C Specification

To read from a buffer object to host memory or to write to a buffer object from host memory call one of the functions

// Provided by CL_VERSION_1_0
cl_int clEnqueueReadBuffer(
    cl_command_queue command_queue,
    cl_mem buffer,
    cl_bool blocking_read,
    size_t offset,
    size_t size,
    void* ptr,
    cl_uint num_events_in_wait_list,
    const cl_event* event_wait_list,
    cl_event* event);
// Provided by CL_VERSION_1_0
cl_int clEnqueueWriteBuffer(
    cl_command_queue command_queue,
    cl_mem buffer,
    cl_bool blocking_write,
    size_t offset,
    size_t size,
    const void* ptr,
    cl_uint num_events_in_wait_list,
    const cl_event* event_wait_list,
    cl_event* event);

Parameters

  • command_queue is a valid host command-queue in which the read / write command will be queued. command_queue and buffer must be created with the same OpenCL context.

  • buffer refers to a valid buffer object.

  • blocking_read and blocking_write indicate if the read and write operations are blocking or non-blocking (see below).

  • offset is the offset in bytes in the buffer object to read from or write to.

  • size is the size in bytes of data being read or written.

  • ptr is the pointer to buffer in host memory where data is to be read into or to be written from.

  • event_wait_list and 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 read / write command and can be used to query or queue a wait for this command to complete. If event is NULL or the enqueue is unsuccessful, no event will be created and therefore it will not be possible to query the status of this command or to wait for this command to complete. If event_wait_list and event are not NULL, event must not refer to an element of the event_wait_list array.

Description

If blocking_read is CL_TRUE i.e. the read command is blocking, clEnqueueReadBuffer does not return until the buffer data has been read and copied into memory pointed to by ptr.

If blocking_read is CL_FALSE i.e. the read command is non-blocking, clEnqueueReadBuffer queues a non-blocking read command and returns. The contents of the buffer that ptr points to cannot be used until the read command has completed. The event argument returns an event object which can be used to query the execution status of the read command. When the read command has completed, the contents of the buffer that ptr points to can be used by the application.

If blocking_write is CL_TRUE, the write command is blocking and does not return until the command is complete, including transfer of the data. The memory pointed to by ptr can be reused by the application after the clEnqueueWriteBuffer call returns.

If blocking_write is CL_FALSE, the OpenCL implementation will use ptr to perform a non-blocking write. As the write is non-blocking the implementation can return immediately. The memory pointed to by ptr cannot be reused by the application after the call returns. The event argument returns an event object which can be used to query the execution status of the write command. When the write command has completed, the memory pointed to by ptr can then be reused by the application.

clEnqueueReadBuffer and clEnqueueWriteBuffer return CL_SUCCESS if the function is executed successfully. Otherwise, they return one of the following errors:

  • 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

    • 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 buffer object

  • CL_INVALID_VALUE

    • if the region being read or written specified by offset and size is out of bounds

    • if ptr is NULL

  • CL_INVALID_EVENT_WAIT_LIST

    • if event_wait_list is NULL and num_events_in_wait_list is greater than zero

    • if event_wait_list is not NULL and num_events_in_wait_list is zero

    • if event objects in event_wait_list are not valid events

  • CL_MISALIGNED_SUB_BUFFER_OFFSET

    • if buffer is a sub-buffer object and the offset specified when the sub-buffer object was created is not aligned to the CL_DEVICE_MEM_BASE_ADDR_ALIGN value for the device associated with command_queue. This error code is missing before version 1.1.

  • CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST

    • if the read or write operations are blocking and the execution status of any of the events in event_wait_list is a negative integer value indicating an error. This error code is missing before version 1.1.

  • CL_MEM_OBJECT_ALLOCATION_FAILURE

    • if there is a failure to allocate memory for the data store associated with buffer

  • CL_INVALID_OPERATION

    • if clEnqueueReadBuffer is called on buffer which has been created with CL_MEM_HOST_WRITE_ONLY or CL_MEM_HOST_NO_ACCESS

    • if clEnqueueWriteBuffer is called on buffer which has been created with CL_MEM_HOST_READ_ONLY or CL_MEM_HOST_NO_ACCESS

    • if clEnqueueWriteBuffer is called on buffer which has been created with CL_MEM_IMMUTABLE_EXT

  • 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

See Also

CL_VERSION_1_0, cl_bool, cl_command_queue, cl_event, cl_mem, cl_uint

Document Notes

For more information, see the OpenCL Specification

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

Copyright

Copyright 2014-2026 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0