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Created April 11, 2019 17:42
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Using the Cling C++ Interpreter on the Bela Platform

Using the Cling C++ Interpreter on the Bela Platform

About Cling

Cling is a C++ interpreter developed at CERN as part of the ROOT project:

## Installation instructions

nano ~/.bashrc
export PATH="/root/cling/bin:$PATH" # add this line!
source ~/.bashrc
  • Try cling, you should see:
****************** CLING ******************
* Type C++ code and press enter to run it *
*             Type .q to exit             *

Using the template

See the comments at the bottom of cling_default.cpp for an explainer of how the template currently works.

In essence:

  • Create a new C++ project
  • Replace render.cpp with cling_default.cpp
  • Follow the instructions in cling_default.cpp

TODO (all experimental)

  • IDE integration and integrated feedback
  • Exposing BelaContext to the REPL
  • Strategies for 'live coding' Bela programs
* Bela Cling demo template
* //
* Jack Armitage 2018
* See comments at the bottom for usage
#include <Bela.h>
#include <cmath>
extern void (*gBelaRender) (BelaContext *, void *);
BelaContext* bela = {}; // If you write to this, weird things **will** happen!
void interrupt_handler (int var) {
gShouldStop = true;
bool setup (BelaContext *context, void *) {
bela = context;
return true;
void cling_render (BelaContext *context, void *) {
void render (BelaContext *context, void *) {
for(unsigned int n = 0; n < context->audioFrames; n++) {
for(unsigned int ch = 0; ch < context->audioInChannels; ch++){
audioWrite(context, n, ch, audioRead(context, n, ch));
void cleanup (BelaContext *context, void *) {
void cling_default() {
BelaInitSettings* settings = Bela_InitSettings_alloc(); // Default settings
settings->setup = setup;
settings->render = render;
settings->cleanup = cleanup;
// Issue: if high performance mode is set and render() is empty,
// you will hear a high-pitched noise:
settings->highPerformanceMode = 1; //
if(Bela_initAudio(settings, 0) != 0) { // Initialise the PRU audio device
fprintf(stderr,"Error: unable to initialise audio\n");
return 1;
if(Bela_startAudio()) { // Start the audio device running
fprintf(stderr,"Error: unable to start real-time audio\n");
Bela_stopAudio(); // Stop the audio device
Bela_cleanupAudio(); // Clean up any resources allocated for audio
return 1;
This code assumes you have already installed Cling on Bela following the instructions in ``.
If not, start there.
Start Cling, then:
.I /root/Bela/include
.L /root/Bela/lib/
.L /root/Bela/lib/
.x /root/Bela/projects/[project_folder]/[project_main].cpp
If you need to load other classes before your main file,
you can load them before the `.x` command,
but make sure you DO NOT include them in [project_main].cpp
as this will no longer be necessary:
.L /root/Bela/projects/[project_folder]/[project_class].cpp
If you rename the function `cling_default()` to the name of your file, Cling will load it automatically.
Then, based on modifications in PRU.cpp, you can swap to a new render function,
or any other function of type `void func (BelaContext *context, void *) {}`:
gBelaRender = cling_render
From Cling you can access anything included / global / public.
You can even read from the main Bela context (writing to it is not advised!) and use the API utilities:
[cling]$ bela->audioSampleRate
(const float) 44100.0f
[cling]$ analogRead(bela, 0, 0)
(float) 0.000259399f
To monitor the performance of Cling, you can run:
watch -n 0.4 cat /proc/xenomai/sched/stat
Note that if you make an error in Cling such as loading an incorrect path,
you will likely need to restart Cling.
You can not currently redefine identifiers in Cling, in that case you first
need to `.undo N` where N is number of steps before you can redefine something.
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