Inline C code

Normal code for Espruino is written in JavaScript. However for some tasks this won't be fast enough, or you may have existing C code that you wish to use.

In this case, you have a few options:

This page covers the first option - follow the links above for other options.

Note: This is an online service that is only provided for official Espruino boards. It won't work on devices like ESP8266 or ESP32.

How do I use it?

You create code of the form var c = E.compiledC in the root scope of your code, giving your C code inside a 'template literal' (using the backtick character at the beginning and end).

var c = E.compiledC(`
// int fibo(int)
int fibo(int n){
 if(n <= 1){
  return n;
 int fibo = 1;
 int fiboPrev = 1;
 for(int i = 2; i < n; ++i){
  int temp = fibo;
  fibo += fiboPrev;
  fiboPrev = temp;
 return fibo;


The first lines should contain comments that describe the arguments and return value for each function that you wish to export to Espruino, in the same form as used for the Assembler. The variable c will then contain an object containing a field for each exported function.


One of the main uses for Inline C is doing something when an interrupt occurs. You can do this by specifying irq:true when using setWatch:

var c = E.compiledC(`
// void press(bool)
// int get()
volatile int data;
void press(bool state){
int get() {
 int r = data;
 data = 0;
 return r;
// Make `` get called on an IRQ
setWatch(, BTN1, {repeat:true, edge:"both", irq:true});
// Every 10 seconds, report back the value
setInterval(function() {
}, 10000);

Note that you can have multiple functions and variables. While Espruino can't access the variables directly, you can write 'getter' functions for them.

Accessing arrays

You'll probably want to use C code to work through large amounts of data quickly. To avoid fragmentation problems, it's not guaranteed that Espruino will always allocate data in one contiguous chunk, so you have two options:


Use Array/TypedArray.forEach to call your function for each element of the array:

var c = E.compiledC(`
// void hash(int)
// int get()
unsigned int data;
void hash(unsigned int value){
  data = ((data<<1) | (data>>31)) ^ value;
int get() {
 int r = data;
 data = 0;
 return r;

var arr = new Uint32Array(1000);
// Fill with random data
for (var i in arr) arr[i]=Math.random()*0xFFFFFFFF;
// Feed each data item through in turn
// Print the result

Note that you can also use .map, .reduce, .filter, etc.

Direct Access

Large typed arrays (eg. Uint8Array) will always be allocated as Flat Strings if possible, but you can use E.toString to force Espruino to create a Flat String (a contiguous array of bytes) from whatever data is supplied as an argument.

Calling E.getAddressOf(variable, true) will return the physical address of the start of that variable's data, or 0 if the memory area is not flat. All you need to do is pass the address from E.getAddressOf into your C code as an integer and you can access the data as a normal array.

// Sum all items in the given array
var c = E.compiledC(`
// int sum(int, int)
int sum(int len, unsigned char *data){
  int s = 0;
  while (len--)
    s += *(data++);
  return s;
// Allocate the data
var data = new Uint8Array(5000);
for (var i in data) data[i]=i;
// Get the address
var addr = E.getAddressOf(data,true);
if (!addr) throw new Error("Not a Flat String");
// Execute the function to sum
print(c.sum(data.length, addr));

Adding code to a module

Adding inline C to a module is easy - just write the Inline C as you would normally do, then export it:

var c = E.compiledC(`
// double lookup(int)

double lut[8] = {
double lookup(unsigned int x)
  return lut[x&7];

exports = c;

(You can of course just export specific functions if you want to, for instance exports.lookup = c.lookup;)

The module itself will contain the C source code, but the IDE will detect the code on upload and will automatically compile it for the board you're uploading to.

What Happens?

Before uploading, the Web IDE scans your code for calls of the form E.compiledC. It then sends those functions to our server which compiles that code with GCC and sends a location-independent binary back so that it can be uploaded to Espruino as a native function.

When you run this native function, the ARM processor in Espruino executes the compiled code directly (with no interpreter in the way). You should see an increase in execution speed of at least 100x for most tasks.


  • Arithmetic with floats, doubles and uint64 may not work since it requires functions that won't be compiled in.
  • There is no C preprocessor - so #define/etc won't work
  • Ctrl+C and Exception handling won't work. If there's a loop in your compiled function then you'll only be able to break out of it by resetting the board.
  • The code that is sent to the Espruino board is specific to that type of board and version of the Espruino firmware. To use it on a different board you'll need to send it again using the Web IDE.

This page is auto-generated from GitHub. If you see any mistakes or have suggestions, please let us know.