Interacting with code¶

Calling compiled C functions from JavaScript using ccall/cwrap¶

The easiest way to call compiled C functions from JavaScript is to use ccall() or cwrap().

ccall() calls a compiled C function with specified parameters and returns the result, while cwrap() “wraps” a compiled C function and returns a JavaScript function you can call normally. cwrap() is therefore more useful if you plan to call a compiled function a number of times.

Consider the tests/hello_function.cpp file shown below. The int_sqrt() function to be compiled is wrapped in extern "C" to prevent C++ name mangling.

#include <math.h>

extern "C" {

int int_sqrt(int x) {
  return sqrt(x);
}

}

To compile this code run the following command in the Emscripten home directory:

./emcc tests/hello_function.cpp -o function.html -s EXPORTED_FUNCTIONS="['_int_sqrt']"

After compiling, you can call this function with cwrap() using the following JavaScript:

int_sqrt = Module.cwrap('int_sqrt', 'number', ['number'])
int_sqrt(12)
int_sqrt(28)

The first parameter is the name of the function to be wrapped, the second is the return type of the function, and the third is an array of parameter types (which may be omitted if there are no parameters). The types are native JavaScript types, “number” (for a C integer, float, or general pointer) or “string” (for a C char* that represents a string).

You can run this yourself by first opening the generated page function.html in a web browser (nothing will happen on page load because there is no main()). Open a JavaScript environment (Control-Shift-K on Firefox, Control-Shift-J on Chrome), and enter the above commands as three separate commands, pressing Enter after each one. You should get the results 3 and 5 — the expected output for these inputs using C++ integer mathematics.

ccall() is similar, but receives another parameter with the parameters to pass to the function:

// Call C from JavaScript
var result = Module.ccall('int_sqrt', // name of C function
  'number', // return type
  ['number'], // argument types
  [28]); // arguments

// result is 5

Call compiled C/C++ code “directly” from JavaScript¶

Functions in the original source become JavaScript functions, so you can call them directly if you do type translations yourself — this will be faster than using ccall() or cwrap(), but a little more complicated.

To call the method directly, you will need to use the full name as it appears in the generated code. This will be the same as the original C function, but with a leading _.

The types of the parameters you pass to functions need to make sense. Integers and floating point values can be passed as is. Pointers are simply integers in the generated code.

Strings in JavaScript must be converted to pointers for compiled code — the relevant function is Pointer_stringify(), which given a pointer returns a JavaScript string. Converting a JavaScript string someString to a pointer can be accomplished using allocate(intArrayFromString(someString), 'i8', ALLOC_STACK).

There are other convenience functions for converting strings and encodings in preamble.js.

Calling JavaScript from C/C++¶

Emscripten provides two main approaches for calling JavaScript from C/C++: running the script using emscripten_run_script() or writing “inline JavaScript”.

The most direct, but slightly slower, way is to use emscripten_run_script(). This effectively runs the specified JavaScript code from C/C++ using eval(). For example, to call the browser’s alert() function with the text ‘hi’, you would call the following JavaScript:

emscripten_run_script("alert('hi')");

A faster way to call JavaScript from C is to write “inline JavaScript”, using EM_ASM() (and related macros). These are used in a similar manner to inline assembly code. The “alert” example above might be written using inline JavaScript as:

#include <emscripten.h>

int main() {
  EM_ASM(
    alert('hello world!');
    throw 'all done';
  );
  return 0;
}

When compiled and run, Emscripten will execute the two lines of JavaScript as if they appeared directly in the generated code. The result would be an alert, followed by an exception. (Note, however, that under the hood Emscripten still does a function call even in this case, which has some amount of overhead.)

You can also send values from C into JavaScript inside EM_ASM_, as well as receive values back (see the linked macro for details. The following example will print out I received: 100 and then 101.

int x = EM_ASM_INT({
  Module.print('I received: ' + $0);
  return $0 + 1;
}, 100);
printf("%d\n", x);

Implement a C API in JavaScript¶

It is possible to implement a C API in JavaScript! This is the approach that was used to write Emscripten’s implementations of SDL and libc.

You can use it to write your own APIs to call from C/C++. To do this you define the interface, decorating with extern to mark the methods in the API as external symbols. You then implement the symbols in JavaScript by simply adding their definition to library.js (by default). When compiling the C code, the compiler looks in the JavaScript libraries for relevant external symbols.

By default, the implementation is added to library.js (and this is where you’ll find the Emscripten implementation of libc). You can put the JavaScript implementation in your own library file and add it using the emcc option --js-library. See test_js_libraries in tests/test_other.py for a complete working example, including the syntax you should use inside the JavaScript library file.

As a simple example, consider the case where you have some C code like this:

extern void my_js();

int main() {
  my_js();
  return 1;
}

extern "C" {
  extern void my_js();
}

Then you can implement my_js in JavaScript by simply adding the implementation to library.js (or your own file). Like our other examples of calling JavaScript from C, the example below just creates a dialog box using a simple alert() function.

my_js: function() {
  alert('hi');
},

If you add it to your own file, you should write something like

mergeInto(LibraryManager.library, {
  my_js: function() {
    alert('hi');
  },
});

mergeInto just copies the properties on the second parameter onto the first, so this add my_js onto LibraryManager.library, the global object where all JavaScript library code should be.

See the library_*.js files for other examples.

Calling JavaScript functions as function pointers from C¶

You can use Runtime.addFunction to return an integer value that represents a function pointer. Passing that integer to C code then lets it call that value as a function pointer, and the JavaScript function you sent to Runtime.addFunction will be called.

See test_add_function in tests/test_core.py for an example.

When using Runtime.addFunction, there is a backing array where these functions are stored. This array must be explicitly sized, which can be done via a compile-time setting, RESERVED_FUNCTION_POINTERS. For example, to reserve space for 20 functions to be added:

emcc ... -s RESERVED_FUNCTION_POINTERS=20 ...

Access memory from JavaScript¶

You can access memory using getValue(ptr, type) and setValue(ptr, value, type). The first argument is a pointer (a number representing a memory address). type must be an LLVM IR type, one of i8, i16, i32, i64, float, double or a pointer type like i8* (or just *).

There are examples of these functions being used in the tests — see tests/core/test_utf.in and tests/test_core.py.

You can also access memory ‘directly’ by manipulating the arrays that represent memory. This is not recommended unless you are sure you know what you are doing, and need the additional speed over getValue() and setValue().

A case where this might be required is if you want to import a large amount of data from JavaScript to be processed by compiled code. For example, the following code allocates a buffer, copies in some data, calls a C function to process the data, and finally frees the buffer.

var buf = Module._malloc(myTypedArray.length*myTypedArray.BYTES_PER_ELEMENT);
Module.HEAPU8.set(myTypedArray, buf);
Module.ccall('my_function', 'number', ['number'], [buf]);
Module._free(buf);

Here my_function is a C function that receives a single integer parameter (or a pointer, they are both just 32-bit integers for us) and returns an integer. This could be something like int my_function(char *buf).

Affect execution behaviour¶

Module is a global JavaScript object, with attributes that Emscripten-generated code calls at various points in its execution.

Developers provide an implementation of Module to control how notifications from Emscripten are displayed, which files that are loaded before the main loop is run, and a number of other behaviours. For more information see Module object.

Environment variables¶

Sometimes compiled code needs to access environment variables (for instance, in C, by calling the getenv() function). Emscripten-generated JavaScript cannot access the computer’s environment variables directly, so a “virtualised” environment is provided.

The JavaScript object ENV contains the virtualised environment variables, and by modifying it you can pass variables to your compiled code. Care must be taken to ensure that the ENV variable has been initialised by Emscripten before it is modified — using Module.preRun is a convenient way to do this.

For example, to set an environment variable MY_FILE_ROOT to be "/usr/lib/test/" you could add the following JavaScript to your Module setup code:

Module.preRun.push(function() {ENV.MY_FILE_ROOT = "/usr/lib/test"})

Binding C++ and JavaScript — WebIDL Binder and Embind¶

The JavaScript methods for calling compiled C functions are efficient, but cannot be used with name-mangled C++ functions.

WebIDL Binder and Embind create bindings between C++ and JavaScript, allowing C++ code entities to be used in a natural manner from JavaScript. Embind additionally supports calling JavaScript code from C++.

Embind can bind almost any C++ code, including sophisticated C++ constructs (e.g. shared_ptr and unique_ptr). The WebIDL Binder supports C++ types that can be expressed in WebIDL. While this subset is smaller than supported by Embind, it is more than sufficient for most use cases — examples of projects that have been ported using the binder include the Box2D and Bullet physics engines.

Both tools allow mapped items to be used from JavaScript in a similar way. However they operate at different levels, and use very different approaches for defining the binding:

• Embind declares bindings within the C/C++ file.
• WebIDL-Binder declares the binding in a separate file. This is run through the binder tool to create “glue” code that is then compiled with the project.