Stream Free Running Kernel (HLS C/C++)

This is simple example which demonstrate how to use and configure a free running kernel.

KEY CONCEPTS: Free Running Kernel

KEYWORDS: ap_ctrl_none, stream_connect

Free-running kernels have no control signal ports, and cannot be started or stopped. The no-control signal feature of the free-running kernel results in the following characteristics:

  • The free-running kernel has no memory input or output port, and therefore it interacts with the host or other kernels (other kernels can be regular kernel or another free running kernel) only through streams.

  • When the FPGA is programmed by the binary container (xclbin), the free-running kernel starts running on the FPGA, and therefore it does not need the clEnqueueTask command from the host code.

  • The kernel works on the stream data as soon as it starts receiving from the host or other kernels, and it stalls when the data is not available.

  • The free-running kernel needs a special interface pragma ap_ctrl_none inside the kernel body.

The free-running kernel only contains hls::stream inputs and outputs. The recommended coding guidelines include:

  • The kernel interface should not have any #pragma HLS interface s_axilite or #pragma HLS interface m_axi (as there should not be any memory or control port).

  • The kernel interface must contain this special pragma:

#pragma HLS interface ap_ctrl_none port=return

For kernel to kernel streaming transfer, kernel to kernel data transfer ports do not need clSetKernelArg from host code.

Producer kernel mem_read streaming output port needs to be connected to consumer kernel increment stream input port during v++ using krnl_incr.cfg file as below:


This stream connection specifies that stream streaming port of mem_read will be connected to input streaming port of increment during system linker run and same for the second stream connection. This connectivity file is provided to v++ command as below:

--config krnl_incr.cfg


Platforms containing following strings in their names are not supported for this example :



Application code is located in the src directory. Accelerator binary files will be compiled to the xclbin directory. The xclbin directory is required by the Makefile and its contents will be filled during compilation. A listing of all the files in this example is shown below


Access these files in the github repo by clicking here.


Once the environment has been configured, the application can be executed by

./streaming_free_running_k2k <krnl_incr XCLBIN>