C Specification

To register a user callback function for a specific command execution status, call the function

cl_int clSetEventCallback(
    cl_event event,
    cl_int command_exec_callback_type,
    void (CL_CALLBACK* pfn_notify)(cl_event event, cl_int event_command_status, void *user_data),
    void* user_data);


  • event is a valid event object.

  • command_exec_callback_type specifies the command execution status for which the callback is registered. The command execution callback values for which a callback can be registered are: CL_​SUBMITTED, CL_​RUNNING, or CL_​COMPLETE21. There is no guarantee that the callback functions registered for various execution status values for an event will be called in the exact order that the execution status of a command changes. Furthermore, it should be noted that receiving a call back for an event with a status other than CL_​COMPLETE, in no way implies that the memory model or execution model as defined by the OpenCL specification has changed. For example, it is not valid to assume that a corresponding memory transfer has completed unless the event is in a state CL_​COMPLETE.

  • pfn_event_notify is the event callback function that can be registered by the application. This callback function may be called asynchronously by the OpenCL implementation. It is the applications responsibility to ensure that the callback function is thread-safe. The parameters to this callback function are:

    • event is the event object for which the callback function is invoked.

    • event_command_exec_status is equal to the command_exec_callback_type used while registering the callback. Refer to the Kernel Argument Queries table for the command execution status values. If the callback is called as the result of the command associated with event being abnormally terminated, an appropriate error code for the error that caused the termination will be passed to event_command_exec_status instead.

    • user_data is a pointer to user supplied data.

  • user_data will be passed as the user_data argument when pfn_notify is called. user_data can be NULL.



The callback function registered for a command_exec_callback_type value of CL_​COMPLETE will be called when the command has completed successfully or is abnormally terminated.

The registered callback function will be called when the execution status of command associated with event changes to an execution status equal to or past the status specified by command_exec_status.

Each call to clSetEventCallback registers the specified user callback function on a callback stack associated with event. The order in which the registered user callback functions are called is undefined.

All callbacks registered for an event object must be called. All enqueued callbacks shall be called before the event object is destroyed. Callbacks must return promptly. The behavior of calling expensive system routines, OpenCL API calls to create contexts or command-queues, or blocking OpenCL operations from the following list below, in a callback is undefined.

  • clFinish,

  • clWaitForEvents,

  • blocking calls to clEnqueueReadBuffer, clEnqueueReadBufferRect,

  • clEnqueueWriteBuffer, clEnqueueWriteBufferRect,

  • blocking calls to clEnqueueReadImage and clEnqueueWriteImage,

  • blocking calls to clEnqueueMapBuffer and clEnqueueMapImage,

  • blocking calls to clBuildProgram, clCompileProgram or clLinkProgram,

  • blocking calls to clEnqueueSVMMemcpy or clEnqueueSVMMap

If an application needs to wait for completion of a routine from the above list in a callback, please use the non-blocking form of the function, and assign a completion callback to it to do the remainder of your work. Note that when a callback (or other code) enqueues commands to a command-queue, the commands are not required to begin execution until the queue is flushed. In standard usage, blocking enqueue calls serve this role by implicitly flushing the queue. Since blocking calls are not permitted in callbacks, those callbacks that enqueue commands on a command queue should either call clFlush on the queue before returning or arrange for clFlush to be called later on another thread.

clSetEventCallback returns CL_​SUCCESS if the function is executed successfully. Otherwise, it returns one of the following errors:

  • CL_​INVALID_​EVENT if event is not a valid event object.

  • CL_​INVALID_​VALUE if pfn_event_notify is NULL or if command_exec_callback_type is not CL_​SUBMITTED, CL_​RUNNING, or CL_​COMPLETE.

  • 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

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 (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.