## C Specification

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 descriptor type used, or how a fixed-function stage accesses memory. Each access type corresponds to a bit flag in VkAccessFlagBits.

Some synchronization commands take sets of access types as parameters to define the access scopes of a memory dependency. If a synchronization command includes a source access mask, its first 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 access scope only includes accesses via the access types specified in that mask.

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

typedef enum VkAccessFlagBits {
VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT = 0x00000100,
VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_WRITE_BIT = 0x00000400,
VK_ACCESS_TRANSFER_WRITE_BIT = 0x00001000,
VK_ACCESS_HOST_WRITE_BIT = 0x00004000,
VK_ACCESS_MEMORY_WRITE_BIT = 0x00010000,
VK_ACCESS_TRANSFORM_FEEDBACK_WRITE_BIT_EXT = 0x02000000,
VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT = 0x08000000,
VK_ACCESS_COMMAND_PROCESS_WRITE_BIT_NVX = 0x00040000,
VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_NV = 0x00400000,
VK_ACCESS_FLAG_BITS_MAX_ENUM = 0x7FFFFFFF
} VkAccessFlagBits;

## Description

• VK_ACCESS_INDIRECT_COMMAND_READ_BIT specifies read access to indirect command data read as part of an indirect drawing or dispatch command.

• VK_ACCESS_INDEX_READ_BIT specifies read access to an index buffer as part of an indexed drawing command, bound by vkCmdBindIndexBuffer.

• VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT specifies read access to a vertex buffer as part of a drawing command, bound by vkCmdBindVertexBuffers.

• VK_ACCESS_UNIFORM_READ_BIT specifies read access to a uniform buffer.

• VK_ACCESS_INPUT_ATTACHMENT_READ_BIT specifies read access to an input attachment within a render pass during fragment shading.

• VK_ACCESS_SHADER_READ_BIT specifies read access to a storage buffer, physical storage buffer, uniform texel buffer, storage texel buffer, sampled image, or storage image.

• VK_ACCESS_SHADER_WRITE_BIT specifies write access to a storage buffer, physical storage buffer, storage texel buffer, or storage image.

• VK_ACCESS_COLOR_ATTACHMENT_READ_BIT specifies read access to a color attachment, such as via blending, logic operations, or via certain subpass load operations. It does not include advanced blend operations.

• VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT specifies write access to a color, resolve, or depth/stencil resolve attachment during a render pass or via certain subpass load and store operations.

• VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_READ_BIT specifies read access to a depth/stencil attachment, via depth or stencil operations or via certain subpass load operations.

• VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_WRITE_BIT specifies write access to a depth/stencil attachment, via depth or stencil operations or via certain subpass load and store operations.

• VK_ACCESS_TRANSFER_READ_BIT specifies read access to an image or buffer in a copy operation.

• VK_ACCESS_TRANSFER_WRITE_BIT specifies write access to an image or buffer in a clear or copy operation.

• 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.

• 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.

• VK_ACCESS_MEMORY_READ_BIT specifies all read accesses. It is always valid in any access mask, and is treated as equivalent to setting all READ access flags that are valid where it is used.

• VK_ACCESS_MEMORY_WRITE_BIT specifies all write accesses. It is always valid in any access mask, and is treated as equivalent to setting all WRITE access flags that are valid where it is used.

• VK_ACCESS_CONDITIONAL_RENDERING_READ_BIT_EXT specifies read access to a predicate as part of conditional rendering.

• VK_ACCESS_TRANSFORM_FEEDBACK_WRITE_BIT_EXT specifies write access to a transform feedback buffer made when transform feedback is active.

• VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT specifies read access to a transform feedback counter buffer which is read when vkCmdBeginTransformFeedbackEXT executes.

• VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT specifies write access to a transform feedback counter buffer which is written when vkCmdEndTransformFeedbackEXT executes.

• VK_ACCESS_COMMAND_PROCESS_READ_BIT_NVX specifies reads from VkBuffer inputs to vkCmdProcessCommandsNVX.

• VK_ACCESS_COMMAND_PROCESS_WRITE_BIT_NVX specifies writes to the target command buffer in vkCmdProcessCommandsNVX.

• VK_ACCESS_COLOR_ATTACHMENT_READ_NONCOHERENT_BIT_EXT is similar to VK_ACCESS_COLOR_ATTACHMENT_READ_BIT, but also includes advanced blend operations.

• VK_ACCESS_SHADING_RATE_IMAGE_READ_BIT_NV specifies read access to a shading rate image as part of a drawing command, as bound by vkCmdBindShadingRateImageNV.

• VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_NV specifies read access to an acceleration structure as part of a trace or build command.

• VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_NV specifies write access to an acceleration structure as part of a build command.

• VK_ACCESS_FRAGMENT_DENSITY_MAP_READ_BIT_EXT specifies read access to a fragment density map attachment during dynamic fragment density map operations

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 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.

Table 1. Supported access types
Access flag Supported pipeline stages

VK_ACCESS_INDIRECT_COMMAND_READ_BIT

VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT

VK_ACCESS_INDEX_READ_BIT

VK_PIPELINE_STAGE_VERTEX_INPUT_BIT

VK_ACCESS_VERTEX_ATTRIBUTE_READ_BIT

VK_PIPELINE_STAGE_VERTEX_INPUT_BIT

VK_ACCESS_UNIFORM_READ_BIT

VK_PIPELINE_STAGE_TASK_SHADER_BIT_NV, VK_PIPELINE_STAGE_MESH_SHADER_BIT_NV, VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_NV, VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT

VK_ACCESS_SHADER_READ_BIT

VK_PIPELINE_STAGE_TASK_SHADER_BIT_NV, VK_PIPELINE_STAGE_MESH_SHADER_BIT_NV, VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_NV, VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT

VK_ACCESS_SHADER_WRITE_BIT

VK_PIPELINE_STAGE_TASK_SHADER_BIT_NV, VK_PIPELINE_STAGE_MESH_SHADER_BIT_NV, VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_NV, VK_PIPELINE_STAGE_VERTEX_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_CONTROL_SHADER_BIT, VK_PIPELINE_STAGE_TESSELLATION_EVALUATION_SHADER_BIT, VK_PIPELINE_STAGE_GEOMETRY_SHADER_BIT, VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT, or VK_PIPELINE_STAGE_COMPUTE_SHADER_BIT

VK_ACCESS_INPUT_ATTACHMENT_READ_BIT

VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT

VK_ACCESS_COLOR_ATTACHMENT_READ_BIT

VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT

VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT

VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT

VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_READ_BIT

VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT, or VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT

VK_ACCESS_DEPTH_STENCIL_ATTACHMENT_WRITE_BIT

VK_PIPELINE_STAGE_EARLY_FRAGMENT_TESTS_BIT, or VK_PIPELINE_STAGE_LATE_FRAGMENT_TESTS_BIT

VK_ACCESS_TRANSFER_READ_BIT

VK_PIPELINE_STAGE_TRANSFER_BIT

VK_ACCESS_TRANSFER_WRITE_BIT

VK_PIPELINE_STAGE_TRANSFER_BIT

VK_ACCESS_HOST_READ_BIT

VK_PIPELINE_STAGE_HOST_BIT

VK_ACCESS_HOST_WRITE_BIT

VK_PIPELINE_STAGE_HOST_BIT

VK_ACCESS_MEMORY_READ_BIT

Any

VK_ACCESS_MEMORY_WRITE_BIT

Any

VK_ACCESS_COLOR_ATTACHMENT_READ_NONCOHERENT_BIT_EXT

VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT

VK_ACCESS_COMMAND_PROCESS_READ_BIT_NVX

VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX

VK_ACCESS_COMMAND_PROCESS_WRITE_BIT_NVX

VK_PIPELINE_STAGE_COMMAND_PROCESS_BIT_NVX

VK_ACCESS_CONDITIONAL_RENDERING_READ_BIT_EXT

VK_PIPELINE_STAGE_CONDITIONAL_RENDERING_BIT_EXT

VK_ACCESS_SHADING_RATE_IMAGE_READ_BIT_NV

VK_PIPELINE_STAGE_SHADING_RATE_IMAGE_BIT_NV

VK_ACCESS_TRANSFORM_FEEDBACK_WRITE_BIT_EXT

VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT

VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_WRITE_BIT_EXT

VK_PIPELINE_STAGE_TRANSFORM_FEEDBACK_BIT_EXT

VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT

VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT

VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_NV

VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_NV, or VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_NV

VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_NV

VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_NV

VK_ACCESS_FRAGMENT_DENSITY_MAP_READ_BIT_EXT

VK_PIPELINE_STAGE_FRAGMENT_DENSITY_PROCESS_BIT_EXT

If a memory object does not have the VK_MEMORY_PROPERTY_HOST_COHERENT_BIT property, then vkFlushMappedMemoryRanges must be called in order to guarantee that writes to the memory object from the host are made available to the host domain, where they can be further made available to the device domain via a domain operation. Similarly, vkInvalidateMappedMemoryRanges must be called to guarantee that writes which are available to the host domain are made visible to host operations.

If the memory object does have the VK_MEMORY_PROPERTY_HOST_COHERENT_BIT property flag, writes to the memory object from the host are automatically made available to the host domain. Similarly, writes made available to the host domain are automatically made visible to the host.

 Note The vkQueueSubmit command automatically performs a domain operation from host to device for all writes performed before the command executes, 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.

## Document Notes

This page is extracted from the Vulkan Specification. Fixes and changes should be made to the Specification, not directly.