attribute to attr:externsync, and
    `implicitunsafeparams` tag to attr:implicitexternsync.
  * 2015-09-07 - Update tag:command tag description to remove the
    attr:threadsafe attribute and replace it with a combination of
    attr:threadunsafe attributes on individual parameters, and
    tag:implicitunsafeparams tags describing additional unsafe objects for
    the command.
  * 2015-08-04 - Add `basetype` and `funcpointer` attr: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 attr:threadsafe, attr:queues, and attr:renderpass
    attributes of <<tag-command,tag:command>> tags, and of attr:modified
    attributes of <<tag-command:param,tag:param>> tags.
  * 2015-06-17 - add descriptions of allowed attr:category attribute values
    of tag:type tags, used to group and sort related categories of
    declarations together in the generated header.
  * 2015-06-04 - Add <<examples,examples of making changes and additions>>
    to the registry.
  * 2015-06-03 - Move location to new `vulkan` Git repository.
    Add definition of tag:type tags for C struct/unions.
    Start adding <<examples,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.