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

// Copyright (c) 2015-2017 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

[[fundamentals]]
= Fundamentals

This chapter introduces fundamental concepts including the Vulkan
architecture and execution model, API syntax, queues, pipeline
configurations, numeric representation, state and state queries, and the
different types of objects and shaders.
It provides a framework for interpreting more specific descriptions of
commands and behavior in the remainder of the Specification.


[[fundamentals-architecture-model]]
== Architecture Model

Vulkan is designed for, and the API is written for, CPU, GPU, and other
hardware accelerator architectures with the following properties:

  * Runtime support for 8, 16, 32 and 64-bit signed and unsigned
    twos-complement integers, all addressable at the granularity of their
    size in bytes.
  * Runtime support for 32- and 64-bit floating-point types satisfying the
    range and precision constraints in the
    <<fundamentals-floatingpoint,Floating Point Computation>> section.
  * The representation and endianness of these types must: be identical for
    the host and the physical devices.

[NOTE]
.Note
====
Since a variety of data types and structures in Vulkan may: be mapped back
and forth between host and physical device memory, host and device
architectures must: both be able to access such data efficiently in order to
write portable and performant applications.
====

Where the Specification leaves choices open that would affect Application
Binary Interface compatibility on a given platform supporting Vulkan, those
choices are usually made to be compliant to the preferred ABI defined by the
platform vendor.
Some choices, such as function calling conventions, may: be made in
platform-specific portions of the +vk_platform.h+ header file.

[NOTE]
.Note
====
For example, the Android ABI is defined by Google, and the Linux ABI is
defined by a combination of gcc defaults, distribution vendor choices, and
external standards such as the Linux Standard Base.
====


[[fundamentals-execmodel]]
== Execution Model

This section outlines the execution model of a Vulkan system.

Vulkan exposes one or more _devices_, each of which exposes one or more
_queues_ which may: process work asynchronously to one another.
The set of queues supported by a device is partitioned into _families_.
Each family supports one or more types of functionality and may: contain
multiple queues with similar characteristics.
Queues within a single family are considered _compatible_ with one another,
and work produced for a family of queues can: be executed on any queue
within that family.
This specification defines four types of functionality that queues may:
support: graphics, compute, transfer, and sparse memory management.

[NOTE]
.Note
====
A single device may: report multiple similar queue families rather than, or
as well as, reporting multiple members of one or more of those families.
This indicates that while members of those families have similar
capabilities, they are _not_ directly compatible with one another.
====

Device memory is explicitly managed by the application.
Each device may: advertise one or more heaps, representing different areas
of memory.
Memory heaps are either device local or host local, but are always visible
to the device.
Further detail about memory heaps is exposed via memory types available on
that heap.
Examples of memory areas that may: be available on an implementation
include:

  * _device local_ is memory that is physically connected to the device.
  * _device local, host visible_ is device local memory that is visible to
    the host.
  * _host local, host visible_ is memory that is local to the host and
    visible to the device and host.

On other architectures, there may: only be a single heap that can: be used
for any purpose.

A Vulkan application controls a set of devices through the submission of
command buffers which have recorded device commands issued via Vulkan
library calls.
The content of command buffers is specific to the underlying hardware and is
opaque to the application.
Once constructed, a command buffer can: be submitted once or many times to a
queue for execution.
Multiple command buffers can: be built in parallel by employing multiple
threads within the application.

Command buffers submitted to different queues may: execute in parallel or
even out of order with respect to one another.
Command buffers submitted to a single queue respect
<<synchronization-submission-order, submission order>>, as described further
in <<synchronization, synchronization chapter>>.
Command buffer execution by the device is also asynchronous to host
execution.
Once a command buffer is submitted to a queue, control may: return to the
application immediately.
Synchronization between the device and host, and between different queues is
the responsibility of the application.


[[fundamentals-queueoperation]]
=== Queue Operation

Vulkan queues provide an interface to the execution engines of a device.
Commands for these execution engines are recorded into command buffers ahead
of execution time.
These command buffers are then submitted to queues with a _queue submission_
command for execution in a number of _batches_.
Once submitted to a queue, these commands will begin and complete execution
without further application intervention, though the order of this execution
is dependent on a number of <<synchronization, implicit and explicit
ordering constraints>>.

Work is submitted to queues using queue submission commands that typically
take the form ftext:vkQueue* (e.g. flink:vkQueueSubmit,
flink:vkQueueBindSparse), and optionally take a list of semaphores upon
which to wait before work begins and a list of semaphores to signal once
work has completed.
The work itself, as well as signaling and waiting on the semaphores are all
_queue operations_.

Queue operations on different queues have no implicit ordering constraints,
and may: execute in any order.
Explicit ordering constraints between queues can: be expressed with
<<synchronization-semaphores,semaphores>> and
<<synchronization-fences,fences>>.

Command buffer submissions to a single queue respect
<<synchronization-submission-order, submission order>> and other
<<synchronization-implicit, implicit ordering guarantees>>, but otherwise
may: overlap or execute out of order.
Other types of batches and queue submissions against a single queue (e.g.
<<sparsemem-memory-binding, sparse memory binding>>) have no implicit
ordering constraints with any other queue submission or batch.
Additional explicit ordering constraints between queue submissions and
individual batches can be expressed with
<<synchronization-semaphores,semaphores>> and
<<synchronization-fences,fences>>.

Before a fence or semaphore is signaled, it is guaranteed that any
previously submitted queue operations have completed execution, and that
memory writes from those queue operations are
<<synchronization-dependencies-available-and-visible,available>> to future
queue operations.
Waiting on a signaled semaphore or fence guarantees that previous writes
that are available are also
<<synchronization-dependencies-available-and-visible,visible>> to subsequent
commands.

Command buffer boundaries, both between primary command buffers of the same
or different batches or submissions as well as between primary and secondary
command buffers, do not introduce any additional ordering constraints.
In other words, submitting the set of command buffers (which can: include
executing secondary command buffers) between any semaphore or fence
operations execute the recorded commands as if they had all been recorded
into a single primary command buffer, except that the current state is
<<commandbuffers-statereset,reset>> on each boundary.
Explicit ordering constraints can: be expressed with <<synchronization,
explicit synchronization primitives>>.

There are a few <<synchronization-implicit, implicit ordering guarantees>>
between commands within a command buffer, but only covering a subset of
execution.
Additional explicit ordering constraints can be expressed with the various
<<synchronization, explicit synchronization primitives>>.

[NOTE]
.Note
====
Implementations have significant freedom to overlap execution of work
submitted to a queue, and this is common due to deep pipelining and
parallelism in Vulkan devices.
====

[[fundamentals-queueoperation-command-types]]
Commands recorded in command buffers either perform actions (draw, dispatch,
clear, copy, query/timestamp operations, begin/end subpass operations), set
state (bind pipelines, descriptor sets, and buffers, set dynamic state, push
constants, set render pass/subpass state), or perform synchronization
(set/wait events, pipeline barrier, render pass/subpass dependencies).
Some commands perform more than one of these tasks.
State setting commands update the _current state_ of the command buffer.
Some commands that perform actions (e.g. draw/dispatch) do so based on the
current state set cumulatively since the start of the command buffer.
The work involved in performing action commands is often allowed to overlap
or to be reordered, but doing so must: not alter the state to be used by
each action command.
In general, action commands are those commands that alter framebuffer
attachments, read/write buffer or image memory, or write to query pools.

Synchronization commands introduce explicit
<<synchronization-dependencies,execution and memory dependencies>> between
two sets of action commands, where the second set of commands depends on the
first set of commands.
These dependencies enforce that both the execution of certain
<<synchronization-pipeline-stages, pipeline stages>> in the later set occur
after the execution of certain stages in the source set, and that the
effects of <<synchronization-global-memory-barriers,memory accesses>>
performed by certain pipeline stages occur in order and are visible to each
other.
When not enforced by an explicit dependency or <<synchronization-implicit,
implicit ordering guarantees>>, action commands may: overlap execution or
execute out of order, and may: not see the side effects of each other's
memory accesses.

The device executes queue operations asynchronously with respect to the
host.
Control is returned to an application immediately following command buffer
submission to a queue.
The application must: synchronize work between the host and device as
needed.


[[fundamentals-objectmodel-overview]]
== Object Model

The devices, queues, and other entities in Vulkan are represented by Vulkan
objects.
At the API level, all objects are referred to by handles.
There are two classes of handles, dispatchable and non-dispatchable.
_Dispatchable_ handle types are a pointer to an opaque type.
This pointer may: be used by layers as part of intercepting API commands,
and thus each API command takes a dispatchable type as its first parameter.
Each object of a dispatchable type must: have a unique handle value during
its lifetime.

_Non-dispatchable_ handle types are a 64-bit integer type whose meaning is
implementation-dependent, and may: encode object information directly in the
handle rather than pointing to a software structure.
Objects of a non-dispatchable type may: not have unique handle values within
a type or across types.
If handle values are not unique, then destroying one such handle must: not
cause identical handles of other types to become invalid, and must: not
cause identical handles of the same type to become invalid if that handle
value has been created more times than it has been destroyed.

All objects created or allocated from a sname:VkDevice (i.e. with a
sname:VkDevice as the first parameter) are private to that device, and must:
not be used on other devices.


[[fundamentals-objectmodel-lifetime]]
=== Object Lifetime

Objects are created or allocated by ftext:vkCreate* and ftext:vkAllocate*
commands, respectively.
Once an object is created or allocated, its "`structure`" is considered to
be immutable, though the contents of certain object types is still free to
change.
Objects are destroyed or freed by ftext:vkDestroy* and ftext:vkFree*
commands, respectively.

Objects that are allocated (rather than created) take resources from an
existing pool object or memory heap, and when freed return resources to that
pool or heap.
While object creation and destruction are generally expected to be
low-frequency occurrences during runtime, allocating and freeing objects
can: occur at high frequency.
Pool objects help accommodate improved performance of the allocations and
frees.

It is an application's responsibility to track the lifetime of Vulkan
objects, and not to destroy them while they are still in use.

Application-owned memory is immediately consumed by any Vulkan command it is
passed into.
The application can: alter or free this memory as soon as the commands that
consume it have returned.

The following object types are consumed when they are passed into a Vulkan
command and not further accessed by the objects they are used to create.
They must: not be destroyed in the duration of any API command they are
passed into:

  * sname:VkShaderModule
  * sname:VkPipelineCache

A sname:VkRenderPass object passed as a parameter to create another object
is not further accessed by that object after the duration of the command it
is passed into.
A sname:VkRenderPass used in a command buffer follows the rules described
below.

A sname:VkPipelineLayout object must: not be destroyed while any command
buffer that uses it is in the recording state.

sname:VkDescriptorSetLayout objects may: be accessed by commands that
operate on descriptor sets allocated using that layout, and those descriptor
sets must: not be updated with flink:vkUpdateDescriptorSets after the
descriptor set layout has been destroyed.
Otherwise, descriptor set layouts can: be destroyed any time they are not in
use by an API command.

The application must: not destroy any other type of Vulkan object until all
uses of that object by the device (such as via command buffer execution)
have completed.

The following Vulkan objects must: not be destroyed while any command
buffers using the object are in the <<commandbuffers-lifecycle, pending
state>>:

  * sname:VkEvent
  * sname:VkQueryPool
  * sname:VkBuffer
  * sname:VkBufferView
  * sname:VkImage
  * sname:VkImageView
  * sname:VkPipeline
  * sname:VkSampler
  * sname:VkDescriptorPool
  * sname:VkFramebuffer
  * sname:VkRenderPass
  * sname:VkCommandBuffer
  * sname:VkCommandPool
  * sname:VkDeviceMemory
  * sname:VkDescriptorSet
ifdef::VK_NVX_device_generated_commands[]
  * sname:VkObjectTableNVX
  * sname:VkIndirectCommandsLayout
endif::VK_NVX_device_generated_commands[]

Destroying these objects will move any command buffers that are in the
<<commandbuffers-lifecycle, recording or executable state>>, and are using
those objects, to the <<commandbuffers-lifecycle, invalid state>>.

The following Vulkan objects must: not be destroyed while any queue is
executing commands that use the object:

  * sname:VkFence
  * sname:VkSemaphore
  * sname:VkCommandBuffer
  * sname:VkCommandPool

In general, objects can: be destroyed or freed in any order, even if the
object being freed is involved in the use of another object (e.g. use of a
resource in a view, use of a view in a descriptor set, use of an object in a
command buffer, binding of a memory allocation to a resource), as long as
any object that uses the freed object is not further used in any way except
to be destroyed or to be reset in such a way that it no longer uses the
other object (such as resetting a command buffer).
If the object has been reset, then it can: be used as if it never used the
freed object.
An exception to this is when there is a parent/child relationship between
objects.
In this case, the application must: not destroy a parent object before its
children, except when the parent is explicitly defined to free its children
when it is destroyed (e.g. for pool objects, as defined below).

sname:VkCommandPool objects are parents of sname:VkCommandBuffer objects.
sname:VkDescriptorPool objects are parents of sname:VkDescriptorSet objects.
sname:VkDevice objects are parents of many object types (all that take a
sname:VkDevice as a parameter to their creation).

The following Vulkan objects have specific restrictions for when they can:
be destroyed:

  * sname:VkQueue objects cannot: be explicitly destroyed.
    Instead, they are implicitly destroyed when the sname:VkDevice object
    they are retrieved from is destroyed.
  * Destroying a pool object implicitly frees all objects allocated from
    that pool.
    Specifically, destroying sname:VkCommandPool frees all
    sname:VkCommandBuffer objects that were allocated from it, and
    destroying sname:VkDescriptorPool frees all sname:VkDescriptorSet
    objects that were allocated from it.
  * sname:VkDevice objects can: be destroyed when all sname:VkQueue objects
    retrieved from them are idle, and all objects created from them have
    been destroyed.
    This includes the following objects:
  ** sname:VkFence
  ** sname:VkSemaphore
  ** sname:VkEvent
  ** sname:VkQueryPool
  ** sname:VkBuffer
  ** sname:VkBufferView
  ** sname:VkImage
  ** sname:VkImageView
  ** sname:VkShaderModule
  ** sname:VkPipelineCache
  ** sname:VkPipeline
  ** sname:VkPipelineLayout
  ** sname:VkSampler
  ** sname:VkDescriptorSetLayout
  ** sname:VkDescriptorPool
  ** sname:VkFramebuffer
  ** sname:VkRenderPass
  ** sname:VkCommandPool
  ** sname:VkCommandBuffer
  ** sname:VkDeviceMemory
  * sname:VkPhysicalDevice objects cannot: be explicitly destroyed.
    Instead, they are implicitly destroyed when the sname:VkInstance object
    they are retrieved from is destroyed.
  * sname:VkInstance objects can: be destroyed once all sname:VkDevice
    objects created from any of its sname:VkPhysicalDevice objects have been
    destroyed.


ifdef::VK_KHR_external_memory_capabilities,VK_KHR_external_semaphore_capabilities,VK_KHR_external_fence_capabilities[]
[[fundamentals-objectmodel-external]]
=== External Object Handles

As defined above, the scope of object handles created or allocated from a
sname:VkDevice is limited to that logical device.
Objects which are not in scope are said to be external.
To bring an external object into scope, an external handle must: be exported
from the object in the source scope and imported into the destination scope.

[NOTE]
.Note
====
The scope of external handles and their associated resources may: vary
according to their type, but they can: generally be shared across process
and API boundaries.
====
endif::VK_KHR_external_memory_capabilities,VK_KHR_external_semaphore_capabilities,VK_KHR_external_fence_capabilities[]


[[fundamentals-commandsyntax]]
== Command Syntax and Duration

The Specification describes Vulkan commands as functions or procedures using
C99 syntax.
Language bindings for other languages such as C++ and JavaScript may: allow
for stricter parameter passing, or object-oriented interfaces.

Vulkan uses the standard C types for the base type of scalar parameters
(e.g. types from +stdint.h+), with exceptions described below, or elsewhere
in the text when appropriate:

[open,refpage='VkBool32',desc='Vulkan boolean type',type='basetypes']
--

basetype:VkBool32 represents boolean `True` and `False` values, since C does
not have a sufficiently portable built-in boolean type:

include::../api/basetypes/VkBool32.txt[]

ename:VK_TRUE represents a boolean *True* (integer 1) value, and
ename:VK_FALSE a boolean *False* (integer 0) value.

All values returned from a Vulkan implementation in a basetype:VkBool32 will
be either ename:VK_TRUE or ename:VK_FALSE.

Applications must: not pass any other values than ename:VK_TRUE or
ename:VK_FALSE into a Vulkan implementation where a basetype:VkBool32 is
expected.

--

[open,refpage='VkDeviceSize',desc='Vulkan device memory size and offsets',type='basetypes']
--

basetype:VkDeviceSize represents device memory size and offset values:

include::../api/basetypes/VkDeviceSize.txt[]

--

Commands that create Vulkan objects are of the form ftext:vkCreate* and take
stext:Vk*CreateInfo structures with the parameters needed to create the
object.
These Vulkan objects are destroyed with commands of the form
ftext:vkDestroy*.

The last in-parameter to each command that creates or destroys a Vulkan
object is pname:pAllocator.
The pname:pAllocator parameter can: be set to a non-`NULL` value such that
allocations for the given object are delegated to an application provided
callback; refer to the <<memory-allocation,Memory Allocation>> chapter for
further details.

Commands that allocate Vulkan objects owned by pool objects are of the form
ftext:vkAllocate*, and take stext:Vk*AllocateInfo structures.
These Vulkan objects are freed with commands of the form ftext:vkFree*.
These objects do not take allocators; if host memory is needed, they will
use the allocator that was specified when their parent pool was created.

Commands are recorded into a command buffer by calling API commands of the
form ftext:vkCmd*.
Each such command may: have different restrictions on where it can: be used:
in a primary and/or secondary command buffer, inside and/or outside a render
pass, and in one or more of the supported queue types.
These restrictions are documented together with the definition of each such
command.

The _duration_ of a Vulkan command refers to the interval between calling
the command and its return to the caller.


[[fundamentals-commandsyntax-results-lifetime]]
=== Lifetime of Retrieved Results

Information is retrieved from the implementation with commands of the form
ftext:vkGet* and ftext:vkEnumerate*.

Unless otherwise specified for an individual command, the results are
_invariant_; that is, they will remain unchanged when retrieved again by
calling the same command with the same parameters, so long as those
parameters themselves all remain valid.


[[fundamentals-threadingbehavior]]
== Threading Behavior

Vulkan is intended to provide scalable performance when used on multiple
host threads.
All commands support being called concurrently from multiple threads, but
certain parameters, or components of parameters are defined to be
_externally synchronized_.
This means that the caller must: guarantee that no more than one thread is
using such a parameter at a given time.

More precisely, Vulkan commands use simple stores to update software
structures representing Vulkan objects.
A parameter declared as externally synchronized may: have its software
structures updated at any time during the host execution of the command.
If two commands operate on the same object and at least one of the commands
declares the object to be externally synchronized, then the caller must:
guarantee not only that the commands do not execute simultaneously, but also
that the two commands are separated by an appropriate memory barrier (if
needed).

[NOTE]
.Note
====
Memory barriers are particularly relevant on the ARM CPU architecture which
is more weakly ordered than many developers are accustomed to from x86/x64
programming.
Fortunately, most higher-level synchronization primitives (like the pthread
library) perform memory barriers as a part of mutual exclusion, so mutexing
Vulkan objects via these primitives will have the desired effect.
====

Many object types are _immutable_, meaning the objects cannot: change once
they have been created.
These types of objects never need external synchronization, except that they
must: not be destroyed while they are in use on another thread.
In certain special cases, mutable object parameters are internally
synchronized such that they do not require external synchronization.
One example of this is the use of a sname:VkPipelineCache in
fname:vkCreateGraphicsPipelines and fname:vkCreateComputePipelines, where
external synchronization around such a heavyweight command would be
impractical.
The implementation must: internally synchronize the cache in this example,
and may: be able to do so in the form of a much finer-grained mutex around
the command.
Any command parameters that are not labeled as externally synchronized are
either not mutated by the command or are internally synchronized.
Additionally, certain objects related to a command's parameters (e.g.
command pools and descriptor pools) may: be affected by a command, and must:
also be externally synchronized.
These implicit parameters are documented as described below.

Parameters of commands that are externally synchronized are listed below.

include::../hostsynctable/parameters.txt[]

There are also a few instances where a command can: take in a user allocated
list whose contents are externally synchronized parameters.
In these cases, the caller must: guarantee that at most one thread is using
a given element within the list at a given time.
These parameters are listed below.

include::../hostsynctable/parameterlists.txt[]

In addition, there are some implicit parameters that need to be externally
synchronized.
For example, all pname:commandBuffer parameters that need to be externally
synchronized imply that the pname:commandPool that was passed in when
creating that command buffer also needs to be externally synchronized.
The implicit parameters and their associated object are listed below.

include::../hostsynctable/implicit.txt[]


[[fundamentals-errors]]
== Errors

Vulkan is a layered API.
The lowest layer is the core Vulkan layer, as defined by this Specification.
The application can: use additional layers above the core for debugging,
validation, and other purposes.

One of the core principles of Vulkan is that building and submitting command
buffers should: be highly efficient.
Thus error checking and validation of state in the core layer is minimal,
although more rigorous validation can: be enabled through the use of layers.

The core layer assumes applications are using the API correctly.
Except as documented elsewhere in the Specification, the behavior of the
core layer to an application using the API incorrectly is undefined, and
may: include program termination.
However, implementations must: ensure that incorrect usage by an application
does not affect the integrity of the operating system, the Vulkan
implementation, or other Vulkan client applications in the system, and does
not allow one application to access data belonging to another application.
Applications can: request stronger robustness guarantees by enabling the
pname:robustBufferAccess feature as described in <<features>>.

Validation of correct API usage is left to validation layers.
Applications should: be developed with validation layers enabled, to help
catch and eliminate errors.
Once validated, released applications should: not enable validation layers
by default.


[[fundamentals-validusage]]
=== Valid Usage

Valid usage defines a set of conditions which must: be met in order to
achieve well-defined run-time behavior in an application.
These conditions depend only on Vulkan state, and the parameters or objects
whose usage is constrained by the condition.

Some valid usage conditions have dependencies on run-time limits or feature
availability.
It is possible to validate these conditions against Vulkan's minimum
supported values for these limits and features, or some subset of other
known values.

Valid usage conditions do not cover conditions where well-defined behavior
(including returning an error code) exists.

Valid usage conditions should: apply to the command or structure where
complete information about the condition would be known during execution of
an application.
This is such that a validation layer or linter can: be written directly
against these statements at the point they are specified.

[NOTE]
.Note
====
This does lead to some non-obvious places for valid usage statements.
For instance, the valid values for a structure might depend on a separate
value in the calling command.
In this case, the structure itself will not reference this valid usage as it
is impossible to determine validity from the structure that it is invalid -
instead this valid usage would be attached to the calling command.

Another example is draw state - the state setters are independent, and can
cause a legitimately invalid state configuration between draw calls; so the
valid usage statements are attached to the place where all state needs to be
valid - at the draw command.
====

Valid usage conditions are described in a block labelled "`Valid Usage`"
following each command or structure they apply to.


[[fundamentals-implicit-validity]]
=== Implicit Valid Usage

Some valid usage conditions apply to all commands and structures in the API,
unless explicitly denoted otherwise for a specific command or structure.
These conditions are considered _implicit_, and are described in a block
labelled "`Valid Usage (Implicit)`" following each command or structure they
apply to.
Implicit valid usage conditions are described in detail below.


[[fundamentals-validusage-handles]]
==== Valid Usage for Object Handles

Any input parameter to a command that is an object handle must: be a valid
object handle, unless otherwise specified.
An object handle is valid if:

  * It has been created or allocated by a previous, successful call to the
    API.
    Such calls are noted in the specification.
  * It has not been deleted or freed by a previous call to the API.
    Such calls are noted in the specification.
  * Any objects used by that object, either as part of creation or
    execution, must: also be valid.

The reserved values dlink:VK_NULL_HANDLE and `NULL` can: be used in place of
valid non-dispatchable handles and dispatchable handles, respectively, when
_explicitly called out in the specification_.
Any command that creates an object successfully must: not return these
values.
It is valid to pass these values to ftext:vkDestroy* or ftext:vkFree*
commands, which will silently ignore these values.


[[fundamentals-validusage-pointers]]
==== Valid Usage for Pointers

Any parameter that is a pointer must: either be a valid pointer, or if
_explicitly called out in the specification_, can: be `NULL`.
A pointer is valid if it points at memory containing values of the number
and type(s) expected by the command, and all fundamental types accessed
through the pointer (e.g. as elements of an array or as members of a
structure) satisfy the alignment requirements of the host processor.


[[fundamentals-validusage-strings]]
==== Valid Usage for Strings

Any parameter that is a pointer to `char` must: be a finite sequence of
values terminated by a null character, or if _explicitly called out in the
specification_, can: be `NULL`.


[[fundamentals-validusage-enums]]
==== Valid Usage for Enumerated Types

Any parameter of an enumerated type must: be a valid enumerant for that
type.
A enumerant is valid if:

  * The enumerant is defined as part of the enumerated type.
  * The enumerant is not one of the special values defined for the
    enumerated type, which are suffixed with etext:_BEGIN_RANGE,
    etext:_END_RANGE, etext:_RANGE_SIZE or etext:_MAX_ENUM.

Any enumerated type returned from a query command or otherwise output from
Vulkan to the application must: not have a reserved value.
Reserved values are values not defined by any extension for that enumerated
type.

[NOTE]
.Note
====
This language is intended to accomodate cases such as "`hidden`" extensions
known only to driver internals, or layers enabling extensions without
knowledge of the application, without allowing return of values not defined
by any extension.
====


[[fundamentals-validusage-flags]]
==== Valid Usage for Flags

[open,refpage='VkFlags',desc='Vulkan bitmasks',type='basetypes',xrefs='VkColorComponentFlags']
--

A collection of flags is represented by a bitmask using the type
basetype:VkFlags:

include::../api/basetypes/VkFlags.txt[]

Bitmasks are passed to many commands and structures to compactly represent
options, but basetype:VkFlags is not used directly in the API.
Instead, a etext:Vk*Flags type which is an alias of basetype:VkFlags, and
whose name matches the corresponding etext:Vk*FlagBits that are valid for
that type, is used.
These aliases are described in the <<boilerplate-flags,Flag Types>> appendix
of the Specification.

--

Any etext:Vk*Flags member or parameter used in the API as an input must: be
a valid combination of bit flags.
A valid combination is either zero or the bitwise OR of valid bit flags.
A bit flag is valid if:

  * The bit flag is defined as part of the etext:Vk*FlagBits type, where the
    bits type is obtained by taking the flag type and replacing the trailing
    etext:Flags with etext:FlagBits.
    For example, a flag value of type elink:VkColorComponentFlags must:
    contain only bit flags defined by elink:VkColorComponentFlagBits.
  * The flag is allowed in the context in which it is being used.
    For example, in some cases, certain bit flags or combinations of bit
    flags are mutually exclusive.

Any etext:Vk*Flags member or parameter returned from a query command or
otherwise output from Vulkan to the application may: contain bit flags
undefined in its corresponding etext:Vk*FlagBits type.
An application cannot: rely on the state of these unspecified bits.


[[fundamentals-validusage-sType]]
==== Valid Usage for Structure Types

Any parameter that is a structure containing a pname:sType member must: have
a value of ptext:sType which is a valid elink:VkStructureType value matching
the type of the structure.
As a general rule, the name of this value is obtained by taking the
structure name, stripping the leading etext:Vk, prefixing each capital
letter with etext:_, converting the entire resulting string to upper case,
and prefixing it with etext:VK_STRUCTURE_TYPE_.
For example, structures of type sname:VkImageCreateInfo must: have a
ptext:sType value of ename:VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO.

The values ename:VK_STRUCTURE_TYPE_LOADER_INSTANCE_CREATE_INFO and
ename:VK_STRUCTURE_TYPE_LOADER_DEVICE_CREATE_INFO are reserved for internal
use by the loader, and do not have corresponding Vulkan structures in this
specification.

The list of supported <<boilerplate-sType,structure types>> is defined in an
appendix.


[[fundamentals-validusage-pNext]]
==== Valid Usage for Structure Pointer Chains

Any parameter that is a structure containing a `void*` ptext:pNext member
must: have a value of pname:pNext that is either `NULL`, or points to a
valid structure defined by an extension, containing ptext:sType and
ptext:pNext members as described in the <<vulkan-styleguide,Vulkan
Documentation and Extensions>> document in the section "`Extension
Interactions`".
The set of structures connected by ptext:pNext pointers is referred to as a
_pname:pNext chain_.
If that extension is supported by the implementation, then it must: be
enabled.

Each type of valid structure must: not appear more than once in a
ptext:pNext chain.

Any component of the implementation (the loader, any enabled layers, and
drivers) must: skip over, without processing (other than reading the
pname:sType and pname:pNext members) any structures in the chain with
pname:sType values not defined by extensions supported by that component.

Extension structures are not described in the base Vulkan specification, but
either in layered specifications incorporating those extensions, or in
separate vendor-provided documents.


[[fundamentals-validusage-nested-structs]]
==== Valid Usage for Nested Structures

The above conditions also apply recursively to members of structures
provided as input to a command, either as a direct argument to the command,
or themselves a member of another structure.

Specifics on valid usage of each command are covered in their individual
sections.


[[fundamentals-returncodes]]
=== Return Codes

[open,refpage='VkResult',desc='Vulkan command return codes',type='enums',xrefs='TBD']
--

While the core Vulkan API is not designed to capture incorrect usage, some
circumstances still require return codes.
Commands in Vulkan return their status via return codes that are in one of
two categories:

  * Successful completion codes are returned when a command needs to
    communicate success or status information.
    All successful completion codes are non-negative values.
  * Run time error codes are returned when a command needs to communicate a
    failure that could only be detected at run time.
    All run time error codes are negative values.

All return codes in Vulkan are reported via elink:VkResult return values.
The possible codes are:

include::../api/enums/VkResult.txt[]

[[fundamentals-successcodes]]
.Success Codes
  * ename:VK_SUCCESS Command successfully completed
  * ename:VK_NOT_READY A fence or query has not yet completed
  * ename:VK_TIMEOUT A wait operation has not completed in the specified
    time
  * ename:VK_EVENT_SET An event is signaled
  * ename:VK_EVENT_RESET An event is unsignaled
  * ename:VK_INCOMPLETE A return array was too small for the result
ifdef::VK_KHR_swapchain[]
  * ename:VK_SUBOPTIMAL_KHR A swapchain no longer matches the surface
    properties exactly, but can: still be used to present to the surface
    successfully.
endif::VK_KHR_swapchain[]

[[fundamentals-errorcodes]]
.Error codes
  * ename:VK_ERROR_OUT_OF_HOST_MEMORY A host memory allocation has failed.
  * ename:VK_ERROR_OUT_OF_DEVICE_MEMORY A device memory allocation has
    failed.
  * ename:VK_ERROR_INITIALIZATION_FAILED Initialization of an object could
    not be completed for implementation-specific reasons.
  * ename:VK_ERROR_DEVICE_LOST The logical or physical device has been lost.
    See <<devsandqueues-lost-device,Lost Device>>
  * ename:VK_ERROR_MEMORY_MAP_FAILED Mapping of a memory object has failed.
  * ename:VK_ERROR_LAYER_NOT_PRESENT A requested layer is not present or
    could not be loaded.
  * ename:VK_ERROR_EXTENSION_NOT_PRESENT A requested extension is not
    supported.
  * ename:VK_ERROR_FEATURE_NOT_PRESENT A requested feature is not supported.
  * ename:VK_ERROR_INCOMPATIBLE_DRIVER The requested version of Vulkan is
    not supported by the driver or is otherwise incompatible for
    implementation-specific reasons.
  * ename:VK_ERROR_TOO_MANY_OBJECTS Too many objects of the type have
    already been created.
  * ename:VK_ERROR_FORMAT_NOT_SUPPORTED A requested format is not supported
    on this device.
  * ename:VK_ERROR_FRAGMENTED_POOL A pool allocation has failed due to
    fragmentation of the pool's memory.
    This must: only be returned if no attempt to allocate host or device
    memory was made to accomodate the new allocation.
ifdef::VK_KHR_maintenance1[]
    This should: be returned in preference to
    ename:VK_ERROR_OUT_OF_POOL_MEMORY_KHR, but only if the implementation is
    certain that the pool allocation failure was due to fragmentation.
endif::VK_KHR_maintenance1[]
ifdef::VK_KHR_surface[]
  * ename:VK_ERROR_SURFACE_LOST_KHR A surface is no longer available.
  * ename:VK_ERROR_NATIVE_WINDOW_IN_USE_KHR The requested window is already
    in use by Vulkan or another API in a manner which prevents it from being
    used again.
endif::VK_KHR_surface[]
ifdef::VK_KHR_swapchain[]
  * ename:VK_ERROR_OUT_OF_DATE_KHR A surface has changed in such a way that
    it is no longer compatible with the swapchain, and further presentation
    requests using the swapchain will fail.
    Applications must: query the new surface properties and recreate their
    swapchain if they wish to continue presenting to the surface.
endif::VK_KHR_swapchain[]
ifdef::VK_KHR_display_swapchain[]
  * ename:VK_ERROR_INCOMPATIBLE_DISPLAY_KHR The display used by a swapchain
    does not use the same presentable image layout, or is incompatible in a
    way that prevents sharing an image.
endif::VK_KHR_display_swapchain[]
ifdef::VK_NV_glsl_shader[]
  * ename:VK_ERROR_INVALID_SHADER_NV One or more shaders failed to compile
    or link.
    More details are reported back to the application via
    +VK_EXT_debug_report+ if enabled.
endif::VK_NV_glsl_shader[]
ifdef::VK_KHR_maintenance1[]
  * ename:VK_ERROR_OUT_OF_POOL_MEMORY_KHR A pool memory allocation has
    failed.
    This must: only be returned if no attempt to allocate host or device
    memory was made to accomodate the new allocation.
    If the failure was definitely due to fragmentation of the pool,
    ename:VK_ERROR_FRAGMENTED_POOL should: be returned instead.
endif::VK_KHR_maintenance1[]
ifdef::VK_KHR_external_memory,VK_KHR_external_semaphore,VK_KHR_external_fence[]
  * ename:VK_ERROR_INVALID_EXTERNAL_HANDLE_KHR An external handle is not a
    valid handle of the specified type.
endif::VK_KHR_external_memory,VK_KHR_external_semaphore,VK_KHR_external_fence[]

If a command returns a run time error, it will leave any result pointers
unmodified, unless other behavior is explicitly defined in the
specification.

Out of memory errors do not damage any currently existing Vulkan objects.
Objects that have already been successfully created can: still be used by
the application.

Performance-critical commands generally do not have return codes.
If a run time error occurs in such commands, the implementation will defer
reporting the error until a specified point.
For commands that record into command buffers (ftext:vkCmd*) run time errors
are reported by fname:vkEndCommandBuffer.

--


[[fundamentals-numerics]]
== Numeric Representation and Computation

Implementations normally perform computations in floating-point, and must:
meet the range and precision requirements defined under "`Floating-Point
Computation`" below.

These requirements only apply to computations performed in Vulkan operations
outside of shader execution, such as texture image specification and
sampling, and per-fragment operations.
Range and precision requirements during shader execution differ and are
specified by the <<spirvenv-precision-operation, Precision and Operation of
SPIR-V Instructions>> section.

In some cases, the representation and/or precision of operations is
implicitly limited by the specified format of vertex or texel data consumed
by Vulkan.
Specific floating-point formats are described later in this section.


[[fundamentals-floatingpoint]]
=== Floating-Point Computation

Most floating-point computation is performed in SPIR-V shader modules.
The properties of computation within shaders are constrained as defined by
the <<spirvenv-precision-operation, Precision and Operation of SPIR-V
Instructions>> section.

Some floating-point computation is performed outside of shaders, such as
viewport and depth range calculations.
For these computations, we do not specify how floating-point numbers are to
be represented, or the details of how operations on them are performed, but
only place minimal requirements on representation and precision as described
in the remainder of this section.

ifdef::editing-notes[]
[NOTE]
.editing-note
====
(Jon, Bug 14966) This is a rat's nest of complexity, both in terms of
describing/enumerating places such computation may: take place (other than
"`not shader code`") and in how implementations may: do it.
We have consciously deferred the resolution of this issue to post-1.0, and
in the meantime, the following language inherited from the OpenGL
Specification is inserted as a placeholder.
Hopefully it can: be tightened up considerably.
====
endif::editing-notes[]

We require simply that numbers' floating-point parts contain enough bits and
that their exponent fields are large enough so that individual results of
floating-point operations are accurate to about 1 part in 10^5^.
The maximum representable magnitude for all floating-point values must: be
at least 2^32^.

  :: [eq]#x {times} 0 = 0 {times} x = 0# for any non-infinite and
     non-[eq]#NaN# [eq]#x#.
  :: [eq]#1 {times} x = x {times} 1 = x#.
  :: [eq]#x {plus} 0 = 0 {plus} x = x#.
  :: [eq]#0^0^ = 1#.

Occasionally, further requirements will be specified.
Most single-precision floating-point formats meet these requirements.

The special values [eq]#Inf# and [eq]#-Inf# encode values with magnitudes
too large to be represented; the special value [eq]#NaN# encodes {ldquo}Not
A Number{rdquo} values resulting from undefined arithmetic operations such
as [eq]#0 / 0#.
Implementations may: support [eq]#Inf# and [eq]#NaN# in their floating-point
computations.

Any representable floating-point value is legal as input to a Vulkan command
that requires floating-point data.
The result of providing a value that is not a floating-point number to such
a command is unspecified, but must: not lead to Vulkan interruption or
termination.
In <<ieee-754,IEEE 754>> arithmetic, for example, providing a negative zero
or a denormalized number to an Vulkan command must: yield deterministic
results, while providing a [eq]#NaN# or [eq]#Inf# yields unspecified
results.


[[fundamentals-fp16]]
=== 16-Bit Floating-Point Numbers

16-bit floating point numbers are defined in the "`16-bit floating point
numbers`" section of the <<data-format,Khronos Data Format Specification>>.

Any representable 16-bit floating-point value is legal as input to a Vulkan
command that accepts 16-bit floating-point data.
The result of providing a value that is not a floating-point number (such as
[eq]#Inf# or [eq]#NaN#) to such a command is unspecified, but must: not lead
to Vulkan interruption or termination.
Providing a denormalized number or negative zero to Vulkan must: yield
deterministic results.


[[fundamentals-fp11]]
=== Unsigned 11-Bit Floating-Point Numbers

Unsigned 11-bit floating point numbers are defined in the "`Unsigned 11-bit
floating point numbers`" section of the <<data-format,Khronos Data Format
Specification>>.

When a floating-point value is converted to an unsigned 11-bit
floating-point representation, finite values are rounded to the closest
representable finite value.

While less accurate, implementations are allowed to always round in the
direction of zero.
This means negative values are converted to zero.
Likewise, finite positive values greater than 65024 (the maximum finite
representable unsigned 11-bit floating-point value) are converted to 65024.
Additionally: negative infinity is converted to zero; positive infinity is
converted to positive infinity; and both positive and negative [eq]#NaN# are
converted to positive [eq]#NaN#.

Any representable unsigned 11-bit floating-point value is legal as input to
a Vulkan command that accepts 11-bit floating-point data.
The result of providing a value that is not a floating-point number (such as
[eq]#Inf# or [eq]#NaN#) to such a command is unspecified, but must: not lead
to Vulkan interruption or termination.
Providing a denormalized number to Vulkan must: yield deterministic results.


[[fundamentals-fp10]]
=== Unsigned 10-Bit Floating-Point Numbers

Unsigned 10-bit floating point numbers are defined in the "`Unsigned 10-bit
floating point numbers`" section of the <<data-format,Khronos Data Format
Specification>>.

When a floating-point value is converted to an unsigned 10-bit
floating-point representation, finite values are rounded to the closest
representable finite value.

While less accurate, implementations are allowed to always round in the
direction of zero.
This means negative values are converted to zero.
Likewise, finite positive values greater than 64512 (the maximum finite
representable unsigned 10-bit floating-point value) are converted to 64512.
Additionally: negative infinity is converted to zero; positive infinity is
converted to positive infinity; and both positive and negative [eq]#NaN# are
converted to positive [eq]#NaN#.

Any representable unsigned 10-bit floating-point value is legal as input to
a Vulkan command that accepts 10-bit floating-point data.
The result of providing a value that is not a floating-point number (such as
[eq]#Inf# or [eq]#NaN#) to such a command is unspecified, but must: not lead
to Vulkan interruption or termination.
Providing a denormalized number to Vulkan must: yield deterministic results.


[[fundamentals-general]]
=== General Requirements

Some calculations require division.
In such cases (including implied divisions performed by vector
normalization), division by zero produces an unspecified result but must:
not lead to Vulkan interruption or termination.


[[fundamentals-fixedconv]]
== Fixed-Point Data Conversions

When generic vertex attributes and pixel color or depth _components_ are
represented as integers, they are often (but not always) considered to be
_normalized_.
Normalized integer values are treated specially when being converted to and
from floating-point values, and are usually referred to as _normalized
fixed-point_.

In the remainder of this section, [eq]#b# denotes the bit width of the
fixed-point integer representation.
When the integer is one of the types defined by the API, [eq]#b# is the bit
width of that type.
When the integer comes from an <<resources-images,image>> containing color
or depth component texels, [eq]#b# is the number of bits allocated to that
component in its <<features-formats,specified image format>>.

The signed and unsigned fixed-point representations are assumed to be
[eq]#b#-bit binary two's-complement integers and binary unsigned integers,
respectively.


[[fundamentals-fixedfpconv]]
=== Conversion from Normalized Fixed-Point to Floating-Point

Unsigned normalized fixed-point integers represent numbers in the range
[eq]#[0,1]#.
The conversion from an unsigned normalized fixed-point value [eq]#c# to the
corresponding floating-point value [eq]#f# is defined as

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
f = { c \over { 2^b - 1 } }
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Signed normalized fixed-point integers represent numbers in the range
[eq]#[-1,1]#.
The conversion from a signed normalized fixed-point value [eq]#c# to the
corresponding floating-point value [eq]#f# is performed using

[latexmath]
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
f = \max\left( {c \over {2^{b-1} - 1}}, -1.0 \right)
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Only the range [eq]#[-2^b-1^ {plus} 1, 2^b-1^ - 1]# is used to represent
signed fixed-point values in the range [eq]#[-1,1]#.
For example, if [eq]#b = 8#, then the integer value [eq]#-127# corresponds
to [eq]#-1.0# and the value 127 corresponds to [eq]#1.0#.
Note that while zero is exactly expressible in this representation, one
value ([eq]#-128# in the example) is outside the representable range, and
must: be clamped before use.
This equation is used everywhere that signed normalized fixed-point values
are converted to floating-point.


[[fundamentals-fpfixedconv]]
=== Conversion from Floating-Point to Normalized Fixed-Point

The conversion from a floating-point value [eq]#f# to the corresponding
unsigned normalized fixed-point value [eq]#c# is defined by first clamping
[eq]#f# to the range [eq]#[0,1]#, then computing

  :: [eq]#c = convertFloatToUint(f {times} (2^b^ - 1), b)#

where [eq]#convertFloatToUint}(r,b)# returns one of the two unsigned binary
integer values with exactly [eq]#b# bits which are closest to the
floating-point value [eq]#r#.
Implementations should: round to nearest.
If [eq]#r# is equal to an integer, then that integer value must: be
returned.
In particular, if [eq]#f# is equal to 0.0 or 1.0, then [eq]#c# must: be
assigned 0 or [eq]#2^b^ - 1#, respectively.

The conversion from a floating-point value [eq]#f# to the corresponding
signed normalized fixed-point value [eq]#c# is performed by clamping [eq]#f#
to the range [eq]#[-1,1]#, then computing

  :: [eq]#c = convertFloatToInt(f {times} (2^b-1^ - 1), b)#

where [eq]#convertFloatToInt(r,b)# returns one of the two signed
two's-complement binary integer values with exactly [eq]#b# bits which are
closest to the floating-point value [eq]#r#.
Implementations should: round to nearest.
If [eq]#r# is equal to an integer, then that integer value must: be
returned.
In particular, if [eq]#f# is equal to -1.0, 0.0, or 1.0, then [eq]#c# must:
be assigned [eq]#-(2^b-1^ - 1)#, 0, or [eq]#2^b-1^ - 1#, respectively.

This equation is used everywhere that floating-point values are converted to
signed normalized fixed-point.


[[fundamentals-versionnum]]
== API Version Numbers and Semantics

The Vulkan version number is used in several places in the API.
In each such use, the API _major version number_, _minor version number_,
and _patch version number_ are packed into a 32-bit integer as follows:

  * The major version number is a 10-bit integer packed into bits 31-22.
  * The minor version number is a 10-bit integer packed into bits 21-12.
  * The patch version number is a 12-bit integer packed into bits 11-0.

Differences in any of the Vulkan version numbers indicates a change to the
API in some way, with each part of the version number indicating a different
scope of changes.

A difference in patch version numbers indicates that some usually small part
of the specification or header has been modified, typically to fix a bug,
and may: have an impact on the behavior of existing functionality.
Differences in this version number should: not affect either _full
compatibility_ or _backwards compatibility_ between two versions, or add
additional interfaces to the API.

A difference in minor version numbers indicates that some amount of new
functionality has been added.
This will usually include new interfaces in the header, and may: also
include behavior changes and bug fixes.
Functionality may: be deprecated in a minor revision, but will not be
removed.
When a new minor version is introduced, the patch version is reset to 0, and
each minor revision maintains its own set of patch versions.
Differences in this version should: not affect backwards compatibility, but
will affect full compatibility.

A difference in major version numbers indicates a large set of changes to
the API, potentially including new functionality and header interfaces,
behavioral changes, removal of deprecated features, modification or outright
replacement of any feature, and is thus very likely to break any and all
compatibility.
Differences in this version will typically require significant modification
to an application in order for it to function.

C language macros for manipulating version numbers are defined in the
<<boilerplate-versions,Version Number Macros>> appendix.


[[fundamentals-common-objects]]
== Common Object Types

Some types of Vulkan objects are used in many different structures and
command parameters, and are described here.
These types include _offsets_, _extents_, and _rectangles_.


=== Offsets

Offsets are used to describe a pixel location within an image or
framebuffer, as an (x,y) location for two-dimensional images, or an (x,y,z)
location for three-dimensional images.

[open,refpage='VkOffset2D',desc='Structure specifying a two-dimensional offset',type='structs']
--

A two-dimensional offsets is defined by the structure:

include::../api/structs/VkOffset2D.txt[]

  * pname:x is the x offset.
  * pname:y is the y offset.

include::../validity/structs/VkOffset2D.txt[]
--

[open,refpage='VkOffset3D',desc='Structure specifying a three-dimensional offset',type='structs']
--

A three-dimensional offset is defined by the structure:

include::../api/structs/VkOffset3D.txt[]

  * pname:x is the x offset.
  * pname:y is the y offset.
  * pname:z is the z offset.

include::../validity/structs/VkOffset3D.txt[]
--


=== Extents

Extents are used to describe the size of a rectangular region of pixels
within an image or framebuffer, as (width,height) for two-dimensional
images, or as (width,height,depth) for three-dimensional images.

[open,refpage='VkExtent2D',desc='Structure specifying a two-dimensional extent',type='structs']
--

A two-dimensional extent is defined by the structure:

include::../api/structs/VkExtent2D.txt[]

  * pname:width is the width of the extent.
  * pname:height is the height of the extent.

include::../validity/structs/VkExtent2D.txt[]
--

[open,refpage='VkExtent3D',desc='Structure specifying a three-dimensional extent',type='structs']
--

A three-dimensional extent is defined by the structure:

include::../api/structs/VkExtent3D.txt[]

  * pname:width is the width of the extent.
  * pname:height is the height of the extent.
  * pname:depth is the depth of the extent.

include::../validity/structs/VkExtent3D.txt[]
--


=== Rectangles

[open,refpage='VkRect2D',desc='Structure specifying a two-dimensional subregion',type='structs']
--

Rectangles are used to describe a specified rectangular region of pixels
within an image or framebuffer.
Rectangles include both an offset and an extent of the same dimensionality,
as described above.
Two-dimensional rectangles are defined by the structure

include::../api/structs/VkRect2D.txt[]

  * pname:offset is a slink:VkOffset2D specifying the rectangle offset.
  * pname:extent is a slink:VkExtent2D specifying the rectangle extent.

include::../validity/structs/VkRect2D.txt[]
--