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)

Parameters

command_queue

Must be a valid host 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 clEnqueueMap{Buffer|Image} is guaranteed to contain the latest bits in the region being mapped when the clEnqueueMap{Buffer|Image} 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 clEnqueueMap{Buffer|Image} is guaranteed to contain the latest bits in the region being mapped when the clEnqueueMap{Buffer|Image} 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 clEnqueueMap{Buffer|Image} 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.

Notes

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 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.

clEnqueueMapBuffer and clEnqueueMapImage 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 on the same memory object will increment this mapped count by appropriate number of calls. clEnqueueUnmapMemObject decrements the mapped count of the memory object.

clEnqueueMapBuffer and clEnqueueMapImage act as synchronization points for a region of the buffer object being mapped.

Accessing mapped regions of a memory object

This section describes the behavior of OpenCL commands that access mapped regions of a memory object.

The contents of the region of a memory object and associated memory objects (sub-buffer objects or 1D image buffer objects that overlap this region) mapped for writing (i.e. CL_MAP_WRITE or CL_MAP_WRITE_INVALIDATE_REGION is set in map_flags argument to clEnqueueMapBuffer or clEnqueueMapImage) are considered to be undefined until this region is unmapped.

Multiple commands in command-queues can map a region or overlapping regions of a memory object and associated memory objects (sub-buffer objects or 1D image buffer objects that overlap this region) for reading (i.e. map_flags = CL_MAP_READ). The contents of the regions of a memory object mapped for reading can also be read by kernels and other OpenCL commands (such as clEnqueueCopyBuffer) executing on a device(s).

Mapping (and unmapping) overlapped regions in a memory object and/or associated memory objects (sub-buffer objects or 1D image buffer objects that overlap this region) for writing is an error and will result in CL_INVALID_OPERATION error returned by clEnqueueMapBuffer or clEnqueueMapImage.

If a memory object is currently mapped for writing, the application must ensure that the memory object is unmapped before any enqueued kernels or commands that read from or write to this memory object or any of its associated memory objects (sub-buffer or 1D image buffer objects) or its parent object (if the memory object is a sub-buffer or 1D image buffer object) begin execution; otherwise the behavior is undefined.

If a memory object is currently mapped for reading, the application must ensure that the memory object is unmapped before any enqueued kernels or commands that write to this memory object or any of its associated memory objects (sub-buffer or 1D image buffer objects) or its parent object (if the memory object is a sub-buffer or 1D image buffer object) begin execution; otherwise the behavior is undefined.

A memory object is considered as mapped if there are one or more active mappings for the memory object irrespective of whether the mapped regions span the entire memory object.

Accessing the contents of the memory region referred to by the mapped pointer that has been unmapped is undefined.

The mapped pointer returned by clEnqueueMapBuffer or clEnqueueMapImage can be used as ptr argument value to clEnqueueReadBuffer, clEnqueueWriteBuffer, clEnqueueReadBufferRect, clEnqueueWriteBufferRect, clEnqueueReadImage, and clEnqueueWriteImage, provided the rules described above are adhered to.

Errors

clEnqueueMapBuffer will return a pointer to the mapped region. 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 host command-queue.
  • CL_INVALID_CONTEXT if the context associated with command_queue and buffer 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 buffer 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 buffer 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.

Specification

OpenCL Specification

Also see

clEnqueueMapImage, clEnqueueUnmapMemObject, clEnqueueSVMMap

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