// 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/ [[fundamentals]] = Fundamentals This chapter introduces fundamental concepts including the Vulkan architecture and execution model, API syntax, queues, pipeline configurations, numeric representation, state and state queries, and the different types of objects and shaders. It provides a framework for interpreting more specific descriptions of commands and behavior in the remainder of the Specification. [[fundamentals-host-environment]] == Host and Device Environment The Vulkan Specification assumes and requires: the following properties of the host environment with respect to Vulkan implementations: * The host must: have runtime support for 8, 16, 32 and 64-bit signed and unsigned twos-complement integers, all addressable at the granularity of their size in bytes. * The host must: have runtime support for 32- and 64-bit floating-point types satisfying the range and precision constraints in the <> section. * The representation and endianness of these types on the host must: match the representation and endianness of the same types on every physical device supported. [NOTE] .Note ==== Since a variety of data types and structures in Vulkan may: be accessible by both host and physical device operations, the implementation should: be able to access such data efficiently in both paths in order to facilitate writing portable and performant applications. ==== [[fundamentals-execmodel]] == Execution Model This section outlines the execution model of a Vulkan system. Vulkan exposes one or more _devices_, each of which exposes one or more _queues_ which may: process work asynchronously to one another. The set of queues supported by a device is partitioned into _families_. Each family supports one or more types of functionality and may: contain multiple queues with similar characteristics. Queues within a single family are considered _compatible_ with one another, and work produced for a family of queues can: be executed on any queue within that family. This Specification defines four types of functionality that queues may: support: graphics, compute, transfer, and sparse memory management. [NOTE] .Note ==== A single device may: report multiple similar queue families rather than, or as well as, reporting multiple members of one or more of those families. This indicates that while members of those families have similar capabilities, they are _not_ directly compatible with one another. ==== Device memory is explicitly managed by the application. Each device may: advertise one or more heaps, representing different areas of memory. Memory heaps are either device local or host local, but are always visible to the device. Further detail about memory heaps is exposed via memory types available on that heap. Examples of memory areas that may: be available on an implementation include: * _device local_ is memory that is physically connected to the device. * _device local, host visible_ is device local memory that is visible to the host. * _host local, host visible_ is memory that is local to the host and visible to the device and host. On other architectures, there may: only be a single heap that can: be used for any purpose. A Vulkan application controls a set of devices through the submission of command buffers which have recorded device commands issued via Vulkan library calls. The content of command buffers is specific to the underlying implementation and is opaque to the application. Once constructed, a command buffer can: be submitted once or many times to a queue for execution. Multiple command buffers can: be built in parallel by employing multiple threads within the application. Command buffers submitted to different queues may: execute in parallel or even out of order with respect to one another. Command buffers submitted to a single queue respect <>, as described further in <>. Command buffer execution by the device is also asynchronous to host execution. Once a command buffer is submitted to a queue, control may: return to the application immediately. Synchronization between the device and host, and between different queues is the responsibility of the application. [[fundamentals-queueoperation]] === Queue Operation Vulkan queues provide an interface to the execution engines of a device. Commands for these execution engines are recorded into command buffers ahead of execution time. These command buffers are then submitted to queues with a _queue submission_ command for execution in a number of _batches_. Once submitted to a queue, these commands will begin and complete execution without further application intervention, though the order of this execution is dependent on a number of <>. Work is submitted to queues using queue submission commands that typically take the form ftext:vkQueue* (e.g. flink:vkQueueSubmit, flink:vkQueueBindSparse), and optionally take a list of semaphores upon which to wait before work begins and a list of semaphores to signal once work has completed. The work itself, as well as signaling and waiting on the semaphores are all _queue operations_. Queue operations on different queues have no implicit ordering constraints, and may: execute in any order. Explicit ordering constraints between queues can: be expressed with <> and <>. Command buffer submissions to a single queue respect <> and other <>, but otherwise may: overlap or execute out of order. Other types of batches and queue submissions against a single queue (e.g. <>) have no implicit ordering constraints with any other queue submission or batch. Additional explicit ordering constraints between queue submissions and individual batches can be expressed with <> and <>. Before a fence or semaphore is signaled, it is guaranteed that any previously submitted queue operations have completed execution, and that memory writes from those queue operations are <> to future queue operations. Waiting on a signaled semaphore or fence guarantees that previous writes that are available are also <> to subsequent commands. Command buffer boundaries, both between primary command buffers of the same or different batches or submissions as well as between primary and secondary command buffers, do not introduce any additional ordering constraints. In other words, submitting the set of command buffers (which can: include executing secondary command buffers) between any semaphore or fence operations execute the recorded commands as if they had all been recorded into a single primary command buffer, except that the current state is <> on each boundary. Explicit ordering constraints can: be expressed with <>. There are a few <> between commands within a command buffer, but only covering a subset of execution. Additional explicit ordering constraints can be expressed with the various <>. [NOTE] .Note ==== Implementations have significant freedom to overlap execution of work submitted to a queue, and this is common due to deep pipelining and parallelism in Vulkan devices. ==== [[fundamentals-queueoperation-command-types]] Commands recorded in command buffers either perform actions (draw, dispatch, clear, copy, query/timestamp operations, begin/end subpass operations), set state (bind pipelines, descriptor sets, and buffers, set dynamic state, push constants, set render pass/subpass state), or perform synchronization (set/wait events, pipeline barrier, render pass/subpass dependencies). Some commands perform more than one of these tasks. State setting commands update the _current state_ of the command buffer. Some commands that perform actions (e.g. draw/dispatch) do so based on the current state set cumulatively since the start of the command buffer. The work involved in performing action commands is often allowed to overlap or to be reordered, but doing so must: not alter the state to be used by each action command. In general, action commands are those commands that alter framebuffer attachments, read/write buffer or image memory, or write to query pools. Synchronization commands introduce explicit <> between two sets of action commands, where the second set of commands depends on the first set of commands. These dependencies enforce that both the execution of certain <> in the later set occur after the execution of certain stages in the source set, and that the effects of <> performed by certain pipeline stages occur in order and are visible to each other. When not enforced by an explicit dependency or <>, action commands may: overlap execution or execute out of order, and may: not see the side effects of each other's memory accesses. The device executes queue operations asynchronously with respect to the host. Control is returned to an application immediately following command buffer submission to a queue. The application must: synchronize work between the host and device as needed. [[fundamentals-objectmodel-overview]] == Object Model The devices, queues, and other entities in Vulkan are represented by Vulkan objects. At the API level, all objects are referred to by handles. There are two classes of handles, dispatchable and non-dispatchable. _Dispatchable_ handle types are a pointer to an opaque type. This pointer may: be used by layers as part of intercepting API commands, and thus each API command takes a dispatchable type as its first parameter. Each object of a dispatchable type must: have a unique handle value during its lifetime. _Non-dispatchable_ handle types are a 64-bit integer type whose meaning is implementation-dependent, and may: encode object information directly in the handle rather than acting as a reference to an underlying object. Objects of a non-dispatchable type may: not have unique handle values within a type or across types. If handle values are not unique, then destroying one such handle must: not cause identical handles of other types to become invalid, and must: not cause identical handles of the same type to become invalid if that handle value has been created more times than it has been destroyed. All objects created or allocated from a sname:VkDevice (i.e. with a sname:VkDevice as the first parameter) are private to that device, and must: not be used on other devices. [[fundamentals-objectmodel-lifetime]] === Object Lifetime Objects are created or allocated by ftext:vkCreate* and ftext:vkAllocate* commands, respectively. Once an object is created or allocated, its "`structure`" is considered to be immutable, though the contents of certain object types is still free to change. Objects are destroyed or freed by ftext:vkDestroy* and ftext:vkFree* commands, respectively. Objects that are allocated (rather than created) take resources from an existing pool object or memory heap, and when freed return resources to that pool or heap. While object creation and destruction are generally expected to be low-frequency occurrences during runtime, allocating and freeing objects can: occur at high frequency. Pool objects help accommodate improved performance of the allocations and frees. It is an application's responsibility to track the lifetime of Vulkan objects, and not to destroy them while they are still in use. [[fundamentals-objectmodel-lifetime-acquire]] The ownership of application-owned memory is immediately acquired by any Vulkan command it is passed into. Ownership of such memory must: be released back to the application at the end of the duration of the command, so that the application can: alter or free this memory as soon as all the commands that acquired it have returned. The following object types are consumed when they are passed into a Vulkan command and not further accessed by the objects they are used to create. They must: not be destroyed in the duration of any API command they are passed into: * sname:VkShaderModule * sname:VkPipelineCache ifdef::VK_EXT_validation_cache[] * sname:VkValidationCacheEXT endif::VK_EXT_validation_cache[] A sname:VkRenderPass object passed as a parameter to create another object is not further accessed by that object after the duration of the command it is passed into. A sname:VkRenderPass used in a command buffer follows the rules described below. A sname:VkPipelineLayout object must: not be destroyed while any command buffer that uses it is in the recording state. sname:VkDescriptorSetLayout objects may: be accessed by commands that operate on descriptor sets allocated using that layout, and those descriptor sets must: not be updated with flink:vkUpdateDescriptorSets after the descriptor set layout has been destroyed. Otherwise, a sname:VkDescriptorSetLayout object passed as a parameter to create another object is not further accessed by that object after the duration of the command it is passed into. The application must: not destroy any other type of Vulkan object until all uses of that object by the device (such as via command buffer execution) have completed. [[fundamentals-objectmodel-lifetime-cmdbuffers]] The following Vulkan objects must: not be destroyed while any command buffers using the object are in the <>: * sname:VkEvent * sname:VkQueryPool * sname:VkBuffer * sname:VkBufferView * sname:VkImage * sname:VkImageView * sname:VkPipeline * sname:VkSampler ifdef::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[] * basetype:VkSamplerYcbcrConversion endif::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[] * sname:VkDescriptorPool * sname:VkFramebuffer * sname:VkRenderPass * sname:VkCommandBuffer * sname:VkCommandPool * sname:VkDeviceMemory * sname:VkDescriptorSet ifdef::VK_NVX_device_generated_commands[] * sname:VkObjectTableNVX * sname:VkIndirectCommandsLayoutNVX endif::VK_NVX_device_generated_commands[] Destroying these objects will move any command buffers that are in the <>, and are using those objects, to the <>. The following Vulkan objects must: not be destroyed while any queue is executing commands that use the object: * sname:VkFence * sname:VkSemaphore * sname:VkCommandBuffer * sname:VkCommandPool In general, objects can: be destroyed or freed in any order, even if the object being freed is involved in the use of another object (e.g. use of a resource in a view, use of a view in a descriptor set, use of an object in a command buffer, binding of a memory allocation to a resource), as long as any object that uses the freed object is not further used in any way except to be destroyed or to be reset in such a way that it no longer uses the other object (such as resetting a command buffer). If the object has been reset, then it can: be used as if it never used the freed object. An exception to this is when there is a parent/child relationship between objects. In this case, the application must: not destroy a parent object before its children, except when the parent is explicitly defined to free its children when it is destroyed (e.g. for pool objects, as defined below). sname:VkCommandPool objects are parents of sname:VkCommandBuffer objects. sname:VkDescriptorPool objects are parents of sname:VkDescriptorSet objects. sname:VkDevice objects are parents of many object types (all that take a sname:VkDevice as a parameter to their creation). The following Vulkan objects have specific restrictions for when they can: be destroyed: * sname:VkQueue objects cannot: be explicitly destroyed. Instead, they are implicitly destroyed when the sname:VkDevice object they are retrieved from is destroyed. * Destroying a pool object implicitly frees all objects allocated from that pool. Specifically, destroying sname:VkCommandPool frees all sname:VkCommandBuffer objects that were allocated from it, and destroying sname:VkDescriptorPool frees all sname:VkDescriptorSet objects that were allocated from it. * sname:VkDevice objects can: be destroyed when all sname:VkQueue objects retrieved from them are idle, and all objects created from them have been destroyed. This includes the following objects: ** sname:VkFence ** sname:VkSemaphore ** sname:VkEvent ** sname:VkQueryPool ** sname:VkBuffer ** sname:VkBufferView ** sname:VkImage ** sname:VkImageView ** sname:VkShaderModule ** sname:VkPipelineCache ** sname:VkPipeline ** sname:VkPipelineLayout ** sname:VkSampler ifdef::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[] ** basetype:VkSamplerYcbcrConversion endif::VK_VERSION_1_1,VK_KHR_sampler_ycbcr_conversion[] ** sname:VkDescriptorSetLayout ** sname:VkDescriptorPool ** sname:VkFramebuffer ** sname:VkRenderPass ** sname:VkCommandPool ** sname:VkCommandBuffer ** sname:VkDeviceMemory ifdef::VK_EXT_validation_cache[] ** sname:VkValidationCacheEXT endif::VK_EXT_validation_cache[] * sname:VkPhysicalDevice objects cannot: be explicitly destroyed. Instead, they are implicitly destroyed when the sname:VkInstance object they are retrieved from is destroyed. * sname:VkInstance objects can: be destroyed once all sname:VkDevice objects created from any of its sname:VkPhysicalDevice objects have been destroyed. ifdef::VK_VERSION_1_1,VK_KHR_external_memory_capabilities,VK_KHR_external_semaphore_capabilities,VK_KHR_external_fence_capabilities[] [[fundamentals-objectmodel-external]] === External Object Handles As defined above, the scope of object handles created or allocated from a sname:VkDevice is limited to that logical device. Objects which are not in scope are said to be external. To bring an external object into scope, an external handle must: be exported from the object in the source scope and imported into the destination scope. [NOTE] .Note ==== The scope of external handles and their associated resources may: vary according to their type, but they can: generally be shared across process and API boundaries. ==== endif::VK_VERSION_1_1,VK_KHR_external_memory_capabilities,VK_KHR_external_semaphore_capabilities,VK_KHR_external_fence_capabilities[] [[fundamentals-abi]] == Application Binary Interface The mechanism by which Vulkan is made available to applications is platform- or implementation- defined. On many platforms the C interface described in this Specification is provided by a shared library. Since shared libraries can be changed independently of the applications that use them, they present particular compatibility challenges, and this Specification places some requirements on them. Shared library implementations must: use the default Application Binary Interface (ABI) of the standard C compiler for the platform, or provide customized API headers that cause application code to use the implementation's non-default ABI. An ABI in this context means the size, alignment, and layout of C data types; the procedure calling convention; and the naming convention for shared library symbols corresponding to C functions. Customizing the calling convention for a platform is usually accomplished by defining <> appropriately in `vk_platform.h`. On platforms where Vulkan is provided as a shared library, library symbols beginning with "`vk`" and followed by a digit or uppercase letter are reserved for use by the implementation. Applications which use Vulkan must: not provide definitions of these symbols. This allows the Vulkan shared library to be updated with additional symbols for new API versions or extensions without causing symbol conflicts with existing applications. Shared library implementations should: provide library symbols for commands in the highest version of this Specification they support, and for ifdef::VK_KHR_surface[] <> endif::VK_KHR_surface[] ifndef::VK_KHR_surface[] Window System Integration endif::VK_KHR_surface[] extensions relevant to the platform. They may: also provide library symbols for commands defined by additional extensions. [NOTE] .Note ==== These requirements and recommendations are intended to allow implementors to take advantage of platform-specific conventions for SDKs, ABIs, library versioning mechanisms, etc. while still minimizing the code changes necessary to port applications or libraries between platforms. Platform vendors, or providers of the _de facto_ standard Vulkan shared library for a platform, are encouraged to document what symbols the shared library provides and how it will be versioned when new symbols are added. Applications should: only rely on shared library symbols for commands in the minimum core version required by the application. flink:vkGetInstanceProcAddr and flink:vkGetDeviceProcAddr should: be used to obtain function pointers for commands in core versions beyond the application's minimum required version. ==== [[fundamentals-commandsyntax]] == Command Syntax and Duration The Specification describes Vulkan commands as functions or procedures using C99 syntax. Language bindings for other languages such as C++ and JavaScript may: allow for stricter parameter passing, or object-oriented interfaces. Vulkan uses the standard C types for the base type of scalar parameters (e.g. types from ``), with exceptions described below, or elsewhere in the text when appropriate: [open,refpage='VkBool32',desc='Vulkan boolean type',type='basetypes'] -- basetype:VkBool32 represents boolean `True` and `False` values, since C does not have a sufficiently portable built-in boolean type: include::../api/basetypes/VkBool32.txt[] ename:VK_TRUE represents a boolean *True* (integer 1) value, and ename:VK_FALSE a boolean *False* (integer 0) value. All values returned from a Vulkan implementation in a basetype:VkBool32 will be either ename:VK_TRUE or ename:VK_FALSE. Applications must: not pass any other values than ename:VK_TRUE or ename:VK_FALSE into a Vulkan implementation where a basetype:VkBool32 is expected. -- [open,refpage='VkDeviceSize',desc='Vulkan device memory size and offsets',type='basetypes'] -- basetype:VkDeviceSize represents device memory size and offset values: include::../api/basetypes/VkDeviceSize.txt[] -- Commands that create Vulkan objects are of the form ftext:vkCreate* and take stext:Vk*CreateInfo structures with the parameters needed to create the object. These Vulkan objects are destroyed with commands of the form ftext:vkDestroy*. The last in-parameter to each command that creates or destroys a Vulkan object is pname:pAllocator. The pname:pAllocator parameter can: be set to a non-`NULL` value such that allocations for the given object are delegated to an application provided callback; refer to the <> chapter for further details. Commands that allocate Vulkan objects owned by pool objects are of the form ftext:vkAllocate*, and take stext:Vk*AllocateInfo structures. These Vulkan objects are freed with commands of the form ftext:vkFree*. These objects do not take allocators; if host memory is needed, they will use the allocator that was specified when their parent pool was created. Commands are recorded into a command buffer by calling API commands of the form ftext:vkCmd*. Each such command may: have different restrictions on where it can: be used: in a primary and/or secondary command buffer, inside and/or outside a render pass, and in one or more of the supported queue types. These restrictions are documented together with the definition of each such command. The _duration_ of a Vulkan command refers to the interval between calling the command and its return to the caller. [[fundamentals-commandsyntax-results-lifetime]] === Lifetime of Retrieved Results Information is retrieved from the implementation with commands of the form ftext:vkGet* and ftext:vkEnumerate*. Unless otherwise specified for an individual command, the results are _invariant_; that is, they will remain unchanged when retrieved again by calling the same command with the same parameters, so long as those parameters themselves all remain valid. [[fundamentals-threadingbehavior]] == Threading Behavior Vulkan is intended to provide scalable performance when used on multiple host threads. All commands support being called concurrently from multiple threads, but certain parameters, or components of parameters are defined to be _externally synchronized_. This means that the caller must: guarantee that no more than one thread is using such a parameter at a given time. More precisely, Vulkan commands use simple stores to update the state of Vulkan objects. A parameter declared as externally synchronized may: have its contents updated at any time during the host execution of the command. If two commands operate on the same object and at least one of the commands declares the object to be externally synchronized, then the caller must: guarantee not only that the commands do not execute simultaneously, but also that the two commands are separated by an appropriate memory barrier (if needed). [NOTE] .Note ==== Memory barriers are particularly relevant for hosts based on the ARM CPU architecture, which is more weakly ordered than many developers are accustomed to from x86/x64 programming. Fortunately, most higher-level synchronization primitives (like the pthread library) perform memory barriers as a part of mutual exclusion, so mutexing Vulkan objects via these primitives will have the desired effect. ==== Similarly the application must: avoid any potential data hazard of application-owned memory that has its <> by a Vulkan command. While the ownership of application-owned memory remains acquired by a command the implementation may: read the memory at any point, and it may: write non-code:const qualified memory at any point. Parameters referring to non-code:const qualified application-owned memory are not marked explicitly as _externally synchronized_ in the Specification. Many object types are _immutable_, meaning the objects cannot: change once they have been created. These types of objects never need external synchronization, except that they must: not be destroyed while they are in use on another thread. In certain special cases mutable object parameters are internally synchronized, making external synchronization unnecessary. One example of this is the use of a sname:VkPipelineCache in fname:vkCreateGraphicsPipelines and fname:vkCreateComputePipelines, where external synchronization around such a heavyweight command would be impractical. The implementation must: internally synchronize the cache in this example, and may: be able to do so in the form of a much finer-grained mutex around the command. Any command parameters that are not labeled as externally synchronized are either not mutated by the command or are internally synchronized. Additionally, certain objects related to a command's parameters (e.g. command pools and descriptor pools) may: be affected by a command, and must: also be externally synchronized. These implicit parameters are documented as described below. Parameters of commands that are externally synchronized are listed below. include::../hostsynctable/parameters.txt[] There are also a few instances where a command can: take in a user allocated list whose contents are externally synchronized parameters. In these cases, the caller must: guarantee that at most one thread is using a given element within the list at a given time. These parameters are listed below. include::../hostsynctable/parameterlists.txt[] In addition, there are some implicit parameters that need to be externally synchronized. For example, all pname:commandBuffer parameters that need to be externally synchronized imply that the pname:commandPool that was passed in when creating that command buffer also needs to be externally synchronized. The implicit parameters and their associated object are listed below. include::../hostsynctable/implicit.txt[] [[fundamentals-errors]] == Errors Vulkan is a layered API. The lowest layer is the core Vulkan layer, as defined by this Specification. The application can: use additional layers above the core for debugging, validation, and other purposes. One of the core principles of Vulkan is that building and submitting command buffers should: be highly efficient. Thus error checking and validation of state in the core layer is minimal, although more rigorous validation can: be enabled through the use of layers. The core layer assumes applications are using the API correctly. Except as documented elsewhere in the Specification, the behavior of the core layer to an application using the API incorrectly is undefined:, and may: include program termination. However, implementations must: ensure that incorrect usage by an application does not affect the integrity of the operating system, the Vulkan implementation, or other Vulkan client applications in the system. In particular, any guarantees made by an operating system about whether memory from one process can: be visible to another process or not must: not be violated by a Vulkan implementation for *any memory allocation*. Vulkan implementations are not required: to make additional security or integrity guarantees beyond those provided by the OS unless explicitly directed by the application's use of a particular feature or extension (e.g. via <>). [NOTE] .Note ==== For instance, if an operating system guarantees that data in all its memory allocations are set to zero when newly allocated, the Vulkan implementation must: make the same guarantees for any allocations it controls (e.g. slink:VkDeviceMemory). ==== Applications can: request stronger robustness guarantees by enabling the pname:robustBufferAccess feature as described in <>. Validation of correct API usage is left to validation layers. Applications should: be developed with validation layers enabled, to help catch and eliminate errors. Once validated, released applications should: not enable validation layers by default. [[fundamentals-validusage]] === Valid Usage Valid usage defines a set of conditions which must: be met in order to achieve well-defined run-time behavior in an application. These conditions depend only on Vulkan state, and the parameters or objects whose usage is constrained by the condition. Some valid usage conditions have dependencies on run-time limits or feature availability. It is possible to validate these conditions against Vulkan's minimum supported values for these limits and features, or some subset of other known values. Valid usage conditions do not cover conditions where well-defined behavior (including returning an error code) exists. Valid usage conditions should: apply to the command or structure where complete information about the condition would be known during execution of an application. This is such that a validation layer or linter can: be written directly against these statements at the point they are specified. [NOTE] .Note ==== This does lead to some non-obvious places for valid usage statements. For instance, the valid values for a structure might depend on a separate value in the calling command. In this case, the structure itself will not reference this valid usage as it is impossible to determine validity from the structure that it is invalid - instead this valid usage would be attached to the calling command. Another example is draw state - the state setters are independent, and can cause a legitimately invalid state configuration between draw calls; so the valid usage statements are attached to the place where all state needs to be valid - at the draw command. ==== Valid usage conditions are described in a block labelled "`Valid Usage`" following each command or structure they apply to. [[fundamentals-implicit-validity]] === Implicit Valid Usage Some valid usage conditions apply to all commands and structures in the API, unless explicitly denoted otherwise for a specific command or structure. These conditions are considered _implicit_, and are described in a block labelled "`Valid Usage (Implicit)`" following each command or structure they apply to. Implicit valid usage conditions are described in detail below. [[fundamentals-validusage-handles]] ==== Valid Usage for Object Handles Any input parameter to a command that is an object handle must: be a valid object handle, unless otherwise specified. An object handle is valid if: * It has been created or allocated by a previous, successful call to the API. Such calls are noted in the Specification. * It has not been deleted or freed by a previous call to the API. Such calls are noted in the Specification. * Any objects used by that object, either as part of creation or execution, must: also be valid. The reserved values dlink:VK_NULL_HANDLE and `NULL` can: be used in place of valid non-dispatchable handles and dispatchable handles, respectively, when _explicitly called out in the Specification_. Any command that creates an object successfully must: not return these values. It is valid to pass these values to ftext:vkDestroy* or ftext:vkFree* commands, which will silently ignore these values. [[fundamentals-validusage-pointers]] ==== Valid Usage for Pointers Any parameter that is a pointer must: be a _valid pointer_ only if it is explicitly called out by a Valid Usage statement. A pointer is "`valid`" if it points at memory containing values of the number and type(s) expected by the command, and all fundamental types accessed through the pointer (e.g. as elements of an array or as members of a structure) satisfy the alignment requirements of the host processor. [[fundamentals-validusage-strings]] ==== Valid Usage for Strings Any parameter that is a pointer to `char` must: be a finite sequence of values terminated by a null character, or if _explicitly called out in the Specification_, can: be `NULL`. [[fundamentals-validusage-enums]] ==== Valid Usage for Enumerated Types Any parameter of an enumerated type must: be a valid enumerant for that type. A enumerant is valid if: * The enumerant is defined as part of the enumerated type. * The enumerant is not one of the special values defined for the enumerated type, which are suffixed with etext:_BEGIN_RANGE, etext:_END_RANGE, etext:_RANGE_SIZE or etext:_MAX_ENUM^1^. 1:: The meaning of these special tokens is not exposed in the Vulkan Specification. They are not part of the API, and they should: not be used by applications. Their original intended use was for internal consumption by Vulkan implementations. Even that use will no longer be supported in the future, but they will be retained for backwards compatibility reasons. Any enumerated type returned from a query command or otherwise output from Vulkan to the application must: not have a reserved value. Reserved values are values not defined by any extension for that enumerated type. [NOTE] .Note ==== This language is intended to accommodate cases such as "`hidden`" extensions known only to driver internals, or layers enabling extensions without knowledge of the application, without allowing return of values not defined by any extension. ==== [[fundamentals-validusage-flags]] ==== Valid Usage for Flags [open,refpage='VkFlags',desc='Vulkan bitmasks',type='basetypes',xrefs='VkColorComponentFlags'] -- A collection of flags is represented by a bitmask using the type basetype:VkFlags: include::../api/basetypes/VkFlags.txt[] Bitmasks are passed to many commands and structures to compactly represent options, but basetype:VkFlags is not used directly in the API. Instead, a etext:Vk*Flags type which is an alias of basetype:VkFlags, and whose name matches the corresponding etext:Vk*FlagBits that are valid for that type, is used. -- Any etext:Vk*Flags member or parameter used in the API as an input must: be a valid combination of bit flags. A valid combination is either zero or the bitwise OR of valid bit flags. A bit flag is valid if: * The bit flag is defined as part of the etext:Vk*FlagBits type, where the bits type is obtained by taking the flag type and replacing the trailing etext:Flags with etext:FlagBits. For example, a flag value of type elink:VkColorComponentFlags must: contain only bit flags defined by elink:VkColorComponentFlagBits. * The flag is allowed in the context in which it is being used. For example, in some cases, certain bit flags or combinations of bit flags are mutually exclusive. Any etext:Vk*Flags member or parameter returned from a query command or otherwise output from Vulkan to the application may: contain bit flags undefined: in its corresponding etext:Vk*FlagBits type. An application cannot: rely on the state of these unspecified bits. [[fundamentals-validusage-sType]] ==== Valid Usage for Structure Types Any parameter that is a structure containing a pname:sType member must: have a value of ptext:sType which is a valid elink:VkStructureType value matching the type of the structure. [open,refpage='VkStructureType',desc='Vulkan structure types (pname:stype)',type='enums'] -- Structure types supported by the Vulkan API include: include::../api/enums/VkStructureType.txt[] Each value corresponds to a particular structure with a pname:sType member with a matching name. As a general rule, the name of each elink:VkStructureType value is obtained by taking the name of the structure, stripping the leading etext:Vk, prefixing each capital letter with etext:_, converting the entire resulting string to upper case, and prefixing it with etext:VK_STRUCTURE_TYPE_. For example, structures of type sname:VkImageCreateInfo correspond to a elink:VkStructureType of ename:VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO, and thus its pname:sType member must: equal that when it is passed to the API. The values ename:VK_STRUCTURE_TYPE_LOADER_INSTANCE_CREATE_INFO and ename:VK_STRUCTURE_TYPE_LOADER_DEVICE_CREATE_INFO are reserved for internal use by the loader, and do not have corresponding Vulkan structures in this Specification. -- [[fundamentals-validusage-pNext]] ==== Valid Usage for Structure Pointer Chains Any parameter that is a structure containing a `void*` ptext:pNext member must: have a value of pname:pNext that is either `NULL`, or points to a valid structure defined by an extension, containing ptext:sType and ptext:pNext members as described in the <> document in the section "`Extension Interactions`". The set of structures connected by ptext:pNext pointers is referred to as a _pname:pNext chain_. If that extension is supported by the implementation, then it must: be enabled. Each type of valid structure must: not appear more than once in a ptext:pNext chain. Any component of the implementation (the loader, any enabled layers, and drivers) must: skip over, without processing (other than reading the pname:sType and pname:pNext members) any structures in the chain with pname:sType values not defined by extensions supported by that component. Extension structures are not described in the base Vulkan Specification, but either in layered Specifications incorporating those extensions, or in separate vendor-provided documents. As a convenience to implementations and layers needing to iterate through a structure pointer chain, the Vulkan API provides the following base structures: include::../api/structs/VkBaseInStructure.txt[] include::../api/structs/VkBaseOutStructure.txt[] sname:VkBaseInStructure can be used to facilitate iterating through a read-only structure pointer chain. sname:VkBaseOutStructure can be used to facilitate iterating through a structure pointer chain that returns data back to the application. These structures allow for some type safety and can be used by Vulkan API functions that operate on generic inputs and outputs. [[fundamentals-validusage-nested-structs]] ==== Valid Usage for Nested Structures The above conditions also apply recursively to members of structures provided as input to a command, either as a direct argument to the command, or themselves a member of another structure. Specifics on valid usage of each command are covered in their individual sections. [[fundamentals-validusage-extensions]] ==== Valid Usage for Extensions Instance-level functionality or behavior added by an <> to the API must: not be used unless that extension is supported by the instance as determined by flink:vkEnumerateInstanceExtensionProperties, and that extension is enabled in slink:VkInstanceCreateInfo. Physical-device-level functionality or behavior added by an <> to the API must: not be used unless that extension is supported by the instance as determined by flink:vkEnumerateInstanceExtensionProperties, and that extension is enabled in slink:VkInstanceCreateInfo. Physical-device-level functionality or behavior added by a <> to the API must: not be used unless the conditions described in <> are met. Device functionality or behavior added by a <> to the API must: not be used unless that extension is supported by the device as determined by flink:vkEnumerateDeviceExtensionProperties, and that extension is enabled in slink:VkDeviceCreateInfo. [[fundamentals-validusage-versions]] ==== Valid Usage for Newer Core Versions ifdef::VK_VERSION_1_1[] Instance-level functionality or behavior added by a <> of the API must: not be used unless it is supported by the instance as determined by flink:vkEnumerateInstanceVersion. endif::VK_VERSION_1_1[] Physical-device-level functionality or behavior added by a <> of the API must: not be used unless it is supported by the physical device as determined by flink:vkGetPhysicalDeviceProperties. Device-level functionality or behavior added by a <> of the API must: not be used unless it is supported by the device as determined by flink:vkGetPhysicalDeviceProperties. [[fundamentals-returncodes]] === Return Codes [open,refpage='VkResult',desc='Vulkan command return codes',type='enums',xrefs='TBD'] -- While the core Vulkan API is not designed to capture incorrect usage, some circumstances still require return codes. Commands in Vulkan return their status via return codes that are in one of two categories: * Successful completion codes are returned when a command needs to communicate success or status information. All successful completion codes are non-negative values. * Run time error codes are returned when a command needs to communicate a failure that could only be detected at run time. All run time error codes are negative values. All return codes in Vulkan are reported via elink:VkResult return values. The possible codes are: include::../api/enums/VkResult.txt[] [[fundamentals-successcodes]] .Success Codes * ename:VK_SUCCESS Command successfully completed * ename:VK_NOT_READY A fence or query has not yet completed * ename:VK_TIMEOUT A wait operation has not completed in the specified time * ename:VK_EVENT_SET An event is signaled * ename:VK_EVENT_RESET An event is unsignaled * ename:VK_INCOMPLETE A return array was too small for the result ifdef::VK_KHR_swapchain[] * ename:VK_SUBOPTIMAL_KHR A swapchain no longer matches the surface properties exactly, but can: still be used to present to the surface successfully. endif::VK_KHR_swapchain[] [[fundamentals-errorcodes]] .Error codes * ename:VK_ERROR_OUT_OF_HOST_MEMORY A host memory allocation has failed. * ename:VK_ERROR_OUT_OF_DEVICE_MEMORY A device memory allocation has failed. * ename:VK_ERROR_INITIALIZATION_FAILED Initialization of an object could not be completed for implementation-specific reasons. * ename:VK_ERROR_DEVICE_LOST The logical or physical device has been lost. See <> * ename:VK_ERROR_MEMORY_MAP_FAILED Mapping of a memory object has failed. * ename:VK_ERROR_LAYER_NOT_PRESENT A requested layer is not present or could not be loaded. * ename:VK_ERROR_EXTENSION_NOT_PRESENT A requested extension is not supported. * ename:VK_ERROR_FEATURE_NOT_PRESENT A requested feature is not supported. * ename:VK_ERROR_INCOMPATIBLE_DRIVER The requested version of Vulkan is not supported by the driver or is otherwise incompatible for implementation-specific reasons. * ename:VK_ERROR_TOO_MANY_OBJECTS Too many objects of the type have already been created. * ename:VK_ERROR_FORMAT_NOT_SUPPORTED A requested format is not supported on this device. * ename:VK_ERROR_FRAGMENTED_POOL A pool allocation has failed due to fragmentation of the pool's memory. This must: only be returned if no attempt to allocate host or device memory was made to accommodate the new allocation. ifdef::VK_VERSION_1_1,VK_KHR_maintenance1[] This should: be returned in preference to ename:VK_ERROR_OUT_OF_POOL_MEMORY, but only if the implementation is certain that the pool allocation failure was due to fragmentation. endif::VK_VERSION_1_1,VK_KHR_maintenance1[] ifdef::VK_KHR_surface[] * ename:VK_ERROR_SURFACE_LOST_KHR A surface is no longer available. * ename:VK_ERROR_NATIVE_WINDOW_IN_USE_KHR The requested window is already in use by Vulkan or another API in a manner which prevents it from being used again. endif::VK_KHR_surface[] ifdef::VK_KHR_swapchain[] * ename:VK_ERROR_OUT_OF_DATE_KHR A surface has changed in such a way that it is no longer compatible with the swapchain, and further presentation requests using the swapchain will fail. Applications must: query the new surface properties and recreate their swapchain if they wish to continue presenting to the surface. endif::VK_KHR_swapchain[] ifdef::VK_KHR_display_swapchain[] * ename:VK_ERROR_INCOMPATIBLE_DISPLAY_KHR The display used by a swapchain does not use the same presentable image layout, or is incompatible in a way that prevents sharing an image. endif::VK_KHR_display_swapchain[] ifdef::VK_NV_glsl_shader[] * ename:VK_ERROR_INVALID_SHADER_NV One or more shaders failed to compile or link. More details are reported back to the application via `<>` if enabled. endif::VK_NV_glsl_shader[] ifdef::VK_VERSION_1_1,VK_KHR_maintenance1[] * ename:VK_ERROR_OUT_OF_POOL_MEMORY A pool memory allocation has failed. This must: only be returned if no attempt to allocate host or device memory was made to accommodate the new allocation. If the failure was definitely due to fragmentation of the pool, ename:VK_ERROR_FRAGMENTED_POOL should: be returned instead. endif::VK_VERSION_1_1,VK_KHR_maintenance1[] ifdef::VK_VERSION_1_1,VK_KHR_external_memory,VK_KHR_external_semaphore,VK_KHR_external_fence[] * ename:VK_ERROR_INVALID_EXTERNAL_HANDLE An external handle is not a valid handle of the specified type. endif::VK_VERSION_1_1,VK_KHR_external_memory,VK_KHR_external_semaphore,VK_KHR_external_fence[] ifdef::VK_EXT_descriptor_indexing[] * ename:VK_ERROR_FRAGMENTATION_EXT A descriptor pool creation has failed due to fragmentation. endif::VK_EXT_descriptor_indexing[] If a command returns a run time error, unless otherwise specified any output parameters will have undefined: contents, except that if the output parameter is a structure with pname:sType and pname:pNext fields, those fields will be unmodified. Any structures chained from pname:pNext will also have undefined: contents, except that pname:sType and pname:pNext will be unmodified. Out of memory errors do not damage any currently existing Vulkan objects. Objects that have already been successfully created can: still be used by the application. Performance-critical commands generally do not have return codes. If a run time error occurs in such commands, the implementation will defer reporting the error until a specified point. For commands that record into command buffers (ftext:vkCmd*) run time errors are reported by fname:vkEndCommandBuffer. -- [[fundamentals-numerics]] == Numeric Representation and Computation Implementations normally perform computations in floating-point, and must: meet the range and precision requirements defined under "`Floating-Point Computation`" below. These requirements only apply to computations performed in Vulkan operations outside of shader execution, such as texture image specification and sampling, and per-fragment operations. Range and precision requirements during shader execution differ and are specified by the <> section. In some cases, the representation and/or precision of operations is implicitly limited by the specified format of vertex or texel data consumed by Vulkan. Specific floating-point formats are described later in this section. [[fundamentals-floatingpoint]] === Floating-Point Computation Most floating-point computation is performed in SPIR-V shader modules. The properties of computation within shaders are constrained as defined by the <> section. Some floating-point computation is performed outside of shaders, such as viewport and depth range calculations. For these computations, we do not specify how floating-point numbers are to be represented, or the details of how operations on them are performed, but only place minimal requirements on representation and precision as described in the remainder of this section. ifdef::editing-notes[] [NOTE] .editing-note ==== (Jon, Bug 14966) This is a rat's nest of complexity, both in terms of describing/enumerating places such computation may: take place (other than "`not shader code`") and in how implementations may: do it. We have consciously deferred the resolution of this issue to post-1.0, and in the meantime, the following language inherited from the OpenGL Specification is inserted as a placeholder. Hopefully it can: be tightened up considerably. ==== endif::editing-notes[] We require simply that numbers`' floating-point parts contain enough bits and that their exponent fields are large enough so that individual results of floating-point operations are accurate to about 1 part in 10^5^. The maximum representable magnitude for all floating-point values must: be at least 2^32^. :: [eq]#x {times} 0 = 0 {times} x = 0# for any non-infinite and non-[eq]#NaN# [eq]#x#. :: [eq]#1 {times} x = x {times} 1 = x#. :: [eq]#x {plus} 0 = 0 {plus} x = x#. :: [eq]#0^0^ = 1#. Occasionally, further requirements will be specified. Most single-precision floating-point formats meet these requirements. The special values [eq]#Inf# and [eq]#-Inf# encode values with magnitudes too large to be represented; the special value [eq]#NaN# encodes "`Not A Number`" values resulting from undefined: arithmetic operations such as [eq]#0 / 0#. Implementations may: support [eq]#Inf# and [eq]#NaN# in their floating-point computations. [[fundamentals-fp-conversion]] === Floating-Point Format Conversions When a value is converted to a defined floating-point representation, finite values falling between two representable finite values are rounded to one or the other. The rounding mode is not defined. Finite values whose magnitude is larger than that of any representable finite value may be rounded either to the closest representable finite value or to the appropriately signed infinity. For unsigned destination formats any negative values are converted to zero. Positive infinity is converted to positive infinity; negative infinity is converted to negative infinity in signed formats and to zero in unsigned formats; and any [eq]#NaN# is converted to a [eq]#NaN#. [[fundamentals-fp16]] === 16-Bit Floating-Point Numbers 16-bit floating point numbers are defined in the "`16-bit floating point numbers`" section of the <>. [[fundamentals-fp11]] === Unsigned 11-Bit Floating-Point Numbers Unsigned 11-bit floating point numbers are defined in the "`Unsigned 11-bit floating point numbers`" section of the <>. [[fundamentals-fp10]] === Unsigned 10-Bit Floating-Point Numbers Unsigned 10-bit floating point numbers are defined in the "`Unsigned 10-bit floating point numbers`" section of the <>. [[fundamentals-general]] === General Requirements Any representable floating-point value in the appropriate format is legal as input to a Vulkan command that requires floating-point data. The result of providing a value that is not a floating-point number to such a command is unspecified, but must: not lead to Vulkan interruption or termination. For example, providing a negative zero (where applicable) or a denormalized number to an Vulkan command must: yield deterministic results, while providing a [eq]#NaN# or [eq]#Inf# yields unspecified results. Some calculations require division. In such cases (including implied divisions performed by vector normalization), division by zero produces an unspecified result but must: not lead to Vulkan interruption or termination. [[fundamentals-fixedconv]] == Fixed-Point Data Conversions When generic vertex attributes and pixel color or depth _components_ are represented as integers, they are often (but not always) considered to be _normalized_. Normalized integer values are treated specially when being converted to and from floating-point values, and are usually referred to as _normalized fixed-point_. In the remainder of this section, [eq]#b# denotes the bit width of the fixed-point integer representation. When the integer is one of the types defined by the API, [eq]#b# is the bit width of that type. When the integer comes from an <> containing color or depth component texels, [eq]#b# is the number of bits allocated to that component in its <>. The signed and unsigned fixed-point representations are assumed to be [eq]#b#-bit binary two's-complement integers and binary unsigned integers, respectively. [[fundamentals-fixedfpconv]] === Conversion from Normalized Fixed-Point to Floating-Point Unsigned normalized fixed-point integers represent numbers in the range [eq]#[0,1]#. The conversion from an unsigned normalized fixed-point value [eq]#c# to the corresponding floating-point value [eq]#f# is defined as [latexmath] ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ f = { c \over { 2^b - 1 } } ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Signed normalized fixed-point integers represent numbers in the range [eq]#[-1,1]#. The conversion from a signed normalized fixed-point value [eq]#c# to the corresponding floating-point value [eq]#f# is performed using [latexmath] ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ f = \max\left( {c \over {2^{b-1} - 1}}, -1.0 \right) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Only the range [eq]#[-2^b-1^ {plus} 1, 2^b-1^ - 1]# is used to represent signed fixed-point values in the range [eq]#[-1,1]#. For example, if [eq]#b = 8#, then the integer value [eq]#-127# corresponds to [eq]#-1.0# and the value 127 corresponds to [eq]#1.0#. Note that while zero is exactly expressible in this representation, one value ([eq]#-128# in the example) is outside the representable range, and must: be clamped before use. This equation is used everywhere that signed normalized fixed-point values are converted to floating-point. [[fundamentals-fpfixedconv]] === Conversion from Floating-Point to Normalized Fixed-Point The conversion from a floating-point value [eq]#f# to the corresponding unsigned normalized fixed-point value [eq]#c# is defined by first clamping [eq]#f# to the range [eq]#[0,1]#, then computing :: [eq]#c = convertFloatToUint(f {times} (2^b^ - 1), b)# where [eq]#convertFloatToUint}(r,b)# returns one of the two unsigned binary integer values with exactly [eq]#b# bits which are closest to the floating-point value [eq]#r#. Implementations should: round to nearest. If [eq]#r# is equal to an integer, then that integer value must: be returned. In particular, if [eq]#f# is equal to 0.0 or 1.0, then [eq]#c# must: be assigned 0 or [eq]#2^b^ - 1#, respectively. The conversion from a floating-point value [eq]#f# to the corresponding signed normalized fixed-point value [eq]#c# is performed by clamping [eq]#f# to the range [eq]#[-1,1]#, then computing :: [eq]#c = convertFloatToInt(f {times} (2^b-1^ - 1), b)# where [eq]#convertFloatToInt(r,b)# returns one of the two signed two's-complement binary integer values with exactly [eq]#b# bits which are closest to the floating-point value [eq]#r#. Implementations should: round to nearest. If [eq]#r# is equal to an integer, then that integer value must: be returned. In particular, if [eq]#f# is equal to -1.0, 0.0, or 1.0, then [eq]#c# must: be assigned [eq]#-(2^b-1^ - 1)#, 0, or [eq]#2^b-1^ - 1#, respectively. This equation is used everywhere that floating-point values are converted to signed normalized fixed-point. [[fundamentals-versionnum]] == API Version Numbers and Semantics The Vulkan version number is used in several places in the API. In each such use, the API _major version number_, _minor version number_, and _patch version number_ are packed into a 32-bit integer as follows: * The major version number is a 10-bit integer packed into bits 31-22. * The minor version number is a 10-bit integer packed into bits 21-12. * The patch version number is a 12-bit integer packed into bits 11-0. Differences in any of the Vulkan version numbers indicates a change to the API in some way, with each part of the version number indicating a different scope of changes. A difference in patch version numbers indicates that some usually small part of the Specification or header has been modified, typically to fix a bug, and may: have an impact on the behavior of existing functionality. Differences in this version number should: not affect either _full compatibility_ or _backwards compatibility_ between two versions, or add additional interfaces to the API. A difference in minor version numbers indicates that some amount of new functionality has been added. This will usually include new interfaces in the header, and may: also include behavior changes and bug fixes. Functionality may: be deprecated in a minor revision, but will not be removed. The patch version will continue to increment through minor version number changes since all minor versions are generated from the same source files, and changes to the source files may affect all minor versions within a major version. Differences in the patch version should: not affect backwards compatibility, but will affect full compatibility. The patch version of the Specification is taken from dlink:VK_HEADER_VERSION. A difference in major version numbers indicates a large set of changes to the API, potentially including new functionality and header interfaces, behavioral changes, removal of deprecated features, modification or outright replacement of any feature, and is thus very likely to break any and all compatibility. Differences in this version will typically require significant modification to an application in order for it to function. C language macros for manipulating version numbers are defined in the <> appendix. [[fundamentals-common-objects]] == Common Object Types Some types of Vulkan objects are used in many different structures and command parameters, and are described here. These types include _offsets_, _extents_, and _rectangles_. === Offsets Offsets are used to describe a pixel location within an image or framebuffer, as an (x,y) location for two-dimensional images, or an (x,y,z) location for three-dimensional images. [open,refpage='VkOffset2D',desc='Structure specifying a two-dimensional offset',type='structs'] -- A two-dimensional offsets is defined by the structure: include::../api/structs/VkOffset2D.txt[] * pname:x is the x offset. * pname:y is the y offset. include::../validity/structs/VkOffset2D.txt[] -- [open,refpage='VkOffset3D',desc='Structure specifying a three-dimensional offset',type='structs'] -- A three-dimensional offset is defined by the structure: include::../api/structs/VkOffset3D.txt[] * pname:x is the x offset. * pname:y is the y offset. * pname:z is the z offset. include::../validity/structs/VkOffset3D.txt[] -- === Extents Extents are used to describe the size of a rectangular region of pixels within an image or framebuffer, as (width,height) for two-dimensional images, or as (width,height,depth) for three-dimensional images. [open,refpage='VkExtent2D',desc='Structure specifying a two-dimensional extent',type='structs'] -- A two-dimensional extent is defined by the structure: include::../api/structs/VkExtent2D.txt[] * pname:width is the width of the extent. * pname:height is the height of the extent. include::../validity/structs/VkExtent2D.txt[] -- [open,refpage='VkExtent3D',desc='Structure specifying a three-dimensional extent',type='structs'] -- A three-dimensional extent is defined by the structure: include::../api/structs/VkExtent3D.txt[] * pname:width is the width of the extent. * pname:height is the height of the extent. * pname:depth is the depth of the extent. include::../validity/structs/VkExtent3D.txt[] -- === Rectangles [open,refpage='VkRect2D',desc='Structure specifying a two-dimensional subregion',type='structs'] -- Rectangles are used to describe a specified rectangular region of pixels within an image or framebuffer. Rectangles include both an offset and an extent of the same dimensionality, as described above. Two-dimensional rectangles are defined by the structure include::../api/structs/VkRect2D.txt[] * pname:offset is a slink:VkOffset2D specifying the rectangle offset. * pname:extent is a slink:VkExtent2D specifying the rectangle extent. include::../validity/structs/VkRect2D.txt[] --