clCreateContext

Creates an OpenCL context.

cl_context clCreateContext( const cl_context_properties *properties,
  cl_uint num_devices,
  const cl_device_id *devices,
  (voidCL_CALLBACK  *pfn_notify) (

const char *errinfo, 
                        const void *private_info, size_t cb, 
                        void *user_data

),
  void *user_data,
  cl_int *errcode_ret)

Description

An OpenCL context is created with one or more devices. Contexts are used by the OpenCL runtime for managing objects such as command-queues, memory, program and kernel objects and for executing kernels on one or more devices specified in the context.

Parameters

properties

Specifies a list of context property names and their corresponding values. Each property name is immediately followed by the corresponding desired value. The list is terminated with 0. properties can be NULL in which case the platform that is selected is implementation-defined. The list of supported properties is described in the table below.

If the extension cl_khr_d3d10_sharing is enabled, then if a property is not specified in properties, then its default value is used (it is said to be specified implicitly). If properties is NULL or empty (points to a list whose first value is zero), all attributes take on their default value.

If the extension cl_khr_gl_sharing is enabled, then properties points to an attribute list, which is a array of ordered <attribute name, value> pairs terminated with zero. If an attribute is not specified in properties, then its default value is used (it is said to be specified implicitly). If properties is NULL or empty (points to a list whose first value is zero), all attributes take on their default values..

cl_context_properties enum Property value Description
CL_CONTEXT_PLATFORM cl_platform_id Specifies the platform to use.
CL_CONTEXT_D3D10_DEVICE_KHR ID3D10Device* If the cl_khr_d3d10_sharing extension is enabled, specifies the ID3D10Device* to use for Direct3D 10 interoperability. The default value is NULL.

CL_GL_CONTEXT_KHR
CL_EGL_DISPLAY_KHR
CL_GLX_DISPLAY_KHR
CL_WGL_HDC_KHR
CL_CGL_SHAREGROUP_KHR

  Available if the cl_khr_gl_sharing extension is enabled.
num_devices

The number of devices specified in the devices argument.

devices

A pointer to a list of unique devices returned by clGetDeviceIDs for a platform. Duplicate devices specified in devices are ignored.

pfn_notify

A callback function that can be registered by the application. This callback function will be used by the OpenCL implementation to report information on errors that occur in this context. This callback function may be called asynchronously by the OpenCL implementation. It is the application's responsibility to ensure that the callback function is thread-safe. If pfn_notify is NULL, no callback function is registered. The parameters to this callback function are:

errinfo is a pointer to an error string.

private_info and cb represent a pointer to binary data that is returned by the OpenCL implementation that can be used to log additional information helpful in debugging the error.

user_data is a pointer to user supplied data.

NOTE: There are a number of cases where error notifications need to be delivered due to an error that occurs outside a context. Such notifications may not be delivered through the pfn_notify callback. Where these notifications go is implementation-defined.

user_data

Passed as the user_data argument when pfn_notify is called. user_data can be NULL.

errcode_ret

Returns an appropriate error code. If errcode_ret is NULL, no error code is returned.

Notes

clCreateContext and clCreateContextFromType perform an implicit retain. This is very helpful for 3rd party libraries, which typically get a context passed to them by the application. However, it is possible that the application may delete the context without informing the library. Allowing functions to attach to (i.e. retain) and release a context solves the problem of a context being used by a library no longer being valid.

Errors

clCreateContext returns a valid non-zero context and errcode_ret is set to CL_SUCCESS if the context is created successfully. Otherwise, it returns NULL value with the following error values returned in errcode_ret:

  • CL_INVALID_PLATFORM if properties is NULL and no platform could be selected or if platform value specified in properties is not a valid platform. (If the extension cl_khr_gl_sharing is enabled, then see error "CL_INVALID_GL_SHAREGROUP_REFERENCE_KHR" below.)
  • CL_INVALID_PROPERTY if context property name in properties is not a supported property name, if the value specified for a supported property name is not valid, or if the same property name is specified more than once. However if the extension cl_khr_gl_sharing is enabled, then CL_INVALID_PROPERTY is returned if an attribute name other than those listed in the table for properties above is specified in properties.
  • CL_INVALID_VALUE if devices is NULL; if num_devices is equal to zero; or if pfn_notify is NULL but user_data is not NULL.
  • CL_INVALID_DEVICE if devices contains an invalid device.
  • CL_INVALID_OPERATION if Direct3D 10 interoperability is specified by setting CL_INVALID_D3D10_DEVICE_KHR to a non-NULL value, and interoperability with another graphics API is also specified (if the cl_khr_d3d10_sharing extension is enabled).
  • CL_DEVICE_NOT_AVAILABLE if a device in devices is currently not available even though the device was returned by clGetDeviceIDs.
  • CL_OUT_OF_RESOURCES if there is a failure to allocate resources required by the OpenCL implementation on the device.
  • CL_OUT_OF_HOST_MEMORY if there is a failure to allocate resources required by the OpenCL implementation on the host.
  • CL_INVALID_D3D10_DEVICE_KHR if the Direct3D 10 device specified for interoperability is not compatible with the devices against which the context is to be created (if the cl_khr_d3d10_sharing extension is enabled).
  • CL_INVALID_D3D10_DEVICE_KHR if the value of the property CL_CONTEXT_D3D10_DEVICE_KHR is non-NULL and does not specify a valid Direct3D 10 device with which the cl_device_ids against which this context is to be created may interoperate (if the cl_khr_d3d10_sharing extension is enabled).
  • CL_INVALID_GL_SHAREGROUP_REFERENCE_KHR when an invalid OpenGL context or share group object handle is specified in properties (only if the cl_khr_gl_sharing extension is enabled).
  • CL_INVALID_GL_SHAREGROUP_REFERENCE_KHR if the cl_khr_gl_sharing extension is enabled and if a context was specified by any of the following means:
    • Context specified for an EGL-based OpenGL ES or OpenGL implementation by setting the attributes CL_GL_CONTEXT_KHR and CL_EGL_DISPLAY_KHR.
    • Context was specified for a GLX-based OpenGL implementation by setting the attributes CL_GL_CONTEXT_KHR and CL_GLX_DISPLAY_KHR.
    • Context was specified for a WGL-based OpenGL implementation by setting the attributes CL_GL_CONTEXT_KHR and CL_WGL_HDC_KHR.
    and any of the following conditions hold:
    • The specified display and context attributes do not identify a valid OpenGL or OpenGL ES context.
    • The specified context does not support buffer and renderbuffer objects.
    • The specified context is not compatible with the OpenCL context being created (for example, it exists in a physically distinct address space, such as another hardware device, or does not support sharing data with OpenCL due to implementation restrictions).
  • CL_INVALID_GL_SHAREGROUP_REFERENCE_KHR if a share group was specified for a CGL-based OpenGL implementation by setting the attribute CL_CGL_SHAREGROUP_KHR, and the specified share group does not identify a valid CGL share group object (only if the cl_khr_gl_sharing extension is enabled).

Specification

OpenCL Specification

Also see

clGetDeviceIDs, clCreateContextFromType, clRetainContext, clReleaseContext, clGetContextInfo, Cardinality Diagram

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