registry/vulkan/chapters/synchronization.txt

// Copyright (c) 2015-2018 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

[[synchronization]]
= Synchronization and Cache Control

Synchronization of access to resources is primarily the responsibility of
the application in Vulkan.
The order of execution of commands with respect to the host and other
commands on the device has few implicit guarantees, and needs to be
explicitly specified.
Memory caches and other optimizations are also explicitly managed, requiring
that the flow of data through the system is largely under application
control.

Whilst some implicit guarantees exist between commands, five explicit
synchronization mechanisms are exposed by Vulkan:

<<synchronization-fences,Fences>>::
    Fences can: be used to communicate to the host that execution of some
    task on the device has completed.

<<synchronization-semaphores,Semaphores>>::
    Semaphores can: be used to control resource access across multiple
    queues.

<<synchronization-events,Events>>::
    Events provide a fine-grained synchronization primitive which can: be
    signaled either within a command buffer or by the host, and can: be
    waited upon within a command buffer or queried on the host.

<<synchronization-pipeline-barriers,Pipeline Barriers>>::
    Pipeline barriers also provide synchronization control within a command
    buffer, but at a single point, rather than with separate signal and wait
    operations.

<<renderpass,Render Passes>>::
    Render passes provide a useful synchronization framework for most
    rendering tasks, built upon the concepts in this chapter.
    Many cases that would otherwise need an application to use other
    synchronization primitives can: be expressed more efficiently as part of
    a render pass.


[[synchronization-dependencies]]
== Execution and Memory Dependencies

An _operation_ is an arbitrary amount of work to be executed on the host, a
device, or an external entity such as a presentation engine.
Synchronization commands introduce explicit _execution dependencies_, and
_memory dependencies_ between two sets of operations defined by the
command's two _synchronization scopes_.

[[synchronization-dependencies-scopes]]
The synchronization scopes define which other operations a synchronization
command is able to create execution dependencies with.
Any type of operation that is not in a synchronization command's
synchronization scopes will not be included in the resulting dependency.
For example, for many synchronization commands, the synchronization scopes
can: be limited to just operations executing in specific
<<synchronization-pipeline-stages,pipeline stages>>, which allows other
pipeline stages to be excluded from a dependency.
Other scoping options are possible, depending on the particular command.

[[synchronization-dependencies-execution]]
An _execution dependency_ is a guarantee that for two sets of operations,
the first set must: _happen-before_ the second set.
If an operation happens-before another operation, then the first operation
must: complete before the second operation is initiated.
More precisely:

  * Let *A* and *B* be separate sets of operations.
  * Let *S* be a synchronization command.
  * Let *A~S~* and *B~S~* be the synchronization scopes of *S*.
  * Let *A'* be the intersection of sets *A* and *A~S~*.
  * Let *B'* be the intersection of sets *B* and *B~S~*.
  * Submitting *A*, *S* and *B* for execution, in that order, will result in
    execution dependency *E* between *A'* and *B'*.
  * Execution dependency *E* guarantees that *A'* happens-before *B'*.

[[synchronization-dependencies-chains]]
An _execution dependency chain_ is a sequence of execution dependencies that
form a happens-before relation between the first dependency's *A'* and the
final dependency's *B'*.
For each consecutive pair of execution dependencies, a chain exists if the
intersection of *B~S~* in the first dependency and *A~S~* in the second
dependency is not an empty set.
The formation of a single execution dependency from an execution dependency
chain can be described by substituting the following in the description of
execution dependencies:

  * Let *S* be a set of synchronization commands that generate an execution
    dependency chain.
  * Let *A~S~* be the first synchronization scope of the first command in
    *S*.
  * Let *B~S~* be the second synchronization scope of the last command in
    *S*.

[NOTE]
.Note
====
An execution dependency is inherently also multiple execution dependencies -
a dependency exists between each subset of *A'* and each subset of *B'*, and
the same is true for execution dependency chains.
For example, a synchronization command with multiple
<<synchronization-pipeline-stages,pipeline stages>> in its stage masks
effectively generates one dependency between each source stage and each
destination stage.
This can be useful to think about when considering how execution chains are
formed if they do not involve all parts of a synchronization command's
dependency.
Similarly, any set of adjacent dependencies in an execution dependency chain
can: be considered an execution dependency chain in its own right.
====

Execution dependencies alone are not sufficient to guarantee that values
resulting from writes in one set of operations can: be read from another set
of operations.

[[synchronization-dependencies-available-and-visible]]
Two additional types of operation are used to control memory access.
_Availability operations_ cause the values generated by specified memory
write accesses to become _available_ for future access.
Any available value remains available until a subsequent write to the same
memory location occurs (whether it is made available or not) or the memory
is freed.
_Visibility operations_ cause any available values to become _visible_ to
specified memory accesses.

[[synchronization-dependencies-memory]]
A _memory dependency_ is an execution dependency which includes availability
and visibility operations such that:

  * The first set of operations happens-before the availability operation.
  * The availability operation happens-before the visibility operation.
  * The visibility operation happens-before the second set of operations.

Once written values are made visible to a particular type of memory access,
they can: be read or written by that type of memory access.
Most synchronization commands in Vulkan define a memory dependency.

[[synchronization-dependencies-access-scopes]]
The specific memory accesses that are made available and visible are defined
by the _access scopes_ of a memory dependency.
Any type of access that is in a memory dependency's first access scope and
occurs in *A'* is made available.
Any type of access that is in a memory dependency's second access scope and
occurs in *B'* has any available writes made visible to it.
Any type of operation that is not in a synchronization command's access
scopes will not be included in the resulting dependency.

A memory dependency enforces availability and visibility of memory accesses
and execution order between two sets of operations.
Adding to the description of <<synchronization-dependencies-chains,
execution dependency chains>>:

  * Let *a* be the set of memory accesses performed by *A'*.
  * Let *b* be the set of memory accesses performed by *B'*.
  * Let *a~S~* be the first access scope of the first command in *S*.
  * Let *b~S~* be the second access scope of the last command in *S*.
  * Let *a'* be the intersection of sets *a* and *a~S~*.
  * Let *b'* be the intersection of sets *b* and *b~S~*.
  * Submitting *A*, *S* and *B* for execution, in that order, will result in
    a memory dependency *m* between *A'* and *B'*.
  * Memory dependency *m* guarantees that:
  ** Memory writes in *a'* are made available.
  ** Available memory writes, including those from *a'*, are made visible to
     *b'*.

[NOTE]
.Note
====
Execution and memory dependencies are used to solve data hazards, i.e. to
ensure that read and write operations occur in a well-defined order.
Write-after-read hazards can be solved with just an execution dependency,
but read-after-write and write-after-write hazards need appropriate memory
dependencies to be included between them.
If an application does not include dependencies to solve these hazards, the
results and execution orders of memory accesses are undefined.
====


[[synchronization-image-layout-transitions]]
=== Image Layout Transitions

Image subresources can: be transitioned from one <<resources-image-layouts,
layout>> to another as part of a <<synchronization-dependencies-memory,
memory dependency>> (e.g. by using an
<<synchronization-image-memory-barriers,image memory barrier>>).
When a layout transition is specified in a memory dependency, it
happens-after the availability operations in the memory dependency, and
happens-before the visibility operations.
Image layout transitions may: perform read and write accesses on all memory
bound to the image subresource range, so applications must: ensure that all
memory writes have been made
<<synchronization-dependencies-available-and-visible, available>> before a
layout transition is executed.
Available memory is automatically made visible to a layout transition, and
writes performed by a layout transition are automatically made available.

Layout transitions always apply to a particular image subresource range, and
specify both an old layout and new layout.
If the old layout does not match the new layout, a transition occurs.
The old layout must: match the current layout of the image subresource
range, with one exception.
The old layout can: always be specified as ename:VK_IMAGE_LAYOUT_UNDEFINED,
though doing so invalidates the contents of the image subresource range.

ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
As image layout transitions may: perform read and write accesses on the
memory bound to the image, if the image subresource affected by the layout
transition is bound to peer memory for any device in the current device mask
then the memory heap the bound memory comes from must: support the
ename:VK_PEER_MEMORY_FEATURE_GENERIC_SRC_BIT and
ename:VK_PEER_MEMORY_FEATURE_GENERIC_DST_BIT capabilities as returned by
flink:vkGetDeviceGroupPeerMemoryFeatures.
endif::VK_VERSION_1_1,VK_KHR_device_group[]

[NOTE]
.Note
====
Setting the old layout to ename:VK_IMAGE_LAYOUT_UNDEFINED implies that the
contents of the image subresource need not be preserved.
Implementations may: use this information to avoid performing expensive data
transition operations.
====

[NOTE]
.Note
====
Applications must: ensure that layout transitions happen-after all
operations accessing the image with the old layout, and happen-before any
operations that will access the image with the new layout.
Layout transitions are potentially read/write operations, so not defining
appropriate memory dependencies to guarantee this will result in a data
race.
====

Image layout transitions interact with <<resources-memory-aliasing,memory
aliasing>>.


[[synchronization-pipeline-stages]]
=== Pipeline Stages

The work performed by an <<fundamentals-queueoperation-command-types, action
or synchronization command>> consists of multiple operations, which are
performed as a sequence of logically independent steps known as _pipeline
stages_.
The exact pipeline stages executed depend on the particular command that is
used, and current command buffer state when the command was recorded.
<<drawing,Drawing commands>>, <<dispatch,dispatching commands>>,
<<copies,copy commands>>, <<clears,clear commands>>, and <<synchronization,
synchronization commands>> all execute in different sets of
<<synchronization-pipeline-stages-types,pipeline stages>>.
<<synchronization, Synchronization commands>> do not execute in a defined
pipeline, but do execute ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT and
ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT.

[NOTE]
.Note
====
Operations performed by synchronization commands (e.g.
<<synchronization-dependencies-available-and-visible, availability and
visibility operations>>) are not executed by a defined pipeline stage.
However other commands can still synchronize with them via the
ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT and
ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT pipeline stages.
====

Execution of operations across pipeline stages must: adhere to
<<synchronization-implicit, implicit ordering guarantees>>, particularly
including <<synchronization-pipeline-stages-order, pipeline stage order>>.
Otherwise, execution across pipeline stages may: overlap or execute out of
order with regards to other stages, unless otherwise enforced by an
execution dependency.

[open,refpage='VkPipelineStageFlagBits',desc='Bitmask specifying pipeline stages',type='enums']
--

Several of the synchronization commands include pipeline stage parameters,
restricting the <<synchronization-dependencies-scopes, synchronization
scopes>> for that command to just those stages.
This allows fine grained control over the exact execution dependencies and
accesses performed by action commands.
Implementations should: use these pipeline stages to avoid unnecessary
stalls or cache flushing.

Bits which can be set, specifying pipeline stages, are:

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

  * ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT specifies the stage of the
    pipeline where any commands are initially received by the queue.
  * ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT specifies the stage of the
    pipeline where Draw/DispatchIndirect data structures are consumed.
ifdef::VK_NVX_device_generated_commands[]
    This stage also includes reading commands written by
    flink:vkCmdProcessCommandsNVX.
endif::VK_NVX_device_generated_commands[]
  * ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT specifies the stage of the
    pipeline where vertex and index buffers are consumed.
  * ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT specifies the vertex shader
    stage.
  * ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT specifies the
    tessellation control shader stage.
  * ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT specifies the
    tessellation evaluation shader stage.
  * ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT specifies the geometry
    shader stage.
  * ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT specifies the fragment
    shader stage.
  * ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT specifies the stage of
    the pipeline where early fragment tests (depth and stencil tests before
    fragment shading) are performed.
    This stage also includes <<renderpass-load-store-ops, subpass load
    operations>> for framebuffer attachments with a depth/stencil format.
  * ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT specifies the stage of
    the pipeline where late fragment tests (depth and stencil tests after
    fragment shading) are performed.
    This stage also includes <<renderpass-load-store-ops, subpass store
    operations>> for framebuffer attachments with a depth/stencil format.
  * ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT specifies the stage
    of the pipeline after blending where the final color values are output
    from the pipeline.
    This stage also includes <<renderpass-load-store-ops, subpass load and
    store operations>> and multisample resolve operations for framebuffer
    attachments with a color format.
  * ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT specifies the execution of a
    compute shader.
  * [[synchronization-pipeline-stages-transfer]]
    ename:VK_PIPELINE_STAGE_TRANSFER_BIT specifies the execution of copy
    commands.
    This includes the operations resulting from all <<copies,copy
    commands>>, <<clears,clear commands>> (with the exception of
    flink:vkCmdClearAttachments), and flink:vkCmdCopyQueryPoolResults.
  * ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT specifies the final stage in
    the pipeline where operations generated by all commands complete
    execution.
  * ename:VK_PIPELINE_STAGE_HOST_BIT specifies a pseudo-stage indicating
    execution on the host of reads/writes of device memory.
    This stage is not invoked by any commands recorded in a command buffer.
  * ename:VK_PIPELINE_STAGE_ALL_GRAPHICS_BIT specifies the execution of all
    graphics pipeline stages, and is equivalent to the logical OR of:
  ** ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT
  ** ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT
  ** ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT
  ** ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT
  ** ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT
  ** ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  ** ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  ** ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT
  ** ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT
  ** ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT
  ** ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
  ** ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT
ifdef::VK_EXT_conditional_rendering[]
  ** ename:VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT
endif::VK_EXT_conditional_rendering[]
  * ename:VK_PIPELINE_STAGE_ALL_COMMANDS_BIT is equivalent to the logical OR
    of every other pipeline stage flag that is supported on the queue it is
    used with.
ifdef::VK_EXT_conditional_rendering[]
  * ename:VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT specifies the
    stage of the pipeline where the predicate of conditional rendering is
    consumed.
endif::VK_EXT_conditional_rendering[]
ifdef::VK_NVX_device_generated_commands[]
  * ename:VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX specifies the stage of
    the pipeline where device-side generation of commands via
    flink:vkCmdProcessCommandsNVX is handled.
endif::VK_NVX_device_generated_commands[]

[NOTE]
.Note
====
An execution dependency with only ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT
in the destination stage mask will only prevent that stage from executing in
subsequently submitted commands.
As this stage does not perform any actual execution, this is not observable
- in effect, it does not delay processing of subsequent commands.
Similarly an execution dependency with only
ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT in the source stage mask will
effectively not wait for any prior commands to complete.

When defining a memory dependency, using only
ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT or
ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT would never make any accesses
available and/or visible because these stages do not access memory.

ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT and
ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT are useful for accomplishing layout
transitions and queue ownership operations when the required execution
dependency is satisfied by other means - for example, semaphore operations
between queues.
====

--

[open,refpage='VkPipelineStageFlags',desc='Bitmask of VkPipelineStageFlagBits',type='enums']
--
include::../api/flags/VkPipelineStageFlags.txt[]

sname:VkPipelineStageFlags is a bitmask type for setting a mask of zero or
more slink:VkPipelineStageFlagBits.
--

[[synchronization-pipeline-stages-masks]]
If a synchronization command includes a source stage mask, its first
<<synchronization-dependencies-scopes, synchronization scope>> only includes
execution of the pipeline stages specified in that mask, and its first
<<synchronization-dependencies-access-scopes, access scope>> only includes
memory access performed by pipeline stages specified in that mask.
If a synchronization command includes a destination stage mask, its second
<<synchronization-dependencies-scopes, synchronization scope>> only includes
execution of the pipeline stages specified in that mask, and its second
<<synchronization-dependencies-access-scopes, access scope>> only includes
memory access performed by pipeline stages specified in that mask.

[NOTE]
.Note
====
Including a particular pipeline stage in the first
<<synchronization-dependencies-scopes, synchronization scope>> of a command
implicitly includes <<synchronization-pipeline-stages-order, logically
earlier>> pipeline stages in the synchronization scope.
Similarly, the second <<synchronization-dependencies-scopes, synchronization
scope>> includes <<synchronization-pipeline-stages-order, logically later>>
pipeline stages.

However, note that <<synchronization-dependencies-access-scopes, access
scopes>> are not affected in this way - only the precise stages specified
are considered part of each access scope.
====

Certain pipeline stages are only available on queues that support a
particular set of operations.
The following table lists, for each pipeline stage flag, which queue
capability flag must: be supported by the queue.
When multiple flags are enumerated in the second column of the table, it
means that the pipeline stage is supported on the queue if it supports any
of the listed capability flags.
For further details on queue capabilities see
<<devsandqueues-physical-device-enumeration,Physical Device Enumeration>>
and <<devsandqueues-queues,Queues>>.

[[synchronization-pipeline-stages-supported]]
.Supported pipeline stage flags
[cols="60%,40%",options="header"]
|====
|Pipeline stage flag                                          | Required queue capability flag
|ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT                      | None required
|ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT                    | ename:VK_QUEUE_GRAPHICS_BIT or ename:VK_QUEUE_COMPUTE_BIT
|ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT                     | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT                    | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT      | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT   | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT                  | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT                  | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT             | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT              | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT          | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT                   | ename:VK_QUEUE_COMPUTE_BIT
|ename:VK_PIPELINE_STAGE_TRANSFER_BIT                         | ename:VK_QUEUE_GRAPHICS_BIT, ename:VK_QUEUE_COMPUTE_BIT or ename:VK_QUEUE_TRANSFER_BIT
|ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT                   | None required
|ename:VK_PIPELINE_STAGE_HOST_BIT                             | None required
|ename:VK_PIPELINE_STAGE_ALL_GRAPHICS_BIT                     | ename:VK_QUEUE_GRAPHICS_BIT
|ename:VK_PIPELINE_STAGE_ALL_COMMANDS_BIT                     | None required
ifdef::VK_EXT_conditional_rendering[]
|ename:VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT        | ename:VK_QUEUE_GRAPHICS_BIT or ename:VK_QUEUE_COMPUTE_BIT
endif::VK_EXT_conditional_rendering[]
ifdef::VK_NVX_device_generated_commands[]
|ename:VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX              | ename:VK_QUEUE_GRAPHICS_BIT or ename:VK_QUEUE_COMPUTE_BIT
endif::VK_NVX_device_generated_commands[]
|====

[[synchronization-pipeline-stages-order]]
Pipeline stages that execute as a result of a command logically complete
execution in a specific order, such that completion of a logically later
pipeline stage must: not happen-before completion of a logically earlier
stage.
This means that including any stage in the source stage mask for a
particular synchronization command also implies that any logically earlier
stages are included in *A~S~* for that command.

Similarly, initiation of a logically earlier pipeline stage must: not
happen-after initiation of a logically later pipeline stage.
Including any given stage in the destination stage mask for a particular
synchronization command also implies that any logically later stages are
included in *B~S~* for that command.

[NOTE]
.Note
====
Implementations may: not support synchronization at every pipeline stage for
every synchronization operation.
If a pipeline stage that an implementation does not support synchronization
for appears in a source stage mask, it may: substitute any logically later
stage in its place for the first synchronization scope.
If a pipeline stage that an implementation does not support synchronization
for appears in a destination stage mask, it may: substitute any logically
earlier stage in its place for the second synchronization scope.

For example, if an implementation is unable to signal an event immediately
after vertex shader execution is complete, it may: instead signal the event
after color attachment output has completed.

If an implementation makes such a substitution, it must: not affect the
semantics of execution or memory dependencies or image and buffer memory
barriers.
====

[[synchronization-pipeline-stages-types]]
The order and set of pipeline stages executed by a given command is
determined by the command's pipeline type, as described below:

For the graphics pipeline, the following stages occur in this order:

  * ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT
  * ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT
  * ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT
  * ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT
  * ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT
  * ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
  * ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT

For the compute pipeline, the following stages occur in this order:

  * ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT
  * ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT
  * ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT

ifdef::VK_EXT_conditional_rendering[]
The conditional rendering stage is formally part of both the graphics, and
the compute pipeline.
The pipeline stage where the predicate read happens has unspecified order
relative to other stages of these pipelines:

  * ename:VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT
endif::VK_EXT_conditional_rendering[]

For the transfer pipeline, the following stages occur in this order:

  * ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT
  * ename:VK_PIPELINE_STAGE_TRANSFER_BIT
  * ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT

For host operations, only one pipeline stage occurs, so no order is
guaranteed:

  * ename:VK_PIPELINE_STAGE_HOST_BIT

ifdef::VK_NVX_device_generated_commands[]
For the command processing pipeline, the following stages occur in this
order:

  * ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT
  * ename:VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX
  * ename:VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT
endif::VK_NVX_device_generated_commands[]


[[synchronization-access-types]]
=== Access Types

[open,refpage='VkAccessFlagBits',desc='Bitmask specifying memory access types that will participate in a memory dependency',type='enums']
--

Memory in Vulkan can: be accessed from within shader invocations and via
some fixed-function stages of the pipeline.
The _access type_ is a function of the <<descriptorsets, descriptor type>>
used, or how a fixed-function stage accesses memory.
Each access type corresponds to a bit flag in slink:VkAccessFlagBits.

[[synchronization-access-masks]]
Some synchronization commands take sets of access types as parameters to
define the <<synchronization-dependencies-access-scopes, access scopes>> of
a memory dependency.
If a synchronization command includes a source access mask, its first
<<synchronization-dependencies-access-scopes, access scope>> only includes
accesses via the access types specified in that mask.
Similarly, if a synchronization command includes a destination access mask,
its second <<synchronization-dependencies-access-scopes, access scope>> only
includes accesses via the access types specified in that mask.

Access types that can: be set in an access mask include:

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

  * ename:VK_ACCESS_INDIRECT_COMMAND_READ_BIT specifies read access to an
    indirect command structure read as part of an indirect drawing or
    dispatch command.
  * ename:VK_ACCESS_INDEX_READ_BIT specifies read access to an index buffer
    as part of an indexed drawing command, bound by
    flink:vkCmdBindIndexBuffer.
  * ename:VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT specifies read access to a
    vertex buffer as part of a drawing command, bound by
    flink:vkCmdBindVertexBuffers.
  * ename:VK_ACCESS_UNIFORM_READ_BIT specifies read access to a
    <<descriptorsets-uniformbuffer, uniform buffer>>.
  * ename:VK_ACCESS_INPUT_ATTACHMENT_READ_BIT specifies read access to an
    <<renderpass, input attachment>> within a render pass during fragment
    shading.
  * ename:VK_ACCESS_SHADER_READ_BIT specifies read access to a
    <<descriptorsets-storagebuffer, storage buffer>>,
    <<descriptorsets-uniformtexelbuffer, uniform texel buffer>>,
    <<descriptorsets-storagetexelbuffer, storage texel buffer>>,
    <<descriptorsets-sampledimage, sampled image>>, or
    <<descriptorsets-storageimage, storage image>>.
  * ename:VK_ACCESS_SHADER_WRITE_BIT specifies write access to a
    <<descriptorsets-storagebuffer, storage buffer>>,
    <<descriptorsets-storagetexelbuffer, storage texel buffer>>, or
    <<descriptorsets-storageimage, storage image>>.
  * ename:VK_ACCESS_COLOR_ATTACHMENT_READ_BIT specifies read access to a
    <<renderpass, color attachment>>, such as via <<framebuffer-blending,
    blending>>, <<framebuffer-logicop, logic operations>>, or via certain
    <<renderpass-load-store-ops, subpass load operations>>.
ifdef::VK_EXT_blend_operation_advanced[]
    It does not include <<framebuffer-blend-advanced,advanced blend
    operations>>.
endif::VK_EXT_blend_operation_advanced[]
  * ename:VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT specifies write access to a
    <<renderpass, color or resolve attachment>> during a <<renderpass,
    render pass>> or via certain <<renderpass-load-store-ops, subpass load
    and store operations>>.
  * ename:VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_READ_BIT specifies read access
    to a <<renderpass, depth/stencil attachment>>, via <<fragops-ds-state,
    depth or stencil operations>> or via certain
    <<renderpass-load-store-ops, subpass load operations>>.
  * ename:VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_WRITE_BIT specifies write
    access to a <<renderpass, depth/stencil attachment>>, via
    <<fragops-ds-state, depth or stencil operations>> or via certain
    <<renderpass-load-store-ops, subpass load and store operations>>.
  * ename:VK_ACCESS_TRANSFER_READ_BIT specifies read access to an image or
    buffer in a <<copies, copy>> operation.
  * ename:VK_ACCESS_TRANSFER_WRITE_BIT specifies write access to an image or
    buffer in a <<clears, clear>> or <<copies, copy>> operation.
  * ename:VK_ACCESS_HOST_READ_BIT specifies read access by a host operation.
    Accesses of this type are not performed through a resource, but directly
    on memory.
  * ename:VK_ACCESS_HOST_WRITE_BIT specifies write access by a host
    operation.
    Accesses of this type are not performed through a resource, but directly
    on memory.
  * ename:VK_ACCESS_MEMORY_READ_BIT specifies read access via non-specific
    entities.
    These entities include the Vulkan device and host, but may: also include
    entities external to the Vulkan device or otherwise not part of the core
    Vulkan pipeline.
    When included in a destination access mask, makes all available writes
    visible to all future read accesses on entities known to the Vulkan
    device.
  * ename:VK_ACCESS_MEMORY_WRITE_BIT specifies write access via non-specific
    entities.
    These entities include the Vulkan device and host, but may: also include
    entities external to the Vulkan device or otherwise not part of the core
    Vulkan pipeline.
    When included in a source access mask, all writes that are performed by
    entities known to the Vulkan device are made available.
    When included in a destination access mask, makes all available writes
    visible to all future write accesses on entities known to the Vulkan
    device.
ifdef::VK_EXT_conditional_rendering[]
  * ename:VK_ACCESS_CONDITIONAL_RENDERING_READ_BIT_EXT specifies read access
    to a predicate as part of conditional rendering.
endif::VK_EXT_conditional_rendering[]
ifdef::VK_NVX_device_generated_commands[]
  * ename:VK_ACCESS_COMMAND_PROCESS_READ_BIT_NVX specifies reads from
    sname:VkBuffer inputs to flink:vkCmdProcessCommandsNVX.
  * ename:VK_ACCESS_COMMAND_PROCESS_WRITE_BIT_NVX specifies writes to the
    target command buffer in flink:vkCmdProcessCommandsNVX.
endif::VK_NVX_device_generated_commands[]
ifdef::VK_EXT_blend_operation_advanced[]
  * ename:VK_ACCESS_COLOR_ATTACHMENT_READ_NONCOHERENT_BIT_EXT is similar to
    ename:VK_ACCESS_COLOR_ATTACHMENT_READ_BIT, but also includes
    <<framebuffer-blend-advanced,advanced blend operations>>.
endif::VK_EXT_blend_operation_advanced[]

Certain access types are only performed by a subset of pipeline stages.
Any synchronization command that takes both stage masks and access masks
uses both to define the <<synchronization-dependencies-access-scopes, access
scopes>> - only the specified access types performed by the specified stages
are included in the access scope.
An application must: not specify an access flag in a synchronization command
if it does not include a pipeline stage in the corresponding stage mask that
is able to perform accesses of that type.
The following table lists, for each access flag, which pipeline stages can:
perform that type of access.

[[synchronization-access-types-supported]]
.Supported access types
[cols="50,50",options="header"]
|====
|Access flag                                                  | Supported pipeline stages
|ename:VK_ACCESS_INDIRECT_COMMAND_READ_BIT                    | ename:VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT
|ename:VK_ACCESS_INDEX_READ_BIT                               | ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT
|ename:VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT                    | ename:VK_PIPELINE_STAGE_VERTEX_INPUT_BIT
|ename:VK_ACCESS_UNIFORM_READ_BIT                             | ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT
|ename:VK_ACCESS_INPUT_ATTACHMENT_READ_BIT                    | ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT
|ename:VK_ACCESS_SHADER_READ_BIT                              | ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT
|ename:VK_ACCESS_SHADER_WRITE_BIT                             | ename:VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or ename:VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT
|ename:VK_ACCESS_COLOR_ATTACHMENT_READ_BIT                    | ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
|ename:VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT                   | ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
|ename:VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_READ_BIT            | ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT, or ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT
|ename:VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_WRITE_BIT           | ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT, or ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT
|ename:VK_ACCESS_TRANSFER_READ_BIT                            | ename:VK_PIPELINE_STAGE_TRANSFER_BIT
|ename:VK_ACCESS_TRANSFER_WRITE_BIT                           | ename:VK_PIPELINE_STAGE_TRANSFER_BIT
|ename:VK_ACCESS_HOST_READ_BIT                                | ename:VK_PIPELINE_STAGE_HOST_BIT
|ename:VK_ACCESS_HOST_WRITE_BIT                               | ename:VK_PIPELINE_STAGE_HOST_BIT
|ename:VK_ACCESS_MEMORY_READ_BIT                              | N/A
|ename:VK_ACCESS_MEMORY_WRITE_BIT                             | N/A
ifdef::VK_EXT_blend_operation_advanced[]
|ename:VK_ACCESS_COLOR_ATTACHMENT_READ_NONCOHERENT_BIT_EXT    | ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
endif::VK_EXT_blend_operation_advanced[]
ifdef::VK_NVX_device_generated_commands[]
|ename:VK_ACCESS_COMMAND_PROCESS_READ_BIT_NVX                 | ename:VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX
|ename:VK_ACCESS_COMMAND_PROCESS_WRITE_BIT_NVX                | ename:VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX
endif::VK_NVX_device_generated_commands[]
ifdef::VK_EXT_conditional_rendering[]
|ename:VK_ACCESS_CONDITIONAL_RENDERING_READ_BIT_EXT           | ename:VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT
endif::VK_EXT_conditional_rendering[]
|====

[[synchronization-host-access-types]]
If a memory object does not have the
ename:VK_MEMORY_PROPERTY_HOST_COHERENT_BIT property, then
flink:vkFlushMappedMemoryRanges must: be called in order to guarantee that
writes to the memory object from the host are made visible to the
ename:VK_ACCESS_HOST_WRITE_BIT <<synchronization-access-types, access
type>>, where it can: be further made available to the device by
<<synchronization, synchronization commands>>.
Similarly, flink:vkInvalidateMappedMemoryRanges must: be called to guarantee
that writes which are visible to the ename:VK_ACCESS_HOST_READ_BIT
<<synchronization-access-types, access type>> are made visible to host
operations.

If the memory object does have the
ename:VK_MEMORY_PROPERTY_HOST_COHERENT_BIT property flag, writes to the
memory object from the host are automatically made visible to the
ename:VK_ACCESS_HOST_WRITE_BIT <<synchronization-access-types,access type>>.
Similarly, writes made visible to the ename:VK_ACCESS_HOST_READ_BIT
<<synchronization-access-types, access type>> are automatically made visible
to the host.

[NOTE]
.Note
====
The flink:vkQueueSubmit command <<synchronization-submission-host-writes,
automatically guarantees that host writes flushed to
ename:VK_ACCESS_HOST_WRITE_BIT are made available>> if they were flushed
before the command executed, so in most cases an explicit memory barrier is
not needed for this case.
In the few circumstances where a submit does not occur between the host
write and the device read access, writes can: be made available by using an
explicit memory barrier.
====

--

[open,refpage='VkAccessFlags',desc='Bitmask of VkAccessFlagBits',type='enums']
--
include::../api/flags/VkAccessFlags.txt[]

sname:VkAccessFlags is a bitmask type for setting a mask of zero or more
slink:VkAccessFlagBits.
--


[[synchronization-framebuffer-regions]]
=== Framebuffer Region Dependencies

<<synchronization-pipeline-stages, Pipeline stages>> that operate on, or
with respect to, the framebuffer are collectively the _framebuffer-space_
pipeline stages.
These stages are:

  * ename:VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT
  * ename:VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT
  * ename:VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT
  * ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT

For these pipeline stages, an execution or memory dependency from the first
set of operations to the second set can: either be a single
_framebuffer-global_ dependency, or split into multiple _framebuffer-local_
dependencies.
A dependency with non-framebuffer-space pipeline stages is neither
framebuffer-global nor framebuffer-local.

A _framebuffer region_ is a set of sample (x, y, layer, sample) coordinates
that is a subset of the entire framebuffer.

Both <<synchronization-dependencies-scopes, synchronization scopes>> of a
framebuffer-local dependency include only the operations performed within
corresponding framebuffer regions (as defined below).
No ordering guarantees are made between different framebuffer regions for a
framebuffer-local dependency.

Both <<synchronization-dependencies-scopes, synchronization scopes>> of a
framebuffer-global dependency include operations on all framebuffer-regions.

If the first synchronization scope includes operations on pixels/fragments
with N samples and the second synchronization scope includes operations on
pixels/fragments with M samples, where N does not equal M, then a
framebuffer region containing all samples at a given (x, y, layer)
coordinate in the first synchronization scope corresponds to a region
containing all samples at the same coordinate in the second synchronization
scope.
In other words, it is a pixel granularity dependency.
If N equals M, then a framebuffer region containing a single (x, y, layer,
sample) coordinate in the first synchronization scope corresponds to a
region containing the same sample at the same coordinate in the second
synchronization scope.
In other words, it is a sample granularity dependency.

[NOTE]
.Note
====
Since fragment invocations are not specified to run in any particular
groupings, the size of a framebuffer region is implementation-dependent, not
known to the application, and must: be assumed to be no larger than
specified above.
====

[NOTE]
.Note
====
Practically, the pixel vs sample granularity dependency means that if an
input attachment has a different number of samples than the pipeline's
pname:rasterizationSamples, then a fragment can: access any sample in the
input attachment's pixel even if it only uses framebuffer-local
dependencies.
If the input attachment has the same number of samples, then the fragment
can: only access the covered samples in its input code:SampleMask (i.e. the
fragment operations happen-after a framebuffer-local dependency for each
sample the fragment covers).
To access samples that are not covered, a framebuffer-global dependency is
required.
====

If a synchronization command includes a pname:dependencyFlags parameter, and
specifies the ename:VK_DEPENDENCY_BY_REGION_BIT flag, then it defines
framebuffer-local dependencies for the framebuffer-space pipeline stages in
that synchronization command, for all framebuffer regions.
If no pname:dependencyFlags parameter is included, or the
ename:VK_DEPENDENCY_BY_REGION_BIT flag is not specified, then a
framebuffer-global dependency is specified for those stages.
The ename:VK_DEPENDENCY_BY_REGION_BIT flag does not affect the dependencies
between non-framebuffer-space pipeline stages, nor does it affect the
dependencies between framebuffer-space and non-framebuffer-space pipeline
stages.

[NOTE]
.Note
====
Framebuffer-local dependencies are more optimal for most architectures;
particularly tile-based architectures - which can keep framebuffer-regions
entirely in on-chip registers and thus avoid external bandwidth across such
a dependency.
Including a framebuffer-global dependency in your rendering will usually
force all implementations to flush data to memory, or to a higher level
cache, breaking any potential locality optimizations.
====


ifdef::VK_VERSION_1_1,VK_KHR_multiview[]

[[synchronization-view-local-dependencies]]
=== View-Local Dependencies

In a render pass instance that has <<renderpass-multiview,multiview>>
enabled, dependencies can: be either view-local or view-global.

A view-local dependency only includes operations from a single
<<renderpass-multiview-view-local,source view>> from the source subpass in
the first synchronization scope, and only includes operations from a single
<<renderpass-multiview-view-local,destination view>> from the destination
subpass in the second synchronization scope.
A view-global dependency includes all views in the view mask of the source
and destination subpasses in the corresponding synchronization scopes.

If a synchronization command includes a pname:dependencyFlags parameter and
specifies the ename:VK_DEPENDENCY_VIEW_LOCAL_BIT flag, then it defines
view-local dependencies for that synchronization command, for all views.
If no pname:dependencyFlags parameter is included or the
ename:VK_DEPENDENCY_VIEW_LOCAL_BIT flag is not specified, then a view-global
dependency is specified.

endif::VK_VERSION_1_1,VK_KHR_multiview[]


ifdef::VK_VERSION_1_1,VK_KHR_device_group[]

[[synchronization-device-local-dependencies]]
=== Device-Local Dependencies

Dependencies can: be either device-local or non-device-local.
A device-local dependency acts as multiple separate dependencies, one for
each physical device that executes the synchronization command, where each
dependency only includes operations from that physical device in both
synchronization scopes.
A non-device-local dependency is a single dependency where both
synchronization scopes include operations from all physical devices that
participate in the synchronization command.
For subpass dependencies, all physical devices in the
slink:VkDeviceGroupRenderPassBeginInfo::pname:deviceMask participate in the
dependency, and for pipeline barriers all physical devices that are set in
the command buffer's current device mask participate in the dependency.

If a synchronization command includes a pname:dependencyFlags parameter and
specifies the ename:VK_DEPENDENCY_DEVICE_GROUP_BIT flag, then it defines a
non-device-local dependency for that synchronization command.
If no pname:dependencyFlags parameter is included or the
ename:VK_DEPENDENCY_DEVICE_GROUP_BIT flag is not specified, then it defines
device-local dependencies for that synchronization command, for all
participating physical devices.

Semaphore and event dependencies are device-local and only execute on the
one physical device that performs the dependency.

endif::VK_VERSION_1_1,VK_KHR_device_group[]


[[synchronization-implicit]]
== Implicit Synchronization Guarantees

A small number of implicit ordering guarantees are provided by Vulkan,
ensuring that the order in which commands are submitted is meaningful, and
avoiding unnecessary complexity in common operations.

[[synchronization-submission-order]]
_Submission order_ is a fundamental ordering in Vulkan, giving meaning to
the order in which <<fundamentals-queueoperation-command-types, action and
synchronization commands>> are recorded and submitted to a single queue.
Explicit and implicit ordering guarantees between commands in Vulkan all
work on the premise that this ordering is meaningful.
This order does not itself define any execution or memory dependencies;
synchronization commands and other orderings within the API use this
ordering to define their scopes.

Submission order for any given set of commands is based on the order in
which they were recorded to command buffers and then submitted.
This order is determined as follows:

  . The initial order is determined by the order in which
    flink:vkQueueSubmit commands are executed on the host, for a single
    queue, from first to last.
  . The order in which slink:VkSubmitInfo structures are specified in the
    pname:pSubmits parameter of flink:vkQueueSubmit, from lowest index to
    highest.
  . The order in which command buffers are specified in the
    pname:pCommandBuffers member of slink:VkSubmitInfo, from lowest index to
    highest.
  . The order in which commands were recorded to a command buffer on the
    host, from first to last:
  ** For commands recorded outside a render pass, this includes all other
     commands recorded outside a render pass, including
     flink:vkCmdBeginRenderPass and flink:vkCmdEndRenderPass commands; it
     does not directly include commands inside a render pass.
  ** For commands recorded inside a render pass, this includes all other
     commands recorded inside the same subpass, including the
     flink:vkCmdBeginRenderPass and flink:vkCmdEndRenderPass commands that
     delimit the same render pass instance; it does not include commands
     recorded to other subpasses.

<<fundamentals-queueoperation-command-types, Action and synchronization
commands>> recorded to a command buffer execute the
ename:VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT pipeline stage in
<<synchronization-submission-order, submission order>> - forming an implicit
execution dependency between this stage in each command.

<<fundamentals-queueoperation-command-types, State commands>> do not execute
any operations on the device, instead they set the state of the command
buffer when they execute on the host, in the order that they are recorded.
<<fundamentals-queueoperation-command-types, Action commands>> consume the
current state of the command buffer when they are recorded, and will execute
state changes on the device as required to match the recorded state.

<<queries-order, Query commands>>, <<drawing-primitive-order, the order of
primitives passing through the graphics pipeline>> and
<<synchronization-image-barrier-layout-transition-order, image layout
transitions as part of an image memory barrier>> provide additional
guarantees based on submission order.

Execution of <<synchronization-pipeline-stages-order, pipeline stages>>
within a given command also has a loose ordering, dependent only on a single
command.


[[synchronization-fences]]
== Fences

[open,refpage='VkFence',desc='Opaque handle to a fence object',type='handles']
--

Fences are a synchronization primitive that can: be used to insert a
dependency from a queue to the host.
Fences have two states - signaled and unsignaled.
A fence can: be signaled as part of the execution of a
<<devsandqueues-submission, queue submission>> command.
Fences can: be unsignaled on the host with flink:vkResetFences.
Fences can: be waited on by the host with the flink:vkWaitForFences command,
and the current state can: be queried with flink:vkGetFenceStatus.

ifdef::VK_VERSION_1_1,VK_KHR_external_fence[]
[[synchronization-fences-payloads]]
As with most objects in Vulkan, fences are an interface to internal data
which is typically opaque to applications.
This internal data is referred to as a fence's _payload_.

However, in order to enable communication with agents outside of the current
device, it is necessary to be able to export that payload to a commonly
understood format, and subsequently import from that format as well.

The internal data of a fence may: include a reference to any resources and
pending work associated with signal or unsignal operations performed on that
fence object.
Mechanisms to import and export that internal data to and from fences are
provided <<VkExportFenceCreateInfo, below>>.
These mechanisms indirectly enable applications to share fence state between
two or more fences and other synchronization primitives across process and
API boundaries.

endif::VK_VERSION_1_1,VK_KHR_external_fence[]

Fences are represented by sname:VkFence handles:

include::../api/handles/VkFence.txt[]

--

[open,refpage='vkCreateFence',desc='Create a new fence object',type='protos']
--

To create a fence, call:

include::../api/protos/vkCreateFence.txt[]

  * pname:device is the logical device that creates the fence.
  * pname:pCreateInfo is a pointer to an instance of the
    sname:VkFenceCreateInfo structure which contains information about how
    the fence is to be created.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pFence points to a handle in which the resulting fence object is
    returned.

include::../validity/protos/vkCreateFence.txt[]
--

[open,refpage='VkFenceCreateInfo',desc='Structure specifying parameters of a newly created fence',type='structs']
--

The sname:VkFenceCreateInfo structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is a bitmask of elink:VkFenceCreateFlagBits specifying the
    initial state and behavior of the fence.

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

[open,refpage='VkFenceCreateFlagBits',desc='Bitmask specifying initial state and behavior of a fence',type='enums']
--

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

  * ename:VK_FENCE_CREATE_SIGNALED_BIT specifies that the fence object is
    created in the signaled state.
    Otherwise, it is created in the unsignaled state.

--

[open,refpage='VkFenceCreateFlags',desc='Bitmask of VkFenceCreateFlagBits',type='enums']
--
include::../api/flags/VkFenceCreateFlags.txt[]

sname:VkFenceCreateFlags is a bitmask type for setting a mask of zero or
more slink:VkFenceCreateFlagBits.
--

ifdef::VK_VERSION_1_1,VK_KHR_external_fence[]

[open,refpage='VkExportFenceCreateInfo',desc='Structure specifying handle types that can be exported from a fence',type='structs']
--

To create a fence whose payload can: be exported to external handles, add
the slink:VkExportFenceCreateInfo structure to the pname:pNext chain of the
slink:VkFenceCreateInfo structure.
The sname:VkExportFenceCreateInfo structure is defined as:

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

ifdef::VK_KHR_external_fence[]
or the equivalent

include::../api/structs/VkExportFenceCreateInfoKHR.txt[]
endif::VK_KHR_external_fence[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:handleTypes is a bitmask of
    elink:VkExternalFenceHandleTypeFlagBits specifying one or more fence
    handle types the application can: export from the resulting fence.
    The application can: request multiple handle types for the same fence.

.Valid Usage
****
  * [[VUID-VkExportFenceCreateInfo-handleTypes-01446]]
    The bits in pname:handleTypes must be supported and compatible, as
    reported by slink:VkExternalFenceProperties.
****

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

endif::VK_VERSION_1_1,VK_KHR_external_fence[]

ifdef::VK_KHR_external_fence_win32[]

[open,refpage='VkExportFenceWin32HandleInfoKHR',desc='Structure specifying additional attributes of Windows handles exported from a fence',type='structs']
--

To specify additional attributes of NT handles exported from a fence, add
the slink:VkExportFenceWin32HandleInfoKHR structure to the pname:pNext chain
of the slink:VkFenceCreateInfo structure.
The sname:VkExportFenceWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkExportFenceWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:pAttributes is a pointer to a Windows code:SECURITY_ATTRIBUTES
    structure specifying security attributes of the handle.
  * pname:dwAccess is a code:DWORD specifying access rights of the handle.
  * pname:name is a NULL-terminated UTF-16 string to associate with the
    underlying synchronization primitive referenced by NT handles exported
    from the created fence.

If this structure is not present, or if pname:pAttributes is set to `NULL`,
default security descriptor values will be used, and child processes created
by the application will not inherit the handle, as described in the MSDN
documentation for "`Synchronization Object Security and Access Rights`"^1^.
Further, if the structure is not present, the access rights will be

code:DXGI_SHARED_RESOURCE_READ | code:DXGI_SHARED_RESOURCE_WRITE

for handles of the following types:

ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_WIN32_BIT

1::
    https://msdn.microsoft.com/en-us/library/windows/desktop/ms686670.aspx

.Valid Usage
****
  * [[VUID-VkExportFenceWin32HandleInfoKHR-handleTypes-01447]]
    If slink:VkExportFenceCreateInfo::pname:handleTypes does not include
    ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_WIN32_BIT,
    sname:VkExportFenceWin32HandleInfoKHR must: not be in the pname:pNext
    chain of slink:VkFenceCreateInfo.
****

include::../validity/structs/VkExportFenceWin32HandleInfoKHR.txt[]
--

[open,refpage='vkGetFenceWin32HandleKHR',desc='Get a Windows HANDLE for a fence',type='protos']
--

To export a Windows handle representing the state of a fence, call:

include::../api/protos/vkGetFenceWin32HandleKHR.txt[]

  * pname:device is the logical device that created the fence being
    exported.
  * pname:pGetWin32HandleInfo is a pointer to an instance of the
    slink:VkFenceGetWin32HandleInfoKHR structure containing parameters of
    the export operation.
  * pname:pHandle will return the Windows handle representing the fence
    state.

For handle types defined as NT handles, the handles returned by
fname:vkGetFenceWin32HandleKHR are owned by the application.
To avoid leaking resources, the application must: release ownership of them
using the fname:CloseHandle system call when they are no longer needed.

Exporting a Windows handle from a fence may: have side effects depending on
the transference of the specified handle type, as described in
<<synchronization-fences-importing,Importing Fence Payloads>>.

include::../validity/protos/vkGetFenceWin32HandleKHR.txt[]
--

[open,refpage='VkFenceGetWin32HandleInfoKHR',desc='Structure describing a Win32 handle fence export operation',type='structs']
--

The sname:VkFenceGetWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkFenceGetWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:fence is the fence from which state will be exported.
  * pname:handleType is the type of handle requested.

The properties of the handle returned depend on the value of
pname:handleType.
See elink:VkExternalFenceHandleTypeFlagBits for a description of the
properties of the defined external fence handle types.

.Valid Usage
****
  * [[VUID-VkFenceGetWin32HandleInfoKHR-handleType-01448]]
    pname:handleType must: have been included in
    slink:VkExportFenceCreateInfo::pname:handleTypes when the pname:fence's
    current payload was created.
  * [[VUID-VkFenceGetWin32HandleInfoKHR-handleType-01449]]
    If pname:handleType is defined as an NT handle,
    flink:vkGetFenceWin32HandleKHR must: be called no more than once for
    each valid unique combination of pname:fence and pname:handleType.
  * [[VUID-VkFenceGetWin32HandleInfoKHR-fence-01450]]
    pname:fence must: not currently have its payload replaced by an imported
    payload as described below in
    <<synchronization-fences-importing,Importing Fence Payloads>> unless
    that imported payload's handle type was included in
    slink:VkExternalFenceProperties::pname:exportFromImportedHandleTypes for
    pname:handleType.
  * [[VUID-VkFenceGetWin32HandleInfoKHR-handleType-01451]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, pname:fence must: be signaled, or have an
    associated <<synchronization-fences-signaling,fence signal operation>>
    pending execution.
  * [[VUID-VkFenceGetWin32HandleInfoKHR-handleType-01452]]
    pname:handleType must: be defined as an NT handle or a global share
    handle.
****

include::../validity/structs/VkFenceGetWin32HandleInfoKHR.txt[]
--

endif::VK_KHR_external_fence_win32[]

ifdef::VK_KHR_external_fence_fd[]

[open,refpage='vkGetFenceFdKHR',desc='Get a POSIX file descriptor handle for a fence',type='protos']
--

To export a POSIX file descriptor representing the payload of a fence, call:

include::../api/protos/vkGetFenceFdKHR.txt[]

  * pname:device is the logical device that created the fence being
    exported.
  * pname:pGetFdInfo is a pointer to an instance of the
    slink:VkFenceGetFdInfoKHR structure containing parameters of the export
    operation.
  * pname:pFd will return the file descriptor representing the fence
    payload.

Each call to fname:vkGetFenceFdKHR must: create a new file descriptor and
transfer ownership of it to the application.
To avoid leaking resources, the application must: release ownership of the
file descriptor when it is no longer needed.

[NOTE]
.Note
====
Ownership can be released in many ways.
For example, the application can call code:close() on the file descriptor,
or transfer ownership back to Vulkan by using the file descriptor to import
a fence payload.
====

If pname:pGetFdInfo\->pname:handleType is
ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_SYNC_FD_BIT and the fence is signaled at
the time fname:vkGetFenceFdKHR is called, pname:pFd may: return the value
`-1` instead of a valid file descriptor.

Where supported by the operating system, the implementation must: set the
file descriptor to be closed automatically when an code:execve system call
is made.

Exporting a file descriptor from a fence may: have side effects depending on
the transference of the specified handle type, as described in
<<synchronization-fences-importing,Importing Fence State>>.

include::../validity/protos/vkGetFenceFdKHR.txt[]
--

[open,refpage='VkFenceGetFdInfoKHR',desc='Structure describing a POSIX FD fence export operation',type='structs']
--

The sname:VkFenceGetFdInfoKHR structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:fence is the fence from which state will be exported.
  * pname:handleType is the type of handle requested.

The properties of the file descriptor returned depend on the value of
pname:handleType.
See elink:VkExternalFenceHandleTypeFlagBits for a description of the
properties of the defined external fence handle types.

.Valid Usage
****
  * [[VUID-VkFenceGetFdInfoKHR-handleType-01453]]
    pname:handleType must: have been included in
    slink:VkExportFenceCreateInfo::pname:handleTypes when pname:fence's
    current payload was created.
  * [[VUID-VkFenceGetFdInfoKHR-handleType-01454]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, pname:fence must: be signaled, or have an
    associated <<synchronization-fences-signaling,fence signal operation>>
    pending execution.
  * [[VUID-VkFenceGetFdInfoKHR-fence-01455]]
    pname:fence must: not currently have its payload replaced by an imported
    payload as described below in
    <<synchronization-fences-importing,Importing Fence Payloads>> unless
    that imported payload's handle type was included in
    slink:VkExternalFenceProperties::pname:exportFromImportedHandleTypes for
    pname:handleType.
  * [[VUID-VkFenceGetFdInfoKHR-handleType-01456]]
    pname:handleType must: be defined as a POSIX file descriptor handle.
****

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

endif::VK_KHR_external_fence_fd[]

[open,refpage='vkDestroyFence',desc='Destroy a fence object',type='protos']
--

To destroy a fence, call:

include::../api/protos/vkDestroyFence.txt[]

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

.Valid Usage
****
  * [[VUID-vkDestroyFence-fence-01120]]
    All <<devsandqueues-submission, queue submission>> commands that refer
    to pname:fence must: have completed execution
  * [[VUID-vkDestroyFence-fence-01121]]
    If sname:VkAllocationCallbacks were provided when pname:fence was
    created, a compatible set of callbacks must: be provided here
  * [[VUID-vkDestroyFence-fence-01122]]
    If no sname:VkAllocationCallbacks were provided when pname:fence was
    created, pname:pAllocator must: be `NULL`
****

include::../validity/protos/vkDestroyFence.txt[]
--

[open,refpage='vkGetFenceStatus',desc='Return the status of a fence',type='protos']
--

To query the status of a fence from the host, call:

include::../api/protos/vkGetFenceStatus.txt[]

  * pname:device is the logical device that owns the fence.
  * pname:fence is the handle of the fence to query.

Upon success, fname:vkGetFenceStatus returns the status of the fence object,
with the following return codes:

.Fence Object Status Codes
[width="80%",options="header"]
|====
| Status | Meaning
| ename:VK_SUCCESS | The fence specified by pname:fence is signaled.
| ename:VK_NOT_READY | The fence specified by pname:fence is unsignaled.
| ename:VK_ERROR_DEVICE_LOST | The device has been lost.  See <<devsandqueues-lost-device,Lost Device>>.
|====

If a <<devsandqueues-submission, queue submission>> command is pending
execution, then the value returned by this command may: immediately be out
of date.

If the device has been lost (see <<devsandqueues-lost-device,Lost Device>>),
fname:vkGetFenceStatus may: return any of the above status codes.
If the device has been lost and fname:vkGetFenceStatus is called repeatedly,
it will eventually return either ename:VK_SUCCESS or
ename:VK_ERROR_DEVICE_LOST.

include::../validity/protos/vkGetFenceStatus.txt[]
--

[[synchronization-fences-unsignaling]]
[open,refpage='vkResetFences',desc='Resets one or more fence objects',type='protos']
--

To set the state of fences to unsignaled from the host, call:

include::../api/protos/vkResetFences.txt[]

  * pname:device is the logical device that owns the fences.
  * pname:fenceCount is the number of fences to reset.
  * pname:pFences is a pointer to an array of fence handles to reset.

ifdef::VK_VERSION_1_1,VK_KHR_external_fence[]

If any member of pname:pFences currently has its
<<synchronization-fences-importing, payload imported>> with temporary
permanence, that fence's prior permanent payload is first restored.
The remaining operations described therefore operate on the restored
payload.

endif::VK_VERSION_1_1,VK_KHR_external_fence[]

When flink:vkResetFences is executed on the host, it defines a _fence
unsignal operation_ for each fence, which resets the fence to the unsignaled
state.

If any member of pname:pFences is already in the unsignaled state when
flink:vkResetFences is executed, then flink:vkResetFences has no effect on
that fence.

.Valid Usage
****
  * [[VUID-vkResetFences-pFences-01123]]
    Each element of pname:pFences must: not be currently associated with any
    queue command that has not yet completed execution on that queue
****

include::../validity/protos/vkResetFences.txt[]
--

[[synchronization-fences-signaling]]
When a fence is submitted to a queue as part of a
<<devsandqueues-submission, queue submission>> command, it defines a memory
dependency on the batches that were submitted as part of that command, and
defines a _fence signal operation_ which sets the fence to the signaled
state.

The first <<synchronization-dependencies-scopes, synchronization scope>>
includes every batch submitted in the same <<devsandqueues-submission, queue
submission>> command.
Fence signal operations that are defined by flink:vkQueueSubmit additionally
include in the first synchronization scope all commands that occur earlier
in <<synchronization-submission-order,submission order>>.

The second <<synchronization-dependencies-scopes, synchronization scope>>
only includes the fence signal operation.

The first <<synchronization-dependencies-access-scopes, access scope>>
includes all memory access performed by the device.

The second <<synchronization-dependencies-access-scopes, access scope>> is
empty.

[open,refpage='vkWaitForFences',desc='Wait for one or more fences to become signaled',type='protos']
--

To wait for one or more fences to enter the signaled state on the host,
call:

include::../api/protos/vkWaitForFences.txt[]

  * pname:device is the logical device that owns the fences.
  * pname:fenceCount is the number of fences to wait on.
  * pname:pFences is a pointer to an array of pname:fenceCount fence
    handles.
  * pname:waitAll is the condition that must: be satisfied to successfully
    unblock the wait.
    If pname:waitAll is ename:VK_TRUE, then the condition is that all fences
    in pname:pFences are signaled.
    Otherwise, the condition is that at least one fence in pname:pFences is
    signaled.
  * pname:timeout is the timeout period in units of nanoseconds.
    pname:timeout is adjusted to the closest value allowed by the
    implementation-dependent timeout accuracy, which may: be substantially
    longer than one nanosecond, and may: be longer than the requested
    period.

If the condition is satisfied when fname:vkWaitForFences is called, then
fname:vkWaitForFences returns immediately.
If the condition is not satisfied at the time fname:vkWaitForFences is
called, then fname:vkWaitForFences will block and wait up to pname:timeout
nanoseconds for the condition to become satisfied.

If pname:timeout is zero, then fname:vkWaitForFences does not wait, but
simply returns the current state of the fences.
ename:VK_TIMEOUT will be returned in this case if the condition is not
satisfied, even though no actual wait was performed.

If the specified timeout period expires before the condition is satisfied,
fname:vkWaitForFences returns ename:VK_TIMEOUT.
If the condition is satisfied before pname:timeout nanoseconds has expired,
fname:vkWaitForFences returns ename:VK_SUCCESS.

If device loss occurs (see <<devsandqueues-lost-device,Lost Device>>) before
the timeout has expired, fname:vkWaitForFences must: return in finite time
with either ename:VK_SUCCESS or ename:VK_ERROR_DEVICE_LOST.

[NOTE]
.Note
====
While we guarantee that fname:vkWaitForFences must: return in finite time,
no guarantees are made that it returns immediately upon device loss.
However, the client can reasonably expect that the delay will be on the
order of seconds and that calling fname:vkWaitForFences will not result in a
permanently (or seemingly permanently) dead process.
====

include::../validity/protos/vkWaitForFences.txt[]
--

[[synchronization-fences-waiting]]
An execution dependency is defined by waiting for a fence to become
signaled, either via flink:vkWaitForFences or by polling on
flink:vkGetFenceStatus.

The first <<synchronization-dependencies-scopes, synchronization scope>>
includes only the fence signal operation.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes the host operations of flink:vkWaitForFences or
flink:vkGetFenceStatus indicating that the fence has become signaled.

[NOTE]
.Note
====
Signaling a fence and waiting on the host does not guarantee that the
results of memory accesses will be visible to the host, as the access scope
of a memory dependency defined by a fence only includes device access.
A <<synchronization-memory-barriers, memory barrier>> or other memory
dependency must: be used to guarantee this.
See the description of <<synchronization-host-access-types, host access
types>> for more information.
====

ifdef::VK_EXT_display_control[]
include::VK_EXT_display_control/fence_events.txt[]
endif::VK_EXT_display_control[]


ifdef::VK_VERSION_1_1,VK_KHR_external_fence[]

[[synchronization-fences-importing]]
=== Importing Fence Payloads

Applications can: import a fence payload into an existing fence using an
external fence handle.
The effects of the import operation will be either temporary or permanent,
as specified by the application.
If the import is temporary, the fence will be _restored_ to its permanent
state the next time that fence is passed to flink:vkResetFences.

[NOTE]
.Note
====
Restoring a fence to its prior permanent payload is a distinct operation
from resetting a fence payload.
See flink:vkResetFences for more detail.
====

Performing a subsequent temporary import on a fence before resetting it has
no effect on this requirement; the next unsignal of the fence must: still
restore its last permanent state.
A permanent payload import behaves as if the target fence was destroyed, and
a new fence was created with the same handle but the imported payload.
Because importing a fence payload temporarily or permanently detaches the
existing payload from a fence, similar usage restrictions to those applied
to fname:vkDestroyFence are applied to any command that imports a fence
payload.
Which of these import types is used is referred to as the import operation's
_permanence_.
Each handle type supports either one or both types of permanence.

The implementation must: perform the import operation by either referencing
or copying the payload referred to by the specified external fence handle,
depending on the handle's type.
The import method used is referred to as the handle type's _transference_.
When using handle types with reference transference, importing a payload to
a fence adds the fence to the set of all fences sharing that payload.
This set includes the fence from which the payload was exported.
Fence signaling, waiting, and resetting operations performed on any fence in
the set must: behave as if the set were a single fence.
Importing a payload using handle types with copy transference creates a
duplicate copy of the payload at the time of import, but makes no further
reference to it.
Fence signaling, waiting, and resetting operations performed on the target
of copy imports must: not affect any other fence or payload.

Export operations have the same transference as the specified handle type's
import operations.
Additionally, exporting a fence payload to a handle with copy transference
has the same side effects on the source fence's payload as executing a fence
reset operation.
If the fence was using a temporarily imported payload, the fence's prior
permanent payload will be restored.

ifdef::VK_KHR_external_fence_win32,VK_KHR_external_fence_fd[]
[NOTE]
.Note
====
The
ifdef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[tables]
ifndef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[table]
ifdef::VK_KHR_external_fence_win32[]
<<synchronization-fence-handletypes-win32,Handle Types Supported by
VkImportFenceWin32HandleInfoKHR>>
endif::VK_KHR_external_fence_win32[]
ifdef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[and]
ifdef::VK_KHR_external_fence_fd[]
<<synchronization-fence-handletypes-fd,Handle Types Supported by
VkImportFenceFdInfoKHR>>
endif::VK_KHR_external_fence_fd[]
ifdef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[define]
ifndef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[defines]
the permanence and transference of each handle type.
====
endif::VK_KHR_external_fence_win32,VK_KHR_external_fence_fd[]

<<fundamentals-threadingbehavior,External synchronization>> allows
implementations to modify an object's internal state, i.e. payload, without
internal synchronization.
However, for fences sharing a payload across processes, satisfying the
external synchronization requirements of fname:VkFence parameters as if all
fences in the set were the same object is sometimes infeasible.
Satisfying valid usage constraints on the state of a fence would similarly
require impractical coordination or levels of trust between processes.
Therefore, these constraints only apply to a specific fence handle, not to
its payload.
For distinct fence objects which share a payload:

* If multiple commands which queue a signal operation, or which unsignal a
fence, are called concurrently, behavior will be as if the commands were
called in an arbitrary sequential order.
* If a queue submission command is called with a fence that is sharing a
payload, and the payload is already associated with another queue command
that has not yet completed execution, either one or both of the commands
will cause the fence to become signaled when they complete execution.
* If a fence payload is reset while it is associated with a queue command
that has not yet completed execution, the payload will become unsignaled,
but may: become signaled again when the command completes execution.
* In the preceding cases, any of the devices associated with the fences
sharing the payload may: be lost, or any of the queue submission or fence
reset commands may: return ename:VK_ERROR_INITIALIZATION_FAILED.

Other than these non-deterministic results, behavior is well defined.
In particular:

* The implementation must: not crash or enter an internally inconsistent
state where future valid Vulkan commands might cause undefined results,
* Timeouts on future wait commands on fences sharing the payload must: be
effective.

[NOTE]
.Note
====
These rules allow processes to synchronize access to shared memory without
trusting each other.
However, such processes must still be cautious not to use the shared fence
for more than synchronizing access to the shared memory.
For example, a process should not use a fence with shared payload to tell
when commands it submitted to a queue have completed and objects used by
those commands may be destroyed, since the other process can accidentally or
maliciously cause the fence to signal before the commands actually complete.
====

When a fence is using an imported payload, its
slink:VkExportFenceCreateInfo::pname:handleTypes value is that specified
when creating the fence from which the payload was exported, rather than
that specified when creating the fence.
Additionally, slink:VkExternalFenceProperties::exportFromImportedHandleTypes
restricts which handle types can: be exported from such a fence based on the
specific handle type used to import the current payload.
ifdef::VK_KHR_swapchain[]
Passing a fence to flink:vkAcquireNextImageKHR is equivalent to temporarily
importing a fence payload to that fence.

[NOTE]
.Note
====
Because the exportable handle types of an imported fence correspond to its
current imported payload, and flink:vkAcquireNextImageKHR behaves the same
as a temporary import operation for which the source fence is opaque to the
application, applications have no way of determining whether any external
handle types can: be exported from a fence in this state.
Therefore, applications must: not attempt to export handles from fences
using a temporarily imported payload from flink:vkAcquireNextImageKHR.
====
endif::VK_KHR_swapchain[]

When importing a fence payload, it is the responsibility of the application
to ensure the external handles meet all valid usage requirements.
However, implementations must: perform sufficient validation of external
handles to ensure that the operation results in a valid fence which will not
cause program termination, device loss, queue stalls, host thread stalls, or
corruption of other resources when used as allowed according to its import
parameters.
If the external handle provided does not meet these requirements, the
implementation must: fail the fence payload import operation with the error
code ename:VK_ERROR_INVALID_EXTERNAL_HANDLE.

endif::VK_VERSION_1_1,VK_KHR_external_fence[]

ifdef::VK_KHR_external_fence_win32[]

[open,refpage='vkImportFenceWin32HandleKHR',desc='Import a fence from a Windows HANDLE',type='protos']
--

To import a fence payload from a Windows handle, call:

include::../api/protos/vkImportFenceWin32HandleKHR.txt[]

  * pname:device is the logical device that created the fence.
  * pname:pImportFenceWin32HandleInfo points to a
    slink:VkImportFenceWin32HandleInfoKHR structure specifying the fence and
    import parameters.

Importing a fence payload from Windows handles does not transfer ownership
of the handle to the Vulkan implementation.
For handle types defined as NT handles, the application must: release
ownership using the fname:CloseHandle system call when the handle is no
longer needed.

Applications can: import the same fence payload into multiple instances of
Vulkan, into the same instance from which it was exported, and multiple
times into a given Vulkan instance.

include::../validity/protos/vkImportFenceWin32HandleKHR.txt[]
--

[open,refpage='VkImportFenceWin32HandleInfoKHR',desc='(None)',type='structs']
--
The sname:VkImportFenceWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkImportFenceWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:fence is the fence into which the state will be imported.
  * pname:flags is a bitmask of elink:VkFenceImportFlagBits specifying
    additional parameters for the fence payload import operation.
  * pname:handleType specifies the type of pname:handle.
  * pname:handle is the external handle to import, or `NULL`.
  * pname:name is the NULL-terminated UTF-16 string naming the underlying
    synchronization primitive to import, or `NULL`.

The handle types supported by pname:handleType are:

[[synchronization-fence-handletypes-win32]]
.Handle Types Supported by VkImportFenceWin32HandleInfoKHR
[width="80%",options="header"]
|====
| Handle Type                                                  | Transference | Permanence Supported
| ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_WIN32_BIT     | Reference    | Temporary,Permanent
| ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_WIN32_KMT_BIT | Reference    | Temporary,Permanent
|====

.Valid Usage
****
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handleType-01457]]
    pname:handleType must: be a value included in the
    <<synchronization-fence-handletypes-win32, Handle Types Supported by
    VkImportFenceWin32HandleInfoKHR>> table.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handleType-01459]]
    If pname:handleType is not
    ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_WIN32_BIT, pname:name must:
    be `NULL`.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handleType-01460]]
    If pname:handleType is not `0` and pname:handle is `NULL`, pname:name
    must: name a valid synchronization primitive of the type specified by
    pname:handleType.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handleType-01461]]
    If pname:handleType is not `0` and pname:name is `NULL`, pname:handle
    must: be a valid handle of the type specified by pname:handleType.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handle-01462]]
    If pname:handle is not `NULL`, pname:name must be `NULL`.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-handle-01539]]
    If pname:handle is not `NULL`, it must: obey any requirements listed for
    pname:handleType in <<external-fence-handle-types-compatibility,external
    fence handle types compatibility>>.
  * [[VUID-VkImportFenceWin32HandleInfoKHR-name-01540]]
    If pname:name is not `NULL`, it must: obey any requirements listed for
    pname:handleType in <<external-fence-handle-types-compatibility,external
    fence handle types compatibility>>.
****

include::../validity/structs/VkImportFenceWin32HandleInfoKHR.txt[]
--

endif::VK_KHR_external_fence_win32[]

ifdef::VK_KHR_external_fence_fd[]

[open,refpage='vkImportFenceFdKHR',desc='Import a fence from a POSIX file descriptor',type='protos']
--

To import a fence payload from a POSIX file descriptor, call:

include::../api/protos/vkImportFenceFdKHR.txt[]

  * pname:device is the logical device that created the fence.
  * pname:pImportFenceFdInfo points to a slink:VkImportFenceFdInfoKHR
    structure specifying the fence and import parameters.

Importing a fence payload from a file descriptor transfers ownership of the
file descriptor from the application to the Vulkan implementation.
The application must: not perform any operations on the file descriptor
after a successful import.

Applications can: import the same fence payload into multiple instances of
Vulkan, into the same instance from which it was exported, and multiple
times into a given Vulkan instance.

.Valid Usage
****
  * [[VUID-vkImportFenceFdKHR-fence-01463]]
    pname:fence must: not be associated with any queue command that has not
    yet completed execution on that queue
****

include::../validity/protos/vkImportFenceFdKHR.txt[]
--

[open,refpage='VkImportFenceFdInfoKHR',desc='(None)',type='structs']
--
The sname:VkImportFenceFdInfoKHR structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:fence is the fence into which the payload will be imported.
  * pname:flags is a bitmask of elink:VkFenceImportFlagBits specifying
    additional parameters for the fence payload import operation.
  * pname:handleType specifies the type of pname:fd.
  * pname:fd is the external handle to import.

The handle types supported by pname:handleType are:

[[synchronization-fence-handletypes-fd]]
.Handle Types Supported by VkImportFenceFdInfoKHR
[width="80%",options="header"]
|====
| Handle Type                                           | Transference | Permanence Supported
| ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_OPAQUE_FD_BIT | Reference    | Temporary,Permanent
| ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_SYNC_FD_BIT   | Copy         | Temporary
|====

.Valid Usage
****
  * [[VUID-VkImportFenceFdInfoKHR-handleType-01464]]
    pname:handleType must: be a value included in the
    <<synchronization-fence-handletypes-fd, Handle Types Supported by
    VkImportFenceFdInfoKHR>> table.
  * [[VUID-VkImportFenceFdInfoKHR-fd-01541]]
    pname:fd must: obey any requirements listed for pname:handleType in
    <<external-fence-handle-types-compatibility,external fence handle types
    compatibility>>.
****

If pname:handleType is ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_SYNC_FD_BIT, the
special value `-1` for pname:fd is treated like a valid sync file descriptor
referring to an object that has already signaled.
The import operation will succeed and the sname:VkFence will have a
temporarily imported payload as if a valid file descriptor had been
provided.

[NOTE]
.Note
====
This special behavior for importing an invalid sync file descriptor allows
easier interoperability with other system APIs which use the convention that
an invalid sync file descriptor represents work that has already completed
and does not need to be waited for.
It is consistent with the option for implementations to return a `-1` file
descriptor when exporting a ename:VK_EXTERNAL_FENCE_HANDLE_TYPE_SYNC_FD_BIT
from a sname:VkFence which is signaled.
====

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

endif::VK_KHR_external_fence_fd[]

ifdef::VK_VERSION_1_1,VK_KHR_external_fence[]
ifdef::VK_KHR_external_fence_win32,VK_KHR_external_fence_fd[]

[open,refpage='VkFenceImportFlagBits',desc='Bitmask specifying additional parameters of fence payload import',type='enums']
--

Bits which can: be set in
ifdef::VK_KHR_external_fence_win32[]
slink:VkImportFenceWin32HandleInfoKHR::pname:flags
endif::VK_KHR_external_fence_win32[]
ifdef::VK_KHR_external_fence_win32+VK_KHR_external_fence_fd[and]
ifdef::VK_KHR_external_fence_fd[]
slink:VkImportFenceFdInfoKHR::pname:flags
endif::VK_KHR_external_fence_fd[]
specifying additional parameters of a fence import operation are:

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

ifdef::VK_KHR_external_fence[]
or the equivalent

include::../api/enums/VkFenceImportFlagBitsKHR.txt[]
endif::VK_KHR_external_fence[]

  * ename:VK_FENCE_IMPORT_TEMPORARY_BIT specifies that the fence payload
    will be imported only temporarily, as described in
    <<synchronization-fences-importing,Importing Fence Payloads>>,
    regardless of the permanence of pname:handleType.

--

[open,refpage='VkFenceImportFlags',desc='Bitmask of VkFenceImportFlagBits',type='enums']
--
include::../api/flags/VkFenceImportFlags.txt[]

ifdef::VK_KHR_external_fence[]
or the equivalent

include::../api/flags/VkFenceImportFlagsKHR.txt[]
endif::VK_KHR_external_fence[]

sname:VkFenceImportFlags is a bitmask type for setting a mask of zero or
more slink:VkFenceImportFlagBits.
--

endif::VK_KHR_external_fence_win32,VK_KHR_external_fence_fd[]
endif::VK_VERSION_1_1,VK_KHR_external_fence[]


[[synchronization-semaphores]]
== Semaphores

[open,refpage='VkSemaphore',desc='Opaque handle to a semaphore object',type='handles']
--

Semaphores are a synchronization primitive that can: be used to insert a
dependency between batches submitted to queues.
Semaphores have two states - signaled and unsignaled.
The state of a semaphore can: be signaled after execution of a batch of
commands is completed.
A batch can: wait for a semaphore to become signaled before it begins
execution, and the semaphore is also unsignaled before the batch begins
execution.

ifdef::VK_VERSION_1_1,VK_KHR_external_semaphore[]
[[synchronization-semaphores-payloads]]
As with most objects in Vulkan, semaphores are an interface to internal data
which is typically opaque to applications.
This internal data is referred to as a semaphore's _payload_.

However, in order to enable communication with agents outside of the current
device, it is necessary to be able to export that payload to a commonly
understood format, and subsequently import from that format as well.

The internal data of a semaphore may: include a reference to any resources
and pending work associated with signal or unsignal operations performed on
that semaphore object.
Mechanisms to import and export that internal data to and from semaphores
are provided <<VkExportSemaphoreCreateInfo, below>>.
These mechanisms indirectly enable applications to share semaphore state
between two or more semaphores and other synchronization primitives across
process and API boundaries.

endif::VK_VERSION_1_1,VK_KHR_external_semaphore[]

Semaphores are represented by sname:VkSemaphore handles:

include::../api/handles/VkSemaphore.txt[]

--

[open,refpage='vkCreateSemaphore',desc='Create a new queue semaphore object',type='protos']
--

To create a semaphore, call:

include::../api/protos/vkCreateSemaphore.txt[]

  * pname:device is the logical device that creates the semaphore.
  * pname:pCreateInfo is a pointer to an instance of the
    sname:VkSemaphoreCreateInfo structure which contains information about
    how the semaphore is to be created.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pSemaphore points to a handle in which the resulting semaphore
    object is returned.

When created, the semaphore is in the unsignaled state.

include::../validity/protos/vkCreateSemaphore.txt[]
--

[open,refpage='VkSemaphoreCreateInfo',desc='Structure specifying parameters of a newly created semaphore',type='structs']
--

The sname:VkSemaphoreCreateInfo structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.

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

[open,refpage='VkSemaphoreCreateFlags',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkSemaphoreCreateFlags.txt[]

sname:VkSemaphoreCreateFlags is a bitmask type for setting a mask, but is
currently reserved for future use.
--

ifdef::VK_VERSION_1_1,VK_KHR_external_semaphore[]

[open,refpage='VkExportSemaphoreCreateInfo',desc='Structure specifying handle types that can be exported from a semaphore',type='structs']
--

To create a semaphore whose payload can: be exported to external handles,
add the slink:VkExportSemaphoreCreateInfo structure to the pname:pNext chain
of the slink:VkSemaphoreCreateInfo structure.
The sname:VkExportSemaphoreCreateInfo structure is defined as:

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

ifdef::VK_KHR_external_semaphore[]
or the equivalent

include::../api/structs/VkExportSemaphoreCreateInfoKHR.txt[]
endif::VK_KHR_external_semaphore[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:handleTypes is a bitmask of
    elink:VkExternalSemaphoreHandleTypeFlagBits specifying one or more
    semaphore handle types the application can: export from the resulting
    semaphore.
    The application can: request multiple handle types for the same
    semaphore.

.Valid Usage
****
  * [[VUID-VkExportSemaphoreCreateInfo-handleTypes-01124]]
    The bits in pname:handleTypes must: be supported and compatible, as
    reported by slink:VkExternalSemaphoreProperties.
****

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

endif::VK_VERSION_1_1,VK_KHR_external_semaphore[]

ifdef::VK_KHR_external_semaphore_win32[]

[open,refpage='VkExportSemaphoreWin32HandleInfoKHR',desc='Structure specifying additional attributes of Windows handles exported from a semaphore',type='structs']
--

To specify additional attributes of NT handles exported from a semaphore,
add the sname:VkExportSemaphoreWin32HandleInfoKHR structure to the
pname:pNext chain of the slink:VkSemaphoreCreateInfo structure.
The sname:VkExportSemaphoreWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkExportSemaphoreWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:pAttributes is a pointer to a Windows code:SECURITY_ATTRIBUTES
    structure specifying security attributes of the handle.
  * pname:dwAccess is a code:DWORD specifying access rights of the handle.
  * pname:name is a NULL-terminated UTF-16 string to associate with the
    underlying synchronization primitive referenced by NT handles exported
    from the created semaphore.

If this structure is not present, or if pname:pAttributes is set to `NULL`,
default security descriptor values will be used, and child processes created
by the application will not inherit the handle, as described in the MSDN
documentation for "`Synchronization Object Security and Access Rights`"^1^.
Further, if the structure is not present, the access rights will be

code:DXGI_SHARED_RESOURCE_READ | code:DXGI_SHARED_RESOURCE_WRITE

for handles of the following types:

ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_WIN32_BIT

And

code:GENERIC_ALL

for handles of the following types:

ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_D3D12_FENCE_BIT

1::
    https://msdn.microsoft.com/en-us/library/windows/desktop/ms686670.aspx

.Valid Usage
****
  * [[VUID-VkExportSemaphoreWin32HandleInfoKHR-handleTypes-01125]]
    If slink:VkExportSemaphoreCreateInfo::pname:handleTypes does not include
    ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_WIN32_BIT or
    ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_D3D12_FENCE_BIT,
    sname:VkExportSemaphoreWin32HandleInfoKHR must: not be in the
    pname:pNext chain of slink:VkSemaphoreCreateInfo.
****

include::../validity/structs/VkExportSemaphoreWin32HandleInfoKHR.txt[]
--

[open,refpage='vkGetSemaphoreWin32HandleKHR',desc='Get a Windows HANDLE for a semaphore',type='protos']
--

To export a Windows handle representing the payload of a semaphore, call:

include::../api/protos/vkGetSemaphoreWin32HandleKHR.txt[]

  * pname:device is the logical device that created the semaphore being
    exported.
  * pname:pGetWin32HandleInfo is a pointer to an instance of the
    slink:VkSemaphoreGetWin32HandleInfoKHR structure containing parameters
    of the export operation.
  * pname:pHandle will return the Windows handle representing the semaphore
    state.

For handle types defined as NT handles, the handles returned by
fname:vkGetSemaphoreWin32HandleKHR are owned by the application.
To avoid leaking resources, the application must: release ownership of them
using the fname:CloseHandle system call when they are no longer needed.

Exporting a Windows handle from a semaphore may: have side effects depending
on the transference of the specified handle type, as described in
<<synchronization-semaphores-importing,Importing Semaphore Payloads>>.

include::../validity/protos/vkGetSemaphoreWin32HandleKHR.txt[]
--

[open,refpage='VkSemaphoreGetWin32HandleInfoKHR',desc='Structure describing a Win32 handle semaphore export operation',type='structs']
--

The sname:VkSemaphoreGetWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkSemaphoreGetWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:semaphore is the semaphore from which state will be exported.
  * pname:handleType is the type of handle requested.

The properties of the handle returned depend on the value of
pname:handleType.
See elink:VkExternalSemaphoreHandleTypeFlagBits for a description of the
properties of the defined external semaphore handle types.

.Valid Usage
****
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-handleType-01126]]
    pname:handleType must: have been included in
    slink:VkExportSemaphoreCreateInfo::pname:handleTypes when the
    pname:semaphore's current payload was created.
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-handleType-01127]]
    If pname:handleType is defined as an NT handle,
    flink:vkGetSemaphoreWin32HandleKHR must: be called no more than once for
    each valid unique combination of pname:semaphore and pname:handleType.
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-semaphore-01128]]
    pname:semaphore must: not currently have its payload replaced by an
    imported payload as described below in
    <<synchronization-semaphores-importing,Importing Semaphore Payloads>>
    unless that imported payload's handle type was included in
    slink:VkExternalSemaphoreProperties::pname:exportFromImportedHandleTypes
    for pname:handleType.
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-handleType-01129]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, as defined below in
    <<synchronization-semaphores-importing,Importing Semaphore Payloads>>,
    there must: be no queue waiting on pname:semaphore.
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-handleType-01130]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, pname:semaphore must: be signaled, or have an
    associated <<synchronization-semaphores-signaling,semaphore signal
    operation>> pending execution.
  * [[VUID-VkSemaphoreGetWin32HandleInfoKHR-handleType-01131]]
    pname:handleType must: be defined as an NT handle or a global share
    handle.
****

include::../validity/structs/VkSemaphoreGetWin32HandleInfoKHR.txt[]
--

endif::VK_KHR_external_semaphore_win32[]

ifdef::VK_KHR_external_semaphore_fd[]

[open,refpage='vkGetSemaphoreFdKHR',desc='Get a POSIX file descriptor handle for a semaphore',type='protos']
--

To export a POSIX file descriptor representing the payload of a semaphore,
call:

include::../api/protos/vkGetSemaphoreFdKHR.txt[]

  * pname:device is the logical device that created the semaphore being
    exported.
  * pname:pGetFdInfo is a pointer to an instance of the
    slink:VkSemaphoreGetFdInfoKHR structure containing parameters of the
    export operation.
  * pname:pFd will return the file descriptor representing the semaphore
    payload.

Each call to fname:vkGetSemaphoreFdKHR must: create a new file descriptor
and transfer ownership of it to the application.
To avoid leaking resources, the application must: release ownership of the
file descriptor when it is no longer needed.

[NOTE]
.Note
====
Ownership can be released in many ways.
For example, the application can call code:close() on the file descriptor,
or transfer ownership back to Vulkan by using the file descriptor to import
a semaphore payload.
====
Where supported by the operating system, the implementation must: set the
file descriptor to be closed automatically when an code:execve system call
is made.

Exporting a file descriptor from a semaphore may: have side effects
depending on the transference of the specified handle type, as described in
<<synchronization-semaphores-importing,Importing Semaphore State>>.

include::../validity/protos/vkGetSemaphoreFdKHR.txt[]
--

[open,refpage='VkSemaphoreGetFdInfoKHR',desc='Structure describing a POSIX FD semaphore export operation',type='structs']
--

The sname:VkSemaphoreGetFdInfoKHR structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:semaphore is the semaphore from which state will be exported.
  * pname:handleType is the type of handle requested.

The properties of the file descriptor returned depend on the value of
pname:handleType.
See elink:VkExternalSemaphoreHandleTypeFlagBits for a description of the
properties of the defined external semaphore handle types.

.Valid Usage
****
  * [[VUID-VkSemaphoreGetFdInfoKHR-handleType-01132]]
    pname:handleType must: have been included in
    slink:VkExportSemaphoreCreateInfo::pname:handleTypes when
    pname:semaphore's current payload was created.
  * [[VUID-VkSemaphoreGetFdInfoKHR-semaphore-01133]]
    pname:semaphore must: not currently have its payload replaced by an
    imported payload as described below in
    <<synchronization-semaphores-importing,Importing Semaphore Payloads>>
    unless that imported payload's handle type was included in
    slink:VkExternalSemaphoreProperties::pname:exportFromImportedHandleTypes
    for pname:handleType.
  * [[VUID-VkSemaphoreGetFdInfoKHR-handleType-01134]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, as defined below in
    <<synchronization-semaphores-importing,Importing Semaphore Payloads>>,
    there must: be no queue waiting on pname:semaphore.
  * [[VUID-VkSemaphoreGetFdInfoKHR-handleType-01135]]
    If pname:handleType refers to a handle type with copy payload
    transference semantics, pname:semaphore must: be signaled, or have an
    associated <<synchronization-semaphores-signaling,semaphore signal
    operation>> pending execution.
  * [[VUID-VkSemaphoreGetFdInfoKHR-handleType-01136]]
    pname:handleType must: be defined as a POSIX file descriptor handle.
****

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

endif::VK_KHR_external_semaphore_fd[]

[open,refpage='vkDestroySemaphore',desc='Destroy a semaphore object',type='protos']
--

To destroy a semaphore, call:

include::../api/protos/vkDestroySemaphore.txt[]

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

.Valid Usage
****
  * [[VUID-vkDestroySemaphore-semaphore-01137]]
    All submitted batches that refer to pname:semaphore must: have completed
    execution
  * [[VUID-vkDestroySemaphore-semaphore-01138]]
    If sname:VkAllocationCallbacks were provided when pname:semaphore was
    created, a compatible set of callbacks must: be provided here
  * [[VUID-vkDestroySemaphore-semaphore-01139]]
    If no sname:VkAllocationCallbacks were provided when pname:semaphore was
    created, pname:pAllocator must: be `NULL`
****

include::../validity/protos/vkDestroySemaphore.txt[]
--


[[synchronization-semaphores-signaling]]
=== Semaphore Signaling

When a batch is submitted to a queue via a <<devsandqueues-submission, queue
submission>>, and it includes semaphores to be signaled, it defines a memory
dependency on the batch, and defines _semaphore signal operations_ which set
the semaphores to the signaled state.

The first <<synchronization-dependencies-scopes, synchronization scope>>
includes every command submitted in the same batch.
Semaphore signal operations that are defined by flink:vkQueueSubmit
additionally include all commands that occur earlier in
<<synchronization-submission-order,submission order>>.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes only the semaphore signal operation.

The first <<synchronization-dependencies-access-scopes, access scope>>
includes all memory access performed by the device.

The second <<synchronization-dependencies-access-scopes, access scope>> is
empty.


[[synchronization-semaphores-waiting]]
=== Semaphore Waiting & Unsignaling

When a batch is submitted to a queue via a <<devsandqueues-submission, queue
submission>>, and it includes semaphores to be waited on, it defines a
memory dependency between prior semaphore signal operations and the batch,
and defines _semaphore unsignal operations_ which set the semaphores to the
unsignaled state.

The first synchronization scope includes all semaphore signal operations
that operate on semaphores waited on in the same batch, and that
happen-before the wait completes.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes every command submitted in the same batch.
In the case of flink:vkQueueSubmit, the second synchronization scope is
limited to operations on the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, destination stage mask>> specified
by the corresponding element of pname:pWaitDstStageMask.
Also, in the case of flink:vkQueueSubmit, the second synchronization scope
additionally includes all commands that occur later in
<<synchronization-submission-order,submission order>>.

The first <<synchronization-dependencies-access-scopes, access scope>> is
empty.

The second <<synchronization-dependencies-access-scopes, access scope>>
includes all memory access performed by the device.

The semaphore unsignal operation happens-after the first set of operations
in the execution dependency, and happens-before the second set of operations
in the execution dependency.

[NOTE]
.Note
====
Unlike fences or events, the act of waiting for a semaphore also unsignals
that semaphore.
If two operations are separately specified to wait for the same semaphore,
and there are no other execution dependencies between those operations,
behaviour is undefined.
An execution dependency must: be present that guarantees that the semaphore
unsignal operation for the first of those waits, happens-before the
semaphore is signalled again, and before the second unsignal operation.
Semaphore waits and signals should thus occur in discrete 1:1 pairs.
====

ifdef::VK_KHR_swapchain[]
[NOTE]
.Note
====
A common scenario for using pname:pWaitDstStageMask with values other than
ename:VK_PIPELINE_STAGE_ALL_COMMANDS_BIT is when synchronizing a window
system presentation operation against subsequent command buffers which
render the next frame.
In this case, a presentation image must: not be overwritten until the
presentation operation completes, but other pipeline stages can: execute
without waiting.
A mask of ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT prevents
subsequent color attachment writes from executing until the semaphore
signals.
Some implementations may: be able to execute transfer operations and/or
vertex processing work before the semaphore is signaled.

If an image layout transition needs to be performed on a presentable image
before it is used in a framebuffer, that can: be performed as the first
operation submitted to the queue after acquiring the image, and should: not
prevent other work from overlapping with the presentation operation.
For example, a sname:VkImageMemoryBarrier could use:

  * pname:srcStageMask = ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
  * pname:srcAccessMask = 0
  * pname:dstStageMask = ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
  * pname:dstAccessMask = ename:VK_ACCESS_COLOR_ATTACHMENT_READ_BIT |
    ename:VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT.
  * pname:oldLayout = ename:VK_IMAGE_LAYOUT_PRESENT_SRC_KHR
  * pname:newLayout = ename:VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL

Alternatively, pname:oldLayout can: be ename:VK_IMAGE_LAYOUT_UNDEFINED, if
the image's contents need not be preserved.

This barrier accomplishes a dependency chain between previous presentation
operations and subsequent color attachment output operations, with the
layout transition performed in between, and does not introduce a dependency
between previous work and any vertex processing stages.
More precisely, the semaphore signals after the presentation operation
completes, the semaphore wait stalls the
ename:VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT stage, and there is a
dependency from that same stage to itself with the layout transition
performed in between.
====
endif::VK_KHR_swapchain[]


[[synchronization-semaphores-waiting-state]]
=== Semaphore State Requirements For Wait Operations

Before waiting on a semaphore, the application must: ensure the semaphore is
in a valid state for a wait operation.
Specifically, when a <<synchronization-semaphores-waiting,semaphore wait and
unsignal operation>> is submitted to a queue:

  * The semaphore must: be signaled, or have an associated
    <<synchronization-semaphores-signaling,semaphore signal operation>> that
    is pending execution.
  * There must: be no other queue waiting on the same semaphore when the
    operation executes.

ifdef::VK_VERSION_1_1,VK_KHR_external_semaphore[]

[[synchronization-semaphores-importing]]
=== Importing Semaphore Payloads

Applications can: import a semaphore payload into an existing semaphore
using an external semaphore handle.
The effects of the import operation will be either temporary or permanent,
as specified by the application.
If the import is temporary, the implementation must: restore the semaphore
to its prior permanent state after submitting the next semaphore wait
operation.
Performing a subsequent temporary import on a semaphore before performing a
semaphore wait has no effect on this requirement; the next wait submitted on
the semaphore must: still restore its last permanent state.
A permanent payload import behaves as if the target semaphore was destroyed,
and a new semaphore was created with the same handle but the imported
payload.
Because importing a semaphore payload temporarily or permanently detaches
the existing payload from a semaphore, similar usage restrictions to those
applied to fname:vkDestroySemaphore are applied to any command that imports
a semaphore payload.
Which of these import types is used is referred to as the import operation's
_permanence_.
Each handle type supports either one or both types of permanence.

The implementation must: perform the import operation by either referencing
or copying the payload referred to by the specified external semaphore
handle, depending on the handle's type.
The import method used is referred to as the handle type's _transference_.
When using handle types with reference transference, importing a payload to
a semaphore adds the semaphore to the set of all semaphores sharing that
payload.
This set includes the semaphore from which the payload was exported.
Semaphore signaling and waiting operations performed on any semaphore in the
set must: behave as if the set were a single semaphore.
Importing a payload using handle types with copy transference creates a
duplicate copy of the payload at the time of import, but makes no further
reference to it.
Semaphore signaling and waiting operations performed on the target of copy
imports must: not affect any other semaphore or payload.

Export operations have the same transference as the specified handle type's
import operations.
Additionally, exporting a semaphore payload to a handle with copy
transference has the same side effects on the source semaphore's payload as
executing a semaphore wait operation.
If the semaphore was using a temporarily imported payload, the semaphore's
prior permanent payload will be restored.

ifdef::VK_KHR_external_semaphore_win32,VK_KHR_external_semaphore_fd[]
[NOTE]
.Note
====
The
ifdef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[tables]
ifndef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[table]
ifdef::VK_KHR_external_semaphore_win32[]
<<synchronization-semaphore-handletypes-win32,Handle Types Supported by
VkImportSemaphoreWin32HandleInfoKHR>>
endif::VK_KHR_external_semaphore_win32[]
ifdef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[and]
ifdef::VK_KHR_external_semaphore_fd[]
<<synchronization-semaphore-handletypes-fd,Handle Types Supported by
VkImportSemaphoreFdInfoKHR>>
endif::VK_KHR_external_semaphore_fd[]
ifdef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[define]
ifndef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[defines]
the permanence and transference of each handle type.
====
endif::VK_KHR_external_semaphore_win32,VK_KHR_external_semaphore_fd[]

<<fundamentals-threadingbehavior,External synchronization>> allows
implementations to modify an object's internal state, i.e. payload, without
internal synchronization.
However, for semaphores sharing a payload across processes, satisfying the
external synchronization requirements of fname:VkSemaphore parameters as if
all semaphores in the set were the same object is sometimes infeasible.
Satisfying the <<synchronization-semaphores-waiting-state,wait operation
state requirements>> would similarly require impractical coordination or
levels of trust between processes.
Therefore, these constraints only apply to a specific semaphore handle, not
to its payload.
For distinct semaphore objects which share a payload, if the semaphores are
passed to separate queue submission commands concurrently, behavior will be
as if the commands were called in an arbitrary sequential order.
If the <<synchronization-semaphores-waiting-state,wait operation state
requirements>> are violated for the shared payload by a queue submission
command, or if a signal operation is queued for a shared payload that is
already signaled or has a pending signal operation, effects must: be limited
to one or more of the following:

  * Returning ename:VK_ERROR_INITIALIZATION_FAILED from the command which
    resulted in the violation.
  * Losing the logical device on which the violation occured immediately or
    at a future time, resulting in a ename:VK_ERROR_DEVICE_LOST error from
    subsequent commands, including the one causing the violation.
  * Continuing execution of the violating command or operation as if the
    semaphore wait completed successfully after an implementation-dependent
    timeout.
    In this case, the state of the payload becomes undefined, and future
    operations on semaphores sharing the payload will be subject to these
    same rules.
    The semaphore must: be destroyed or have its payload replaced by an
    import operation to again have a well-defined state.

[NOTE]
.Note
====
These rules allow processes to synchronize access to shared memory without
trusting each other.
However, such processes must still be cautious not to use the shared
semaphore for more than synchronizing access to the shared memory.
For example, a process should not use a shared semaphore as part of an
execution dependency chain that, when complete, leads to objects being
destroyed, if it does not trust other processes sharing the semaphore
payload.
====

When a semaphore is using an imported payload, its
slink:VkExportSemaphoreCreateInfo::pname:handleTypes value is that specified
when creating the semaphore from which the payload was exported, rather than
that specified when creating the semaphore.
Additionally,
slink:VkExternalSemaphoreProperties::exportFromImportedHandleTypes restricts
which handle types can: be exported from such a semaphore based on the
specific handle type used to import the current payload.
ifdef::VK_KHR_swapchain[]
Passing a semaphore to flink:vkAcquireNextImageKHR is equivalent to
temporarily importing a semaphore payload to that semaphore.

[NOTE]
.Note
====
Because the exportable handle types of an imported semaphore correspond to
its current imported payload, and flink:vkAcquireNextImageKHR behaves the
same as a temporary import operation for which the source semaphore is
opaque to the application, applications have no way of determining whether
any external handle types can: be exported from a semaphore in this state.
Therefore, applications must: not attempt to export external handles from
semaphores using a temporarily imported payload from
flink:vkAcquireNextImageKHR.
====
endif::VK_KHR_swapchain[]

When importing a semaphore payload, it is the responsibility of the
application to ensure the external handles meet all valid usage
requirements.
However, implementations must: perform sufficient validation of external
handles to ensure that the operation results in a valid semaphore which will
not cause program termination, device loss, queue stalls, or corruption of
other resources when used as allowed according to its import parameters, and
excepting those side effects allowed for violations of the
<<synchronization-semaphores-waiting-state,valid semaphore state for wait
operations>> rules.
If the external handle provided does not meet these requirements, the
implementation must: fail the semaphore payload import operation with the
error code ename:VK_ERROR_INVALID_EXTERNAL_HANDLE.

endif::VK_VERSION_1_1,VK_KHR_external_semaphore[]

ifdef::VK_KHR_external_semaphore_win32[]

[open,refpage='vkImportSemaphoreWin32HandleKHR',desc='Import a semaphore from a Windows HANDLE',type='protos']
--

To import a semaphore payload from a Windows handle, call:

include::../api/protos/vkImportSemaphoreWin32HandleKHR.txt[]

  * pname:device is the logical device that created the semaphore.
  * pname:pImportSemaphoreWin32HandleInfo points to a
    slink:VkImportSemaphoreWin32HandleInfoKHR structure specifying the
    semaphore and import parameters.

Importing a semaphore payload from Windows handles does not transfer
ownership of the handle to the Vulkan implementation.
For handle types defined as NT handles, the application must: release
ownership using the fname:CloseHandle system call when the handle is no
longer needed.

Applications can: import the same semaphore payload into multiple instances
of Vulkan, into the same instance from which it was exported, and multiple
times into a given Vulkan instance.

include::../validity/protos/vkImportSemaphoreWin32HandleKHR.txt[]
--

[open,refpage='VkImportSemaphoreWin32HandleInfoKHR',desc='Structure specifying Windows handle to import to a semaphore',type='structs']
--

The sname:VkImportSemaphoreWin32HandleInfoKHR structure is defined as:

include::../api/structs/VkImportSemaphoreWin32HandleInfoKHR.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:semaphore is the semaphore into which the payload will be
    imported.
  * pname:flags is a bitmask of elink:VkSemaphoreImportFlagBits specifying
    additional parameters for the semaphore payload import operation.
  * pname:handleType specifies the type of pname:handle.
  * pname:handle is the external handle to import, or `NULL`.
  * pname:name is a NULL-terminated UTF-16 string naming the underlying
    synchronization primitive to import, or `NULL`.

The handle types supported by pname:handleType are:

[[synchronization-semaphore-handletypes-win32]]
.Handle Types Supported by VkImportSemaphoreWin32HandleInfoKHR
[width="80%",options="header"]
|====
| Handle Type                                                      | Transference | Permanence Supported
| ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_WIN32_BIT     | Reference    | Temporary,Permanent
| ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_WIN32_KMT_BIT | Reference    | Temporary,Permanent
| ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_D3D12_FENCE_BIT      | Reference    | Temporary,Permanent
|====

.Valid Usage
****
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handleType-01140]]
    pname:handleType must: be a value included in the
    <<synchronization-semaphore-handletypes-win32,Handle Types Supported by
    VkImportSemaphoreWin32HandleInfoKHR>> table.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handleType-01466]]
    If pname:handleType is not
    ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_WIN32_BIT or
    ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_D3D12_FENCE_BIT, pname:name
    must: be `NULL`.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handleType-01467]]
    If pname:handleType is not `0` and pname:handle is `NULL`, pname:name
    must: name a valid synchronization primitive of the type specified by
    pname:handleType.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handleType-01468]]
    If pname:handleType is not `0` and pname:name is `NULL`, pname:handle
    must: be a valid handle of the type specified by pname:handleType.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handle-01469]]
    If pname:handle is not `NULL`, pname:name must be `NULL`.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-handle-01542]]
    If pname:handle is not `NULL`, it must: obey any requirements listed for
    pname:handleType in
    <<external-semaphore-handle-types-compatibility,external semaphore
    handle types compatibility>>.
  * [[VUID-VkImportSemaphoreWin32HandleInfoKHR-name-01543]]
    If pname:name is not `NULL`, it must: obey any requirements listed for
    pname:handleType in
    <<external-semaphore-handle-types-compatibility,external semaphore
    handle types compatibility>>.
****

include::../validity/structs/VkImportSemaphoreWin32HandleInfoKHR.txt[]
--

endif::VK_KHR_external_semaphore_win32[]

ifdef::VK_KHR_external_semaphore_fd[]

[open,refpage='vkImportSemaphoreFdKHR',desc='Import a semaphore from a POSIX file descriptor',type='protos']
--

To import a semaphore payload from a POSIX file descriptor, call:

include::../api/protos/vkImportSemaphoreFdKHR.txt[]

  * pname:device is the logical device that created the semaphore.
  * pname:pImportSemaphoreFdInfo points to a
    slink:VkImportSemaphoreFdInfoKHR structure specifying the semaphore and
    import parameters.

Importing a semaphore payload from a file descriptor transfers ownership of
the file descriptor from the application to the Vulkan implementation.
The application must: not perform any operations on the file descriptor
after a successful import.

Applications can: import the same semaphore payload into multiple instances
of Vulkan, into the same instance from which it was exported, and multiple
times into a given Vulkan instance.

.Valid Usage
****
  * [[VUID-vkImportSemaphoreFdKHR-semaphore-01142]]
    pname:semaphore must: not be associated with any queue command that has
    not yet completed execution on that queue
****

include::../validity/protos/vkImportSemaphoreFdKHR.txt[]
--

[open,refpage='VkImportSemaphoreFdInfoKHR',desc='Structure specifying POSIX file descriptor to import to a semaphore',type='structs']
--

The sname:VkImportSemaphoreFdInfoKHR structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:semaphore is the semaphore into which the payload will be
    imported.
  * pname:flags is a bitmask of elink:VkSemaphoreImportFlagBits specifying
    additional parameters for the semaphore payload import operation.
  * pname:handleType specifies the type of pname:fd.
  * pname:fd is the external handle to import.

The handle types supported by pname:handleType are:

[[synchronization-semaphore-handletypes-fd]]
.Handle Types Supported by VkImportSemaphoreFdInfoKHR
[width="80%",options="header"]
|====
| Handle Type                                               | Transference | Permanence Supported
| ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_OPAQUE_FD_BIT | Reference    | Temporary,Permanent
| ename:VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT   | Copy         | Temporary
|====

.Valid Usage
****
  * [[VUID-VkImportSemaphoreFdInfoKHR-handleType-01143]]
    pname:handleType must: be a value included in the
    <<synchronization-semaphore-handletypes-fd,Handle Types Supported by
    VkImportSemaphoreFdInfoKHR>> table.
  * [[VUID-VkImportSemaphoreFdInfoKHR-fd-01544]]
    pname:fd must: obey any requirements listed for pname:handleType in
    <<external-semaphore-handle-types-compatibility,external semaphore
    handle types compatibility>>.
****

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

endif::VK_KHR_external_semaphore_fd[]

ifdef::VK_VERSION_1_1,VK_KHR_external_semaphore[]
ifdef::VK_KHR_external_semaphore_win32,VK_KHR_external_semaphore_fd[]

[open,refpage='VkSemaphoreImportFlagBits',desc='Bitmask specifying additional parameters of semaphore payload import',type='enums']
--

Additional parameters of a semaphore import operation are specified by
ifdef::VK_KHR_external_semaphore_win32[]
slink:VkImportSemaphoreWin32HandleInfoKHR::pname:flags
endif::VK_KHR_external_semaphore_win32[]
ifdef::VK_KHR_external_semaphore_win32+VK_KHR_external_semaphore_fd[or]
ifdef::VK_KHR_external_semaphore_fd[]
slink:VkImportSemaphoreFdInfoKHR::pname:flags
endif::VK_KHR_external_semaphore_fd[]
.
Bits which can be set include:

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

ifdef::VK_KHR_external_semaphore[]
or the equivalent

include::../api/enums/VkSemaphoreImportFlagBitsKHR.txt[]
endif::VK_KHR_external_semaphore[]

These bits have the following meanings:

  * ename:VK_SEMAPHORE_IMPORT_TEMPORARY_BIT specifies that the semaphore
    payload will be imported only temporarily, as described in
    <<synchronization-semaphores-importing,Importing Semaphore Payloads>>,
    regardless of the permanence of pname:handleType.

--

[open,refpage='VkSemaphoreImportFlags',desc='Bitmask of VkSemaphoreImportFlagBits',type='enums']
--
include::../api/flags/VkSemaphoreImportFlags.txt[]

ifdef::VK_KHR_external_semaphore[]
or the equivalent

include::../api/flags/VkSemaphoreImportFlagsKHR.txt[]
endif::VK_KHR_external_semaphore[]

sname:VkSemaphoreImportFlags is a bitmask type for setting a mask of zero or
more slink:VkSemaphoreImportFlagBits.
--

endif::VK_KHR_external_semaphore_win32,VK_KHR_external_semaphore_fd[]
endif::VK_VERSION_1_1,VK_KHR_external_semaphore[]


[[synchronization-events]]
== Events

[open,refpage='VkEvent',desc='Opaque handle to an event object',type='handles']
--

Events are a synchronization primitive that can: be used to insert a
fine-grained dependency between commands submitted to the same queue, or
between the host and a queue.
Events must: not be used to insert a dependency between commands submitted
to different queues.
Events have two states - signaled and unsignaled.
An application can: signal an event, or unsignal it, on either the host or
the device.
A device can: wait for an event to become signaled before executing further
operations.
No command exists to wait for an event to become signaled on the host, but
the current state of an event can: be queried.

Events are represented by sname:VkEvent handles:

include::../api/handles/VkEvent.txt[]

--

[open,refpage='vkCreateEvent',desc='Create a new event object',type='protos']
--

To create an event, call:

include::../api/protos/vkCreateEvent.txt[]

  * pname:device is the logical device that creates the event.
  * pname:pCreateInfo is a pointer to an instance of the
    sname:VkEventCreateInfo structure which contains information about how
    the event is to be created.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pEvent points to a handle in which the resulting event object is
    returned.

When created, the event object is in the unsignaled state.

include::../validity/protos/vkCreateEvent.txt[]
--

[open,refpage='VkEventCreateInfo',desc='Structure specifying parameters of a newly created event',type='structs']
--

The sname:VkEventCreateInfo structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.

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

[open,refpage='VkEventCreateFlags',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkEventCreateFlags.txt[]

sname:VkEventCreateFlags is a bitmask type for setting a mask, but is
currently reserved for future use.
--

[open,refpage='vkDestroyEvent',desc='Destroy an event object',type='protos']
--

To destroy an event, call:

include::../api/protos/vkDestroyEvent.txt[]

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

.Valid Usage
****
  * [[VUID-vkDestroyEvent-event-01145]]
    All submitted commands that refer to pname:event must: have completed
    execution
  * [[VUID-vkDestroyEvent-event-01146]]
    If sname:VkAllocationCallbacks were provided when pname:event was
    created, a compatible set of callbacks must: be provided here
  * [[VUID-vkDestroyEvent-event-01147]]
    If no sname:VkAllocationCallbacks were provided when pname:event was
    created, pname:pAllocator must: be `NULL`
****

include::../validity/protos/vkDestroyEvent.txt[]
--

[open,refpage='vkGetEventStatus',desc='Retrieve the status of an event object',type='protos']
--

To query the state of an event from the host, call:

include::../api/protos/vkGetEventStatus.txt[]

  * pname:device is the logical device that owns the event.
  * pname:event is the handle of the event to query.

Upon success, fname:vkGetEventStatus returns the state of the event object
with the following return codes:

.Event Object Status Codes
[width="80%",options="header"]
|====
| Status | Meaning
| ename:VK_EVENT_SET | The event specified by pname:event is signaled.
| ename:VK_EVENT_RESET | The event specified by pname:event is unsignaled.
|====

If a fname:vkCmdSetEvent or fname:vkCmdResetEvent command is in a command
buffer that is in the <<commandbuffers-lifecycle, pending state>>, then the
value returned by this command may: immediately be out of date.

The state of an event can: be updated by the host.
The state of the event is immediately changed, and subsequent calls to
fname:vkGetEventStatus will return the new state.
If an event is already in the requested state, then updating it to the same
state has no effect.

include::../validity/protos/vkGetEventStatus.txt[]
--

[[synchronization-events-signaling-host]]
[open,refpage='vkSetEvent',desc='Set an event to signaled state',type='protos']
--

To set the state of an event to signaled from the host, call:

include::../api/protos/vkSetEvent.txt[]

  * pname:device is the logical device that owns the event.
  * pname:event is the event to set.

When flink:vkSetEvent is executed on the host, it defines an _event signal
operation_ which sets the event to the signaled state.

If pname:event is already in the signaled state when flink:vkSetEvent is
executed, then flink:vkSetEvent has no effect, and no event signal operation
occurs.

include::../validity/protos/vkSetEvent.txt[]
--

[[synchronization-events-unsignaling-host]]
[open,refpage='vkResetEvent',desc='Reset an event to non-signaled state',type='protos']
--

To set the state of an event to unsignaled from the host, call:

include::../api/protos/vkResetEvent.txt[]

  * pname:device is the logical device that owns the event.
  * pname:event is the event to reset.

When flink:vkResetEvent is executed on the host, it defines an _event
unsignal operation_ which resets the event to the unsignaled state.

If pname:event is already in the unsignaled state when flink:vkResetEvent is
executed, then flink:vkResetEvent has no effect, and no event unsignal
operation occurs.

.Valid Usage
****
  * [[VUID-vkResetEvent-event-01148]]
    pname:event must: not be waited on by a fname:vkCmdWaitEvents command
    that is currently executing
****

include::../validity/protos/vkResetEvent.txt[]
--


The state of an event can: also be updated on the device by commands
inserted in command buffers.

[[synchronization-events-signaling-device]]
[open,refpage='vkCmdSetEvent',desc='Set an event object to signaled state',type='protos']
--

To set the state of an event to signaled from a device, call:

include::../api/protos/vkCmdSetEvent.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:event is the event that will be signaled.
  * pname:stageMask specifies the <<synchronization-pipeline-stages,source
    stage mask>> used to determine when the pname:event is signaled.

When flink:vkCmdSetEvent is submitted to a queue, it defines an execution
dependency on commands that were submitted before it, and defines an event
signal operation which sets the event to the signaled state.

The first <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands that occur earlier in
<<synchronization-submission-order,submission order>>.
The synchronization scope is limited to operations on the pipeline stages
determined by the <<synchronization-pipeline-stages-masks, source stage
mask>> specified by pname:stageMask.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes only the event signal operation.

If pname:event is already in the signaled state when flink:vkCmdSetEvent is
executed on the device, then flink:vkCmdSetEvent has no effect, no event
signal operation occurs, and no execution dependency is generated.

.Valid Usage
****
  * [[VUID-vkCmdSetEvent-stageMask-01149]]
    pname:stageMask must: not include ename:VK_PIPELINE_STAGE_HOST_BIT
  * [[VUID-vkCmdSetEvent-stageMask-01150]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:stageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdSetEvent-stageMask-01151]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:stageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
  * [[VUID-vkCmdSetEvent-commandBuffer-01152]]
    pname:commandBuffer's current device mask must: include exactly one
    physical device.
endif::VK_VERSION_1_1,VK_KHR_device_group[]
****

include::../validity/protos/vkCmdSetEvent.txt[]
--

[[synchronization-events-unsignaling-device]]
[open,refpage='vkCmdResetEvent',desc='Reset an event object to non-signaled state',type='protos']
--

To set the state of an event to unsignaled from a device, call:

include::../api/protos/vkCmdResetEvent.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:event is the event that will be unsignaled.
  * pname:stageMask is a bitmask of elink:VkPipelineStageFlagBits specifying
    the <<synchronization-pipeline-stages, source stage mask>> used to
    determine when the pname:event is unsignaled.

When flink:vkCmdResetEvent is submitted to a queue, it defines an execution
dependency on commands that were submitted before it, and defines an event
unsignal operation which resets the event to the unsignaled state.

The first <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands that occur earlier in
<<synchronization-submission-order,submission order>>.
The synchronization scope is limited to operations on the pipeline stages
determined by the <<synchronization-pipeline-stages-masks, source stage
mask>> specified by pname:stageMask.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes only the event unsignal operation.

If pname:event is already in the unsignaled state when flink:vkCmdResetEvent
is executed on the device, then flink:vkCmdResetEvent has no effect, no
event unsignal operation occurs, and no execution dependency is generated.

.Valid Usage
****
  * [[VUID-vkCmdResetEvent-stageMask-01153]]
    pname:stageMask must: not include ename:VK_PIPELINE_STAGE_HOST_BIT
  * [[VUID-vkCmdResetEvent-stageMask-01154]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:stageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdResetEvent-stageMask-01155]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:stageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * [[VUID-vkCmdResetEvent-event-01156]]
    When this command executes, pname:event must: not be waited on by a
    fname:vkCmdWaitEvents command that is currently executing
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
  * [[VUID-vkCmdResetEvent-commandBuffer-01157]]
    pname:commandBuffer's current device mask must: include exactly one
    physical device.
endif::VK_VERSION_1_1,VK_KHR_device_group[]
****

include::../validity/protos/vkCmdResetEvent.txt[]
--

[open,refpage='vkCmdWaitEvents',desc='Wait for one or more events and insert a set of memory',type='protos']
--

To wait for one or more events to enter the signaled state on a device,
call:

[[synchronization-events-waiting-device]]
include::../api/protos/vkCmdWaitEvents.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:eventCount is the length of the pname:pEvents array.
  * pname:pEvents is an array of event object handles to wait on.
  * pname:srcStageMask is a bitmask of elink:VkPipelineStageFlagBits
    specifying the <<synchronization-pipeline-stages, source stage mask>>.
  * pname:dstStageMask is a bitmask of elink:VkPipelineStageFlagBits
    specifying the <<synchronization-pipeline-stages, destination stage
    mask>>.
  * pname:memoryBarrierCount is the length of the pname:pMemoryBarriers
    array.
  * pname:pMemoryBarriers is a pointer to an array of slink:VkMemoryBarrier
    structures.
  * pname:bufferMemoryBarrierCount is the length of the
    pname:pBufferMemoryBarriers array.
  * pname:pBufferMemoryBarriers is a pointer to an array of
    slink:VkBufferMemoryBarrier structures.
  * pname:imageMemoryBarrierCount is the length of the
    pname:pImageMemoryBarriers array.
  * pname:pImageMemoryBarriers is a pointer to an array of
    slink:VkImageMemoryBarrier structures.

When fname:vkCmdWaitEvents is submitted to a queue, it defines a memory
dependency between prior event signal operations on the same queue or the
host, and subsequent commands.
fname:vkCmdWaitEvents must: not be used to wait on event signal operations
occuring on other queues.

The first synchronization scope only includes event signal operations that
operate on members of pname:pEvents, and the operations that happened-before
the event signal operations.
Event signal operations performed by flink:vkCmdSetEvent that occur earlier
in <<synchronization-submission-order,submission order>> are included in the
first synchronization scope, if the <<synchronization-pipeline-stages-order,
logically latest>> pipeline stage in their pname:stageMask parameter is
<<synchronization-pipeline-stages-order, logically earlier>> than or equal
to the <<synchronization-pipeline-stages-order, logically latest>> pipeline
stage in pname:srcStageMask.
Event signal operations performed by flink:vkSetEvent are only included in
the first synchronization scope if ename:VK_PIPELINE_STAGE_HOST_BIT is
included in pname:srcStageMask.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands that occur later in
<<synchronization-submission-order,submission order>>.
The second synchronization scope is limited to operations on the pipeline
stages determined by the <<synchronization-pipeline-stages-masks,
destination stage mask>> specified by pname:dstStageMask.

The first <<synchronization-dependencies-access-scopes, access scope>> is
limited to access in the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, source stage mask>> specified by
pname:srcStageMask.
Within that, the first access scope only includes the first access scopes
defined by elements of the pname:pMemoryBarriers,
pname:pBufferMemoryBarriers and pname:pImageMemoryBarriers arrays, which
each define a set of <<synchronization-memory-barriers, memory barriers>>.
If no memory barriers are specified, then the first access scope includes no
accesses.

The second <<synchronization-dependencies-access-scopes, access scope>> is
limited to access in the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, destination stage mask>> specified
by pname:dstStageMask.
Within that, the second access scope only includes the second access scopes
defined by elements of the pname:pMemoryBarriers,
pname:pBufferMemoryBarriers and pname:pImageMemoryBarriers arrays, which
each define a set of <<synchronization-memory-barriers, memory barriers>>.
If no memory barriers are specified, then the second access scope includes
no accesses.

[NOTE]
.Note
====
flink:vkCmdWaitEvents is used with flink:vkCmdSetEvent to define a memory
dependency between two sets of action commands, roughly in the same way as
pipeline barriers, but split into two commands such that work between the
two may: execute unhindered.
====

[NOTE]
.Note
====
Applications should: be careful to avoid race conditions when using events.
There is no direct ordering guarantee between a flink:vkCmdResetEvent
command and a flink:vkCmdWaitEvents command submitted after it, so some
other execution dependency must: be included between these commands (e.g. a
semaphore).
====

.Valid Usage
****
  * [[VUID-vkCmdWaitEvents-srcStageMask-01158]]
    pname:srcStageMask must: be the bitwise OR of the pname:stageMask
    parameter used in previous calls to fname:vkCmdSetEvent with any of the
    members of pname:pEvents and ename:VK_PIPELINE_STAGE_HOST_BIT if any of
    the members of pname:pEvents was set using fname:vkSetEvent
  * [[VUID-vkCmdWaitEvents-srcStageMask-01159]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:srcStageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdWaitEvents-dstStageMask-01160]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:dstStageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdWaitEvents-srcStageMask-01161]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:srcStageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * [[VUID-vkCmdWaitEvents-dstStageMask-01162]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:dstStageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * [[VUID-vkCmdWaitEvents-pEvents-01163]]
    If pname:pEvents includes one or more events that will be signaled by
    fname:vkSetEvent after pname:commandBuffer has been submitted to a
    queue, then fname:vkCmdWaitEvents must: not be called inside a render
    pass instance
  * [[VUID-vkCmdWaitEvents-srcStageMask-01164]]
    Any pipeline stage included in pname:srcStageMask or pname:dstStageMask
    must: be supported by the capabilities of the queue family specified by
    the pname:queueFamilyIndex member of the slink:VkCommandPoolCreateInfo
    structure that was used to create the sname:VkCommandPool that
    pname:commandBuffer was allocated from, as specified in the
    <<synchronization-pipeline-stages-supported, table of supported pipeline
    stages>>.
  * [[VUID-vkCmdWaitEvents-pMemoryBarriers-01165]]
    Each element of pname:pMemoryBarriers, pname:pBufferMemoryBarriers or
    pname:pImageMemoryBarriers must: not have any access flag included in
    its pname:srcAccessMask member if that bit is not supported by any of
    the pipeline stages in pname:srcStageMask, as specified in the
    <<synchronization-access-types-supported, table of supported access
    types>>.
  * [[VUID-vkCmdWaitEvents-pMemoryBarriers-01166]]
    Each element of pname:pMemoryBarriers, pname:pBufferMemoryBarriers or
    pname:pImageMemoryBarriers must: not have any access flag included in
    its pname:dstAccessMask member if that bit is not supported by any of
    the pipeline stages in pname:dstStageMask, as specified in the
    <<synchronization-access-types-supported, table of supported access
    types>>.
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
  * [[VUID-vkCmdWaitEvents-commandBuffer-01167]]
    pname:commandBuffer's current device mask must: include exactly one
    physical device.
endif::VK_VERSION_1_1,VK_KHR_device_group[]
****

include::../validity/protos/vkCmdWaitEvents.txt[]
--


[[synchronization-pipeline-barriers]]
== Pipeline Barriers

flink:vkCmdPipelineBarrier is a synchronization command that inserts a
dependency between commands submitted to the same queue, or between commands
in the same subpass.

[open,refpage='vkCmdPipelineBarrier',desc='Insert a memory dependency',type='protos']
--

To record a pipeline barrier, call:

include::../api/protos/vkCmdPipelineBarrier.txt[]

  * pname:commandBuffer is the command buffer into which the command is
    recorded.
  * pname:srcStageMask is a bitmask of elink:VkPipelineStageFlagBits
    specifying the <<synchronization-pipeline-stages-masks, source stage
    mask>>.
  * pname:dstStageMask is a bitmask of elink:VkPipelineStageFlagBits
    specifying the <<synchronization-pipeline-stages-masks, destination
    stage mask>>.
  * pname:dependencyFlags is a bitmask of elink:VkDependencyFlagBits
    specifying how execution and memory dependencies are formed.
  * pname:memoryBarrierCount is the length of the pname:pMemoryBarriers
    array.
  * pname:pMemoryBarriers is a pointer to an array of slink:VkMemoryBarrier
    structures.
  * pname:bufferMemoryBarrierCount is the length of the
    pname:pBufferMemoryBarriers array.
  * pname:pBufferMemoryBarriers is a pointer to an array of
    slink:VkBufferMemoryBarrier structures.
  * pname:imageMemoryBarrierCount is the length of the
    pname:pImageMemoryBarriers array.
  * pname:pImageMemoryBarriers is a pointer to an array of
    slink:VkImageMemoryBarrier structures.

When flink:vkCmdPipelineBarrier is submitted to a queue, it defines a memory
dependency between commands that were submitted before it, and those
submitted after it.

If flink:vkCmdPipelineBarrier was recorded outside a render pass instance,
the first <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands that occur earlier in
<<synchronization-submission-order,submission order>>.
If flink:vkCmdPipelineBarrier was recorded inside a render pass instance,
the first synchronization scope includes only commands that occur earlier in
<<synchronization-submission-order,submission order>> within the same
subpass.
In either case, the first synchronization scope is limited to operations on
the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, source stage mask>> specified by
pname:srcStageMask.

If flink:vkCmdPipelineBarrier was recorded outside a render pass instance,
the second <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands that occur later in
<<synchronization-submission-order,submission order>>.
If flink:vkCmdPipelineBarrier was recorded inside a render pass instance,
the second synchronization scope includes only commands that occur later in
<<synchronization-submission-order,submission order>> within the same
subpass.
In either case, the second synchronization scope is limited to operations on
the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, destination stage mask>> specified
by pname:dstStageMask.

The first <<synchronization-dependencies-access-scopes, access scope>> is
limited to access in the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, source stage mask>> specified by
pname:srcStageMask.
Within that, the first access scope only includes the first access scopes
defined by elements of the pname:pMemoryBarriers,
pname:pBufferMemoryBarriers and pname:pImageMemoryBarriers arrays, which
each define a set of <<synchronization-memory-barriers, memory barriers>>.
If no memory barriers are specified, then the first access scope includes no
accesses.

The second <<synchronization-dependencies-access-scopes, access scope>> is
limited to access in the pipeline stages determined by the
<<synchronization-pipeline-stages-masks, destination stage mask>> specified
by pname:dstStageMask.
Within that, the second access scope only includes the second access scopes
defined by elements of the pname:pMemoryBarriers,
pname:pBufferMemoryBarriers and pname:pImageMemoryBarriers arrays, which
each define a set of <<synchronization-memory-barriers, memory barriers>>.
If no memory barriers are specified, then the second access scope includes
no accesses.

If pname:dependencyFlags includes ename:VK_DEPENDENCY_BY_REGION_BIT, then
any dependency between <<synchronization-framebuffer-regions,
framebuffer-space>> pipeline stages is
<<synchronization-framebuffer-regions, framebuffer-local>> - otherwise it is
<<synchronization-framebuffer-regions, framebuffer-global>>.

.Valid Usage
****
  * [[VUID-vkCmdPipelineBarrier-srcStageMask-01168]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:srcStageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdPipelineBarrier-dstStageMask-01169]]
    If the <<features-features-geometryShader,geometry shaders>> feature is
    not enabled, pname:dstStageMask must: not contain
    ename:VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT
  * [[VUID-vkCmdPipelineBarrier-srcStageMask-01170]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:srcStageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * [[VUID-vkCmdPipelineBarrier-dstStageMask-01171]]
    If the <<features-features-tessellationShader,tessellation shaders>>
    feature is not enabled, pname:dstStageMask must: not contain
    ename:VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT or
    ename:VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT
  * [[VUID-vkCmdPipelineBarrier-pDependencies-02024]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    the render pass must: have been created with at least one
    sname:VkSubpassDependency instance in
    sname:VkRenderPassCreateInfo::pname:pDependencies that expresses a
    dependency from the current subpass to itself, and for which
    pname:srcStageMask contains a subset of the bit values in
    sname:VkSubpassDependency::pname:srcStageMask, pname:dstStageMask
    contains a subset of the bit values in
    sname:VkSubpassDependency::pname:dstStageMask, and pname:dependencyFlags
    is equal to sname:VkSubpassDependency::pname:dependencyFlags.
  * [[VUID-vkCmdPipelineBarrier-pMemoryBarriers-02026]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    for each element of pname:pMemoryBarriers and
    pname:pImageMemoryBarriers, the render pass must: have been defined with
    a sname:VkSubpassDependency self-dependency for the current subpass with
    valid pname:srcStageMask, pname:dstStageMask, and pname:dependencyFlags
    values such that stext:Vk*Barrier::pname:srcAccessMask contains a subset
    of the bit values in sname:VkSubpassDependency::pname:srcAccessMask and
    stext:Vk*Barrier::pname:dstAccessMask contains a subset of the bit
    values in sname:VkSubpassDependency::pname:dstAccessMask.
  * [[VUID-vkCmdPipelineBarrier-bufferMemoryBarrierCount-01178]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    pname:bufferMemoryBarrierCount must: be `0`
  * [[VUID-vkCmdPipelineBarrier-image-01179]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    the pname:image member of any element of pname:pImageMemoryBarriers
    must: be equal to one of the elements of pname:pAttachments that the
    current pname:framebuffer was created with, that is also referred to by
    one of the elements of the pname:pColorAttachments,
    pname:pResolveAttachments or pname:pDepthStencilAttachment members of
    the sname:VkSubpassDescription instance that the current subpass was
    created with
  * [[VUID-vkCmdPipelineBarrier-oldLayout-01180]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    the pname:oldLayout and pname:newLayout members of any element of
    pname:pImageMemoryBarriers must: be equal to the pname:layout member of
    an element of the pname:pColorAttachments, pname:pResolveAttachments or
    pname:pDepthStencilAttachment members of the sname:VkSubpassDescription
    instance that the current subpass was created with, that refers to the
    same pname:image
  * [[VUID-vkCmdPipelineBarrier-oldLayout-01181]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    the pname:oldLayout and pname:newLayout members of an element of
    pname:pImageMemoryBarriers must: be equal
  * [[VUID-vkCmdPipelineBarrier-srcQueueFamilyIndex-01182]]
    If fname:vkCmdPipelineBarrier is called within a render pass instance,
    the pname:srcQueueFamilyIndex and pname:dstQueueFamilyIndex members of
    any element of pname:pImageMemoryBarriers must: be
    ename:VK_QUEUE_FAMILY_IGNORED
  * [[VUID-vkCmdPipelineBarrier-srcStageMask-01183]]
    Any pipeline stage included in pname:srcStageMask or pname:dstStageMask
    must: be supported by the capabilities of the queue family specified by
    the pname:queueFamilyIndex member of the slink:VkCommandPoolCreateInfo
    structure that was used to create the sname:VkCommandPool that
    pname:commandBuffer was allocated from, as specified in the
    <<synchronization-pipeline-stages-supported, table of supported pipeline
    stages>>.
  * [[VUID-vkCmdPipelineBarrier-pMemoryBarriers-01184]]
    Each element of pname:pMemoryBarriers, pname:pBufferMemoryBarriers and
    pname:pImageMemoryBarriers must: not have any access flag included in
    its pname:srcAccessMask member if that bit is not supported by any of
    the pipeline stages in pname:srcStageMask, as specified in the
    <<synchronization-access-types-supported, table of supported access
    types>>.
  * [[VUID-vkCmdPipelineBarrier-pMemoryBarriers-01185]]
    Each element of pname:pMemoryBarriers, pname:pBufferMemoryBarriers and
    pname:pImageMemoryBarriers must: not have any access flag included in
    its pname:dstAccessMask member if that bit is not supported by any of
    the pipeline stages in pname:dstStageMask, as specified in the
    <<synchronization-access-types-supported, table of supported access
    types>>.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
  * [[VUID-vkCmdPipelineBarrier-dependencyFlags-01186]]
    If fname:vkCmdPipelineBarrier is called outside of a render pass
    instance, pname:dependencyFlags must: not include
    ename:VK_DEPENDENCY_VIEW_LOCAL_BIT
endif::VK_VERSION_1_1,VK_KHR_multiview[]
****

include::../validity/protos/vkCmdPipelineBarrier.txt[]
--

[open,refpage='VkDependencyFlagBits',desc='Bitmask specifying how execution and memory dependencies are formed',type='enums']
--

Bits which can: be set in fname:vkCmdPipelineBarrier::pname:dependencyFlags,
specifying how execution and memory dependencies are formed, are:

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

  * ename:VK_DEPENDENCY_BY_REGION_BIT specifies that dependencies will be
    <<synchronization-framebuffer-regions, framebuffer-local>>.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
  * ename:VK_DEPENDENCY_VIEW_LOCAL_BIT specifies that a
    <<synchronization-pipeline-barriers-subpass-self-dependencies, subpass
    has more than one view>>.
endif::VK_VERSION_1_1,VK_KHR_multiview[]
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
  * ename:VK_DEPENDENCY_DEVICE_GROUP_BIT specifies that dependencies are
    <<synchronization-device-local-dependencies, non-device-local
    dependency>>.
endif::VK_VERSION_1_1,VK_KHR_device_group[]

--

[open,refpage='VkDependencyFlags',desc='Bitmask of VkDependencyFlagBits',type='enums']
--
include::../api/flags/VkDependencyFlags.txt[]

sname:VkDependencyFlags is a bitmask type for setting a mask of zero or more
slink:VkDependencyFlagBits.
--


[[synchronization-pipeline-barriers-subpass-self-dependencies]]
=== Subpass Self-dependency

If fname:vkCmdPipelineBarrier is called inside a render pass instance, the
following restrictions apply.
For a given subpass to allow a pipeline barrier, the render pass must:
declare a _self-dependency_ from that subpass to itself.
That is, there must: exist a sname:VkSubpassDependency in the subpass
dependency list for the render pass with pname:srcSubpass and
pname:dstSubpass equal to that subpass index.
More than one self-dependency can: be declared for each subpass.
Self-dependencies must: only include pipeline stage bits that are graphics
stages.
Self-dependencies must: not have any earlier pipeline stages depend on any
later pipeline stages (according to the order of
<<synchronization-pipeline-stages-types,graphics pipeline stages>>), unless
all of the stages are
<<synchronization-framebuffer-regions,framebuffer-space stages>>.
If the source and destination stage masks both include framebuffer-space
stages, then pname:dependencyFlags must: include
ename:VK_DEPENDENCY_BY_REGION_BIT.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass has more than one view, then pname:dependencyFlags must:
include ename:VK_DEPENDENCY_VIEW_LOCAL_BIT.
endif::VK_VERSION_1_1,VK_KHR_multiview[]

A fname:vkCmdPipelineBarrier command inside a render pass instance must: be
a _subset_ of one of the self-dependencies of the subpass it is used in,
meaning that the stage masks and access masks must: each include only a
subset of the bits of the corresponding mask in that self-dependency.
If the self-dependency has ename:VK_DEPENDENCY_BY_REGION_BIT
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
or ename:VK_DEPENDENCY_VIEW_LOCAL_BIT
endif::VK_VERSION_1_1,VK_KHR_multiview[]
set, then so must: the pipeline barrier.
Pipeline barriers within a render pass instance can: only be types
sname:VkMemoryBarrier or sname:VkImageMemoryBarrier.
If a sname:VkImageMemoryBarrier is used, the image and image subresource
range specified in the barrier must: be a subset of one of the image views
used by the framebuffer in the current subpass.
Additionally, pname:oldLayout must: be equal to pname:newLayout, and both
the pname:srcQueueFamilyIndex and pname:dstQueueFamilyIndex must: be
ename:VK_QUEUE_FAMILY_IGNORED.


[[synchronization-memory-barriers]]
== Memory Barriers

_Memory barriers_ are used to explicitly control access to buffer and image
subresource ranges.
Memory barriers are used to <<synchronization-queue-transfers, transfer
ownership between queue families>>,
<<synchronization-image-layout-transitions, change image layouts>>, and
define <<synchronization-dependencies-available-and-visible, availability
and visibility operations>>.
They explicitly define the <<synchronization-access-types, access types>>
and buffer and image subresource ranges that are included in the
<<synchronization-dependencies-access-scopes, access scopes>> of a memory
dependency that is created by a synchronization command that includes them.


[[synchronization-global-memory-barriers]]
=== Global Memory Barriers

Global memory barriers apply to memory accesses involving all memory objects
that exist at the time of its execution.

[open,refpage='VkMemoryBarrier',desc='Structure specifying a global memory barrier',type='structs']
--

The sname:VkMemoryBarrier structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:srcAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, source access mask>>.
  * pname:dstAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, destination access mask>>.

The first <<synchronization-dependencies-access-scopes, access scope>> is
limited to access types in the <<synchronization-access-masks, source access
mask>> specified by pname:srcAccessMask.

The second <<synchronization-dependencies-access-scopes, access scope>> is
limited to access types in the <<synchronization-access-masks, destination
access mask>> specified by pname:dstAccessMask.

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


[[synchronization-buffer-memory-barriers]]
=== Buffer Memory Barriers

Buffer memory barriers only apply to memory accesses involving a specific
buffer range.
That is, a memory dependency formed from a buffer memory barrier is
<<synchronization-dependencies-access-scopes, scoped>> to access via the
specified buffer range.
Buffer memory barriers can: also be used to define a
<<synchronization-queue-transfers, queue family ownership transfer>> for the
specified buffer range.

[open,refpage='VkBufferMemoryBarrier',desc='Structure specifying a buffer memory barrier',type='structs']
--

The sname:VkBufferMemoryBarrier structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:srcAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, source access mask>>.
  * pname:dstAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, destination access mask>>.
  * pname:srcQueueFamilyIndex is the source queue family for a
    <<synchronization-queue-transfers, queue family ownership transfer>>.
  * pname:dstQueueFamilyIndex is the destination queue family for a
    <<synchronization-queue-transfers, queue family ownership transfer>>.
  * pname:buffer is a handle to the buffer whose backing memory is affected
    by the barrier.
  * pname:offset is an offset in bytes into the backing memory for
    pname:buffer; this is relative to the base offset as bound to the buffer
    (see flink:vkBindBufferMemory).
  * pname:size is a size in bytes of the affected area of backing memory for
    pname:buffer, or ename:VK_WHOLE_SIZE to use the range from pname:offset
    to the end of the buffer.

The first <<synchronization-dependencies-access-scopes, access scope>> is
limited to access to memory through the specified buffer range, via access
types in the <<synchronization-access-masks, source access mask>> specified
by pname:srcAccessMask.
If pname:srcAccessMask includes ename:VK_ACCESS_HOST_WRITE_BIT, memory
writes performed by that access type are also made visible, as that access
type is not performed through a resource.

The second <<synchronization-dependencies-access-scopes, access scope>> is
limited to access to memory through the specified buffer range, via access
types in the <<synchronization-access-masks, destination access mask>>.
specified by pname:dstAccessMask.
If pname:dstAccessMask includes ename:VK_ACCESS_HOST_WRITE_BIT or
ename:VK_ACCESS_HOST_READ_BIT, available memory writes are also made visible
to accesses of those types, as those access types are not performed through
a resource.

If pname:srcQueueFamilyIndex is not equal to pname:dstQueueFamilyIndex, and
pname:srcQueueFamilyIndex is equal to the current queue family, then the
memory barrier defines a <<synchronization-queue-transfers-release, queue
family release operation>> for the specified buffer range, and the second
access scope includes no access, as if pname:dstAccessMask was `0`.

If pname:dstQueueFamilyIndex is not equal to pname:srcQueueFamilyIndex, and
pname:dstQueueFamilyIndex is equal to the current queue family, then the
memory barrier defines a <<synchronization-queue-transfers-acquire, queue
family acquire operation>> for the specified buffer range, and the first
access scope includes no access, as if pname:srcAccessMask was `0`.

.Valid Usage
****
  * [[VUID-VkBufferMemoryBarrier-offset-01187]]
    pname:offset must: be less than the size of pname:buffer
  * [[VUID-VkBufferMemoryBarrier-size-01188]]
    If pname:size is not equal to ename:VK_WHOLE_SIZE, pname:size must: be
    greater than `0`
  * [[VUID-VkBufferMemoryBarrier-size-01189]]
    If pname:size is not equal to ename:VK_WHOLE_SIZE, pname:size must: be
    less than or equal to than the size of pname:buffer minus pname:offset
ifndef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkBufferMemoryBarrier-buffer-01190]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex must: both be ename:VK_QUEUE_FAMILY_IGNORED
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifdef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkBufferMemoryBarrier-buffer-01191]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, at least one of
    pname:srcQueueFamilyIndex and pname:dstQueueFamilyIndex must: be
    ename:VK_QUEUE_FAMILY_IGNORED
  * [[VUID-VkBufferMemoryBarrier-buffer-01763]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, and one of pname:srcQueueFamilyIndex
    and pname:dstQueueFamilyIndex is ename:VK_QUEUE_FAMILY_IGNORED, the
    other must: be ename:VK_QUEUE_FAMILY_IGNORED or a special queue family
    reserved for external memory ownership transfers, as described in
    <<synchronization-queue-transfers>>.
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifndef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkBufferMemoryBarrier-buffer-01192]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE, pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex must: either both be
    ename:VK_QUEUE_FAMILY_IGNORED, or both be a valid queue family (see
    <<devsandqueues-queueprops>>)
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifdef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkBufferMemoryBarrier-buffer-01193]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:srcQueueFamilyIndex is
    ename:VK_QUEUE_FAMILY_IGNORED, pname:dstQueueFamilyIndex must: also be
    ename:VK_QUEUE_FAMILY_IGNORED
  * [[VUID-VkBufferMemoryBarrier-buffer-01764]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:srcQueueFamilyIndex is not
    ename:VK_QUEUE_FAMILY_IGNORED, it must: be a valid queue family or a
    special queue family reserved for external memory transfers, as
    described in <<synchronization-queue-transfers>>.
  * [[VUID-VkBufferMemoryBarrier-buffer-01765]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:dstQueueFamilyIndex is not
    ename:VK_QUEUE_FAMILY_IGNORED, it must: be a valid queue family or a
    special queue family reserved for external memory transfers, as
    described in <<synchronization-queue-transfers>>.
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkBufferMemoryBarrier-buffer-01196]]
    If pname:buffer was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE, and pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex are not ename:VK_QUEUE_FAMILY_IGNORED, at
    least one of them must: be the same as the family of the queue that will
    execute this barrier
  * [[VUID-VkBufferMemoryBarrier-buffer-01931]]
    If pname:buffer is non-sparse then it must: be bound completely and
    contiguously to a single sname:VkDeviceMemory object
****

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


[[synchronization-image-memory-barriers]]
=== Image Memory Barriers

Image memory barriers only apply to memory accesses involving a specific
image subresource range.
That is, a memory dependency formed from an image memory barrier is
<<synchronization-dependencies-access-scopes, scoped>> to access via the
specified image subresource range.
Image memory barriers can: also be used to define
<<synchronization-image-layout-transitions, image layout transitions>> or a
<<synchronization-queue-transfers, queue family ownership transfer>> for the
specified image subresource range.

[open,refpage='VkImageMemoryBarrier',desc='Structure specifying the parameters of an image memory barrier',type='structs']
--

The sname:VkImageMemoryBarrier structure is defined as:

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

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:srcAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, source access mask>>.
  * pname:dstAccessMask is a bitmask of elink:VkAccessFlagBits specifying a
    <<synchronization-access-masks, destination access mask>>.
  * pname:oldLayout is the old layout in an
    <<synchronization-image-layout-transitions, image layout transition>>.
  * pname:newLayout is the new layout in an
    <<synchronization-image-layout-transitions, image layout transition>>.
  * pname:srcQueueFamilyIndex is the source queue family for a
    <<synchronization-queue-transfers, queue family ownership transfer>>.
  * pname:dstQueueFamilyIndex is the destination queue family for a
    <<synchronization-queue-transfers, queue family ownership transfer>>.
  * pname:image is a handle to the image affected by this barrier.
  * pname:subresourceRange describes the <<resources-image-views, image
    subresource range>> within pname:image that is affected by this barrier.

The first <<synchronization-dependencies-access-scopes, access scope>> is
limited to access to memory through the specified image subresource range,
via access types in the <<synchronization-access-masks, source access mask>>
specified by pname:srcAccessMask.
If pname:srcAccessMask includes ename:VK_ACCESS_HOST_WRITE_BIT, memory
writes performed by that access type are also made visible, as that access
type is not performed through a resource.

The second <<synchronization-dependencies-access-scopes, access scope>> is
limited to access to memory through the specified image subresource range,
via access types in the <<synchronization-access-masks, destination access
mask>> specified by pname:dstAccessMask.
If pname:dstAccessMask includes ename:VK_ACCESS_HOST_WRITE_BIT or
ename:VK_ACCESS_HOST_READ_BIT, available memory writes are also made visible
to accesses of those types, as those access types are not performed through
a resource.

If pname:srcQueueFamilyIndex is not equal to pname:dstQueueFamilyIndex, and
pname:srcQueueFamilyIndex is equal to the current queue family, then the
memory barrier defines a <<synchronization-queue-transfers-release, queue
family release operation>> for the specified image subresource range, and
the second access scope includes no access, as if pname:dstAccessMask was
`0`.

If pname:dstQueueFamilyIndex is not equal to pname:srcQueueFamilyIndex, and
pname:dstQueueFamilyIndex is equal to the current queue family, then the
memory barrier defines a <<synchronization-queue-transfers-acquire, queue
family acquire operation>> for the specified image subresource range, and
the first access scope includes no access, as if pname:srcAccessMask was
`0`.

If pname:oldLayout is not equal to pname:newLayout, then the memory barrier
defines an <<synchronization-image-layout-transitions, image layout
transition>> for the specified image subresource range.

[[synchronization-image-barrier-layout-transition-order]]
Layout transitions that are performed via image memory barriers execute in
their entirety in <<synchronization-submission-order, submission order>>,
relative to other image layout transitions submitted to the same queue,
including those performed by <<renderpass, render passes>>.
In effect there is an implicit execution dependency from each such layout
transition to all layout transitions previously submitted to the same queue.

ifdef::VK_EXT_sample_locations[]

The image layout of each image subresource of a depth/stencil image created
with ename:VK_IMAGE_CREATE_SAMPLE_LOCATIONS_COMPATIBLE_DEPTH_BIT_EXT is
dependent on the last sample locations used to render to the image
subresource as a depth/stencil attachment, thus when the pname:image member
of an sname:VkImageMemoryBarrier is an image created with this flag the
application can: chain a slink:VkSampleLocationsInfoEXT structure to the
pname:pNext chain of sname:VkImageMemoryBarrier to specify the sample
locations to use during the image layout transition.

If the sname:VkSampleLocationsInfoEXT structure in the pname:pNext chain of
sname:VkImageMemoryBarrier does not match the sample location state last
used to render to the image subresource range specified by
pname:subresourceRange or if no sname:VkSampleLocationsInfoEXT structure is
in the pname:pNext chain of sname:VkImageMemoryBarrier then the contents of
the given image subresource range becomes undefined as if pname:oldLayout
would equal ename:VK_IMAGE_LAYOUT_UNDEFINED.

endif::VK_EXT_sample_locations[]

ifdef::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[]

If pname:image has a multi-planar format and the image is _disjoint_, then
including ename:VK_IMAGE_ASPECT_COLOR_BIT in the pname:aspectMask member of
pname:subresourceRange is equivalent to including
ename:VK_IMAGE_ASPECT_PLANE_0_BIT, ename:VK_IMAGE_ASPECT_PLANE_1_BIT, and
(for three-plane formats only) ename:VK_IMAGE_ASPECT_PLANE_2_BIT.

endif::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[]

.Valid Usage
****
  * [[VUID-VkImageMemoryBarrier-oldLayout-01197]]
    pname:oldLayout must: be ename:VK_IMAGE_LAYOUT_UNDEFINED or the current
    layout of the image subresources affected by the barrier
  * [[VUID-VkImageMemoryBarrier-newLayout-01198]]
    pname:newLayout must: not be ename:VK_IMAGE_LAYOUT_UNDEFINED or
    ename:VK_IMAGE_LAYOUT_PREINITIALIZED
ifndef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkImageMemoryBarrier-image-01199]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex must: both be ename:VK_QUEUE_FAMILY_IGNORED
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifdef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkImageMemoryBarrier-image-01381]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, at least one of
    pname:srcQueueFamilyIndex and pname:dstQueueFamilyIndex must: be
    ename:VK_QUEUE_FAMILY_IGNORED
  * [[VUID-VkImageMemoryBarrier-image-01766]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_CONCURRENT, and one of pname:srcQueueFamilyIndex
    and pname:dstQueueFamilyIndex is ename:VK_QUEUE_FAMILY_IGNORED, the
    other must: be ename:VK_QUEUE_FAMILY_IGNORED or a special queue family
    reserved for external memory transfers, as described in
    <<synchronization-queue-transfers>>.
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifndef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkImageMemoryBarrier-image-01200]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE, pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex must: either both be
    ename:VK_QUEUE_FAMILY_IGNORED, or both be a valid queue family (see
    <<devsandqueues-queueprops>>).
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
ifdef::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkImageMemoryBarrier-image-01201]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:srcQueueFamilyIndex is
    ename:VK_QUEUE_FAMILY_IGNORED, pname:dstQueueFamilyIndex must: also be
    ename:VK_QUEUE_FAMILY_IGNORED.
  * [[VUID-VkImageMemoryBarrier-image-01767]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:srcQueueFamilyIndex is not
    ename:VK_QUEUE_FAMILY_IGNORED, it must: be a valid queue family or a
    special queue family reserved for external memory transfers, as
    described in <<synchronization-queue-transfers>>.
  * [[VUID-VkImageMemoryBarrier-image-01768]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE and pname:dstQueueFamilyIndex is not
    ename:VK_QUEUE_FAMILY_IGNORED, it must: be a valid queue family or a
    special queue family reserved for external memory transfers, as
    described in <<synchronization-queue-transfers>>.
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
  * [[VUID-VkImageMemoryBarrier-image-01205]]
    If pname:image was created with a sharing mode of
    ename:VK_SHARING_MODE_EXCLUSIVE, and pname:srcQueueFamilyIndex and
    pname:dstQueueFamilyIndex are not ename:VK_QUEUE_FAMILY_IGNORED, at
    least one of them must: be the same as the family of the queue that will
    execute this barrier
  * [[VUID-VkImageMemoryBarrier-subresourceRange-01486]]
    pname:subresourceRange.baseMipLevel must: be less than the
    pname:mipLevels specified in slink:VkImageCreateInfo when pname:image
    was created
  * [[VUID-VkImageMemoryBarrier-subresourceRange-01724]]
    If pname:subresourceRange.levelCount is not
    ename:VK_REMAINING_MIP_LEVELS, [eq]#pname:subresourceRange.baseMipLevel
    {plus} pname:subresourceRange.levelCount# must: be less than or equal to
    the pname:mipLevels specified in slink:VkImageCreateInfo when
    pname:image was created
  * [[VUID-VkImageMemoryBarrier-subresourceRange-01488]]
    pname:subresourceRange.baseArrayLayer must: be less than the
    pname:arrayLayers specified in slink:VkImageCreateInfo when pname:image
    was created
  * [[VUID-VkImageMemoryBarrier-subresourceRange-01725]]
    If pname:subresourceRange.layerCount is not
    ename:VK_REMAINING_ARRAY_LAYERS,
    [eq]#pname:subresourceRange.baseArrayLayer {plus}
    pname:subresourceRange.layerCount# must: be less than or equal to the
    pname:arrayLayers specified in slink:VkImageCreateInfo when pname:image
    was created
  * [[VUID-VkImageMemoryBarrier-image-01207]]
    If pname:image has a depth/stencil format with both depth and stencil
    components, then the pname:aspectMask member of pname:subresourceRange
    must: include both ename:VK_IMAGE_ASPECT_DEPTH_BIT and
    ename:VK_IMAGE_ASPECT_STENCIL_BIT
ifdef::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[]
  * [[VUID-VkImageMemoryBarrier-image-01671]]
    If pname:image has a single-plane color format or is not _disjoint_,
    then the pname:aspectMask member of pname:subresourceRange must: be
    ename:VK_IMAGE_ASPECT_COLOR_BIT
  * [[VUID-VkImageMemoryBarrier-image-01672]]
    If pname:image has a multi-planar format and the image is _disjoint_,
    then the pname:aspectMask member of pname:subresourceRange must: include
    either at least one of ename:VK_IMAGE_ASPECT_PLANE_0_BIT,
    ename:VK_IMAGE_ASPECT_PLANE_1_BIT, and
    ename:VK_IMAGE_ASPECT_PLANE_2_BIT; or must: include
    ename:VK_IMAGE_ASPECT_COLOR_BIT
  * [[VUID-VkImageMemoryBarrier-image-01673]]
    If pname:image has a multi-planar format with only two planes, then the
    pname:aspectMask member of pname:subresourceRange must: not include
    ename:VK_IMAGE_ASPECT_PLANE_2_BIT
endif::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[]
  * [[VUID-VkImageMemoryBarrier-oldLayout-01208]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL then pname:image must:
    have been created with ename:VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT set
  * [[VUID-VkImageMemoryBarrier-oldLayout-01209]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL then pname:image
    must: have been created with
    ename:VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT set
  * [[VUID-VkImageMemoryBarrier-oldLayout-01210]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_DEPTH_STENCIL_READ_ONLY_OPTIMAL then pname:image
    must: have been created with
    ename:VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT set
ifdef::VK_VERSION_1_1,VK_KHR_maintenance2[]
  * [[VUID-VkImageMemoryBarrier-oldLayout-01658]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_DEPTH_READ_ONLY_STENCIL_ATTACHMENT_OPTIMAL then
    pname:image must: have been created with
    ename:VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT set
  * [[VUID-VkImageMemoryBarrier-oldLayout-01659]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_DEPTH_ATTACHMENT_STENCIL_READ_ONLY_OPTIMAL then
    pname:image must: have been created with
    ename:VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT set
endif::VK_VERSION_1_1,VK_KHR_maintenance2[]
  * [[VUID-VkImageMemoryBarrier-oldLayout-01211]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL then pname:image must:
    have been created with ename:VK_IMAGE_USAGE_SAMPLED_BIT or
    ename:VK_IMAGE_USAGE_INPUT_ATTACHMENT_BIT set
  * [[VUID-VkImageMemoryBarrier-oldLayout-01212]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL then pname:image must: have
    been created with ename:VK_IMAGE_USAGE_TRANSFER_SRC_BIT set
  * [[VUID-VkImageMemoryBarrier-oldLayout-01213]]
    If either pname:oldLayout or pname:newLayout is
    ename:VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL then pname:image must: have
    been created with ename:VK_IMAGE_USAGE_TRANSFER_DST_BIT set
  * [[VUID-VkImageMemoryBarrier-image-01932]]
    If pname:image is non-sparse then it must: be bound completely and
    contiguously to a single sname:VkDeviceMemory object
****

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


[[synchronization-queue-transfers]]
=== Queue Family Ownership Transfer

Resources created with a elink:VkSharingMode of
ename:VK_SHARING_MODE_EXCLUSIVE must: have their ownership explicitly
transferred from one queue family to another in order to access their
content in a well-defined manner on a queue in a different queue family.
ifdef::VK_VERSION_1_1,VK_KHR_external_memory[]
Resources shared with external APIs or instances using external memory must:
also explicitly manage ownership transfers between local and external queues
(or equivalent constructs in external APIs) regardless of the
elink:VkSharingMode specified when creating them.
The special queue family index ename:VK_QUEUE_FAMILY_EXTERNAL represents any
queue external to the resource's current Vulkan instance, as long as the
queue uses the same underlying physical device
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
or device group
endif::VK_VERSION_1_1,VK_KHR_device_group[]
and uses the same driver version as the resource's slink:VkDevice, as
indicated by slink:VkPhysicalDeviceIDProperties::pname:deviceUUID and
slink:VkPhysicalDeviceIDProperties::pname:driverUUID.
ifdef::VK_EXT_queue_family_foreign[]
The special queue family index ename:VK_QUEUE_FAMILY_FOREIGN_EXT represents
any queue external to the resource's current Vulkan instance, regardless of
the queue's underlying physical device or driver version.
This includes, for example, queues for fixed-function image processing
devices, media codec devices, and display devices, as well as all queues
that use the same underlying physical device
ifdef::VK_VERSION_1_1,VK_KHR_device_group[]
(or device group)
endif::VK_VERSION_1_1,VK_KHR_device_group[]
and driver version as the resource's slink:VkDevice.
endif::VK_EXT_queue_family_foreign[]
endif::VK_VERSION_1_1,VK_KHR_external_memory[]
If memory dependencies are correctly expressed between uses of such a
resource between two queues in different families, but no ownership transfer
is defined, the contents of that resource are undefined for any read
accesses performed by the second queue family.

[NOTE]
.Note
====
If an application does not need the contents of a resource to remain valid
when transferring from one queue family to another, then the ownership
transfer should: be skipped.
====

ifdef::VK_EXT_queue_family_foreign[]
[NOTE]
.Note
====
Applications should expect transfers to/from
ename:VK_QUEUE_FAMILY_FOREIGN_EXT to be more expensive than transfers
to/from ename:VK_QUEUE_FAMILY_EXTERNAL_KHR.
====
endif::VK_EXT_queue_family_foreign[]

A queue family ownership transfer consists of two distinct parts:

  . Release exclusive ownership from the source queue family
  . Acquire exclusive ownership for the destination queue family

An application must: ensure that these operations occur in the correct order
by defining an execution dependency between them, e.g. using a semaphore.

[[synchronization-queue-transfers-release]] A _release operation_ is used to
release exclusive ownership of a range of a buffer or image subresource
range.
A release operation is defined by executing a
<<synchronization-buffer-memory-barriers, buffer memory barrier>> (for a
buffer range) or an <<synchronization-image-memory-barriers, image memory
barrier>> (for an image subresource range), on a queue from the source queue
family.
The pname:srcQueueFamilyIndex parameter of the barrier must: be set to the
source queue family index, and the pname:dstQueueFamilyIndex parameter to
the destination queue family index.
pname:dstStageMask is ignored for such a barrier, such that no visibility
operation is executed - the value of this mask does not affect the validity
of the barrier.
The release operation happens-after the availability operation.

[[synchronization-queue-transfers-acquire]] An _acquire operation_ is used
to acquire exclusive ownership of a range of a buffer or image subresource
range.
An acquire operation is defined by executing a
<<synchronization-buffer-memory-barriers, buffer memory barrier>> (for a
buffer range) or an <<synchronization-image-memory-barriers, image memory
barrier>> (for an image subresource range), on a queue from the destination
queue family.
The buffer range or image subresource range specified in an acquire
operation must: match exactly that of a previous release operation.
The pname:srcQueueFamilyIndex parameter of the barrier must: be set to the
source queue family index, and the pname:dstQueueFamilyIndex parameter to
the destination queue family index.
pname:srcStageMask is ignored for such a barrier, such that no availability
operation is executed - the value of this mask does not affect the validity
of the barrier.
The acquire operation happens-before the visibility operation.

[NOTE]
.Note
====
Whilst it is not invalid to provide destination or source access masks for
memory barriers used for release or acquire operations, respectively, they
have no practical effect.
Access after a release operation has undefined results, and so visibility
for those accesses has no practical effect.
Similarly, write access before an acquire operation will produce undefined
results for future access, so availability of those writes has no practical
use.
In an earlier version of the specification, these were required to match on
both sides - but this was subsequently relaxed.
These masks should: be set to 0.
====

If the transfer is via an image memory barrier, and an
<<synchronization-image-layout-transitions, image layout transition>> is
desired, then the values of pname:oldLayout and pname:newLayout in the
release memory barrier must: be equal to values of pname:oldLayout and
pname:newLayout in the acquire memory barrier.
Although the image layout transition is submitted twice, it will only be
executed once.
A layout transition specified in this way happens-after the release
operation and happens-before the acquire operation.

If the values of pname:srcQueueFamilyIndex and pname:dstQueueFamilyIndex are
equal, no ownership transfer is performed, and the barrier operates as if
they were both set to ename:VK_QUEUE_FAMILY_IGNORED.

Queue family ownership transfers may: perform read and write accesses on all
memory bound to the image subresource or buffer range, so applications must:
ensure that all memory writes have been made
<<synchronization-dependencies-available-and-visible, available>> before a
queue family ownership transfer is executed.
Available memory is automatically made visible to queue family release and
acquire operations, and writes performed by those operations are
automatically made available.

Once a queue family has acquired ownership of a buffer range or image
subresource range of an ename:VK_SHARING_MODE_EXCLUSIVE resource, its
contents are undefined to other queue families unless ownership is
transferred.
The contents of any portion of another resource which aliases memory that is
bound to the transferred buffer or image subresource range are undefined
after a release or acquire operation.


[[synchronization-wait-idle]]
== Wait Idle Operations

[open,refpage='vkQueueWaitIdle',desc='Wait for a queue to become idle',type='protos']
--

To wait on the host for the completion of outstanding queue operations for a
given queue, call:

include::../api/protos/vkQueueWaitIdle.txt[]

  * pname:queue is the queue on which to wait.

fname:vkQueueWaitIdle is equivalent to submitting a fence to a queue and
waiting with an infinite timeout for that fence to signal.

include::../validity/protos/vkQueueWaitIdle.txt[]
--

[open,refpage='vkDeviceWaitIdle',desc='Wait for a device to become idle',type='protos']
--

To wait on the host for the completion of outstanding queue operations for
all queues on a given logical device, call:

include::../api/protos/vkDeviceWaitIdle.txt[]

  * pname:device is the logical device to idle.

fname:vkDeviceWaitIdle is equivalent to calling fname:vkQueueWaitIdle for
all queues owned by pname:device.

include::../validity/protos/vkDeviceWaitIdle.txt[]
--


[[synchronization-submission-host-writes]]
== Host Write Ordering Guarantees

When batches of command buffers are submitted to a queue via
flink:vkQueueSubmit, it defines a memory dependency with prior host
operations, and execution of command buffers submitted to the queue.

The first <<synchronization-dependencies-scopes, synchronization scope>> is
defined by the host execution model, but includes execution of
flink:vkQueueSubmit on the host and anything that happened-before it.

The second <<synchronization-dependencies-scopes, synchronization scope>>
includes all commands submitted in the same <<devsandqueues-submission,
queue submission>>, and all commands that occur later in
<<synchronization-submission-order,submission order>>.

The first <<synchronization-dependencies-access-scopes, access scope>>
includes all host writes to mappable device memory that are either coherent,
or have been flushed with flink:vkFlushMappedMemoryRanges.

The second <<synchronization-dependencies-access-scopes, access scope>>
includes all memory access performed by the device.


ifdef::VK_VERSION_1_1,VK_KHR_device_group[]

[[synchronization-device-group]]
== Synchronization and Multiple Physical Devices

If a logical device includes more than one physical device, then fences,
semaphores, and events all still have a single instance of the signaled
state.

A fence becomes signaled when all physical devices complete the necessary
queue operations.

Semaphore wait and signal operations all include a device index that is the
sole physical device that performs the operation.
These indices are provided in the slink:VkDeviceGroupSubmitInfo and
slink:VkDeviceGroupBindSparseInfo structures.
Semaphores are not exclusively owned by any physical device.
For example, a semaphore can be signaled by one physical device and then
waited on by a different physical device.

An event can: only be waited on by the same physical device that signaled it
(or the host).

endif::VK_VERSION_1_1,VK_KHR_device_group[]