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clEnqueueMapBuffer - Enqueues a command to map a region of the buffer object given by buffer into the host address space and returns a pointer to this mapped region. void * clEnqueueMapBuffer(cl_command_queue command_queue, cl_mem buffer, cl_bool blocking_map, cl_map_flags map_flags, size_t offset, size_t size, cl_uint num_events_in_wait_list, const cl_event *event_wait_list, cl_event *event, cl_int *errcode_ret);
command_queue Must be a valid command-queue. blocking_map Indicates if the map operation is blocking or non-blocking. If blocking_map is CL_TRUE, clEnqueueMapBuffer does not return until the specified region in buffer is mapped into the host address space and the application can access the contents of the mapped region using the pointer returned by clEnqueueMapBuffer. If blocking_map is CL_FALSE i.e. map operation is non-blocking, the pointer to the mapped region returned by clEnqueueMapBuffer cannot be used until the map command has completed. The event argument returns an event object which can be used to query the execution status of the map command. When the map command is completed, the application can access the contents of the mapped region using the pointer returned by clEnqueueMapBuffer. map_flags A bit-bield with the following supported values. ┌───────────────────────────────┬──────────────────────────────────┐ │cl_map_flags │ Description │ ├───────────────────────────────┼──────────────────────────────────┤ │CL_MAP_READ │ This flag specifies that the │ │ │ region being mapped in the │ │ │ memory object is being mapped │ │ │ for reading. │ │ │ │ │ │ The pointer returned by │ │ │ clEnqueueMapBuffer or │ │ │ clEnqueueMapImage(3clc) is │ │ │ guaranteed to contain the latest │ │ │ bits in the region being mapped │ │ │ when the clEnqueueMapBuffer or │ │ │ clEnqueueMapImage(3clc) command │ │ │ has completed. │ ├───────────────────────────────┼──────────────────────────────────┤ │CL_MAP_WRITE │ This flag specifies that the │ │ │ region being mapped in the │ │ │ memory object is being mapped │ │ │ for writing. │ │ │ │ │ │ The pointer returned by │ │ │ clEnqueueMapBuffer or │ │ │ clEnqueueMapImage(3clc) is │ │ │ guaranteed to contain the latest │ │ │ bits in the region being mapped │ │ │ when the clEnqueueMapBuffer or │ │ │ clEnqueueMapImage(3clc) command │ │ │ has completed. │ ├───────────────────────────────┼──────────────────────────────────┤ │CL_MAP_WRITE_INVALIDATE_REGION │ This flag specifies that the │ │ │ region being mapped in the │ │ │ memory object is being mapped │ │ │ for writing. │ │ │ │ │ │ The contents of the region being │ │ │ mapped are to be discarded. This │ │ │ is typically the case when the │ │ │ region being mapped is │ │ │ overwritten by the host. This │ │ │ flag allows the implementation │ │ │ to no longer guarantee that the │ │ │ pointer returned by │ │ │ clEnqueueMapBuffer or │ │ │ clEnqueueMapImage(3clc) contains │ │ │ the latest bits in the region │ │ │ being mapped which can be a │ │ │ significant performance │ │ │ enhancement. │ │ │ │ │ │ CL_MAP_READ or CL_MAP_WRITE and │ │ │ CL_MAP_WRITE_INVALIDATE_REGION │ │ │ are mutually exclusive. │ └───────────────────────────────┴──────────────────────────────────┘ buffer A valid buffer object. The OpenCL context associated with command_queue and buffer must be the same. offset, size The offset in bytes and the size of the region in the buffer object that is being mapped. event_wait_list, num_events_in_wait_list Specify events that need to complete before this particular command can be executed. If event_wait_list is NULL, then this particular command does not wait on any event to complete. If event_wait_list is NULL, num_events_in_wait_list must be 0. If event_wait_list is not NULL, the list of events pointed to by event_wait_list must be valid and num_events_in_wait_list must be greater than 0. The events specified in event_wait_list act as synchronization points. The context associated with events in event_wait_list and command_queue must be the same. The memory associated with event_wait_list can be reused or freed after the function returns. event Returns an event object that identifies this particular copy command and can be used to query or queue a wait for this particular command to complete. event can be NULL in which case it will not be possible for the application to query the status of this command or queue a wait for this command to complete. If the event_wait_list and the event arguments are not NULL, the event argument should not refer to an element of the event_wait_list array. errcode_ret Returns an appropriate error code. If errcode_ret is NULL, no error code is returned.
The returned pointer maps a region starting at offset and is at least size bytes in size. The result of a memory access outside this region is undefined. If the buffer object is created with CL_MEM_USE_HOST_PTR set in mem_flags, the following will be true: · The host_ptr specified in clCreateBuffer(3clc) is guaranteed to contain the latest bits in the region being mapped when the clEnqueueMapBuffer command has completed. · The pointer value returned by clEnqueueMapBuffer will be derived from the host_ptr specified when the buffer object is created. Mapped buffer objects are unmapped using clEnqueueUnmapMemObject(3clc). clEnqueueMapBuffer and clEnqueueMapImage(3clc) increment the mapped count of the memory object. The initial mapped count value of a memory object is zero. Multiple calls to clEnqueueMapBuffer or clEnqueueMapImage(3clc) on the same memory object will increment this mapped count by appropriate number of calls. clEnqueueUnmapMemObject(3clc) decrements the mapped count of the memory object. clEnqueueMapBuffer and clEnqueueMapImage(3clc) act as synchronization points for a region of the buffer object being mapped.
clEnqueueMapBuffer will return a pointer to the mapped region if the function is executed successfully. The errcode_ret is set to CL_SUCCESS. A NULL pointer is returned otherwise with one of the following error values returned in errcode_ret: · CL_INVALID_COMMAND_QUEUE if command_queue is not a valid command-queue. · CL_INVALID_CONTEXT if the context associated with command_queue and image are not the same or if the context associated with command_queue and events in event_wait_list are not the same. · CL_INVALID_MEM_OBJECT if image is not a valid image object. · CL_INVALID_VALUE if region being mapped given by (offset, size) is out of bounds or if size is 0 or values specified in map_flags are not valid. · CL_INVALID_EVENT_WAIT_LIST if event_wait_list is NULL and num_events_in_wait_list > 0, or event_wait_list is not NULL and num_events_in_wait_list is 0, or if event objects in event_wait_list are not valid events. · CL_MISALIGNED_SUB_BUFFER_OFFSET if buffer is a sub-buffer object and offset specified when the sub-buffer object is created is not aligned to CL_DEVICE_MEM_BASE_ADDR_ALIGN value for device associated with queue. · CL_MAP_FAILURE if there is a failure to map the requested region into the host address space. This error cannot occur for image objects created with CL_MEM_USE_HOST_PTR or CL_MEM_ALLOC_HOST_PTR. · CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST if the map operation is blocking and the execution status of any of the events in event_wait_list is a negative integer value. · CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to allocate memory for data store associated with buffer. · CL_INVALID_OPERATION if buffer has been created with CL_MEM_HOST_WRITE_ONLY or CL_MEM_HOST_NO_ACCESS and CL_MAP_READ is set in map_flags or if buffer has been created with CL_MEM_HOST_READ_ONL or CL_MEM_HOST_NO_ACCESS and CL_MAP_WRITE or CL_MAP_WRITE_INVALIDATE_REGION is set in map_flags. · 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.
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1. OpenCL Specification page 86, section 5.2.3 - Mapping Buffer Objects