# Interacting with code

Emscripten provides numerous methods to connect and interact between JavaScript and compiled C or C++:

- Call compiled C functions from normal JavaScript:

  - Using ccall or cwrap.

  - Using direct function calls (faster but more complicated).

- Call compiled C++ classes from JavaScript using bindings created with:

  - Embind or WebIDL-Binder

- Call JavaScript functions from C/C++:

  - Using emscripten_run_script().

  - Using EM_JS() (faster).

  - Using EM_ASM() (faster).

  - Using a C API implemented in JavaScript.

  - As function pointers from C.

  - Using the Embind val class.

- Access compiled code memory from JavaScript.

- Affect execution behaviour.

- Access environment variables.

This article explains each of the methods listed above, and provides links to more detailed information.

Note

For information on how compiled code interacts with the browser environment, see Emscripten Runtime Environment. For file system related manners, see the File System Overview.

Note

Before you can call your code, the runtime environment may need to load a memory initialization file, preload files, or do other asynchronous operations depending on optimization and build settings. See How can I tell when the page is fully loaded and it is safe to call compiled functions? in the FAQ.

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

    // Copyright 2012 The Emscripten Authors.  All rights reserved.
    // Emscripten is available under two separate licenses, the MIT license and the
    // University of Illinois/NCSA Open Source License.  Both these licenses can be
    // found in the LICENSE file.

    #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 test/hello_function.cpp -o function.html -sEXPORTED_FUNCTIONS=_int_sqrt -sEXPORTED_RUNTIME_METHODS=ccall,cwrap

`EXPORTED_FUNCTIONS` tells the compiler what we want to be accessible from the compiled code (everything else might be removed if it is not used), and `EXPORTED_RUNTIME_METHODS` tells the compiler that we want to use the runtime functions `ccall` and `cwrap` (otherwise, it will not include them).

Note

EXPORTED_FUNCTIONS affects compilation to JavaScript. If you first compile to an object file, then compile the object to JavaScript, you need that option on the second command. If you do it all together as in the example here (source straight to JavaScript) then this just works, of course.

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 (or a JavaScript null value if there isn’t one), and the third is an array of parameter types (which may be omitted if there are no parameters). The types are “number” (for a JavaScript number corresponding to a C integer, float, or general pointer), “string” (for a JavaScript string that corresponds to a C `char*` that represents a string) or “array” (for a JavaScript array or typed array that corresponds to a C array; for typed arrays, it must be a Uint8Array or Int8Array).

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

Note

This example illustrates a few other points, which you should remember when using `ccall()` or `cwrap()`:

- These methods can be used with compiled C functions — name-mangled C++ functions won’t work.

- We highly recommended that you export functions that are to be called from JavaScript:

  - Exporting is done at compile time. For example: `-sEXPORTED_FUNCTIONS=_main,_other_function` exports `main()` and `other_function()`.

  - Note that you need `_` at the beginning of the function names in the `EXPORTED_FUNCTIONS` list.

  - Note that `_main` is mentioned in that list. If you don’t have it there, the compiler will eliminate it as dead code. The list of exported functions is the entire list that will be kept alive (unless other code was kept alive in another manner).

  - Emscripten does dead code elimination to minimize code size — exporting ensures the functions you need aren’t removed.

  - At higher optimisation levels (`-O2` and above), code is minified, including function names. Exporting functions allows you to continue to access them using the original name through the global `Module` object.

- The compiler will remove code it does not see is used, to improve code size. If you use `ccall` in a place it sees, like code in a `--pre-js` or `--post-js`, it will just work. If you use it in a place the compiler didn’t see, like another script tag on the HTML or in the JS console like we did in this tutorial, then because of optimizations and minification you should export ccall from the runtime, using `EXPORTED_RUNTIME_METHODS`, for example using `-sEXPORTED_RUNTIME_METHODS=ccall,cwrap`, and call it on `Module` (which contains everything exported, in a safe way that is not influenced by minification or optimizations).

## Interacting with an API written in C/C++ from NodeJS

Say you have a C library that exposes some procedures:

    //api_example.c
    #include <stdio.h>
    #include <emscripten.h>

    EMSCRIPTEN_KEEPALIVE
    void sayHi() {
      printf("Hi!\n");
    }

    EMSCRIPTEN_KEEPALIVE
    int daysInWeek() {
      return 7;
    }

Compile the library with emcc:

    emcc api_example.c -o api_example.js -sMODULARIZE -sEXPORTED_RUNTIME_METHODS=ccall

Require the library and call its procedures from node:

    var factory = require('./api_example.js');

    factory().then((instance) => {
      instance._sayHi(); // direct calling works
      instance.ccall("sayHi"); // using ccall etc. also work
      console.log(instance._daysInWeek()); // values can be returned, etc.
    });

The `MODULARIZE` option makes `emcc` emit code in a modular format that is easy to import and use with `require()`: `require()` of the module returns a factory function that can instantiate the compiled code, returning a `Promise` to tell us when it is ready, and giving us the instance of the module as a parameter.

(Note that we use `ccall` here, so we need to add it to the exported runtime methods, as before.)

## 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 `_`.

Note

If you use `ccall()` or `cwrap()`, you do not need to prefix function calls with `_` – just use the C name.

The parameters you pass to and receive from functions need to be primitive values:

> - Integer and floating point numbers can be passed as-is.

> - Pointers can be passed as-is also, as they are simply integers in the generated code.

> - JavaScript string `someString` can be converted to a `char *` using `ptr = stringToNewUTF8(someString)`.

>   Note

>   The conversion to a pointer allocates memory, which needs to be freed up via a call to `free(ptr)` afterwards (`_free` in JavaScript side) -

> - `char *` received from C/C++ can be converted to a JavaScript string using `UTF8ToString()`.

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

> - Other values can be passed via `emscripten::val`. Check out examples on as_handle and take_ownership methods.

## 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')");

Note

The function `alert` is present in browsers, but not in node or other JavaScript shells. A more generic alternative is to call console.log.

A faster way to call JavaScript from C is to write “inline JavaScript”, using `EM_JS()` or `EM_ASM()` (and related macros).

EM_JS is used to declare JavaScript functions from inside a C file. The “alert” example might be written using EM_JS like:

    #include <emscripten.h>

    EM_JS(void, call_alert, (), {
      alert('hello world!');
      throw 'all done';
    });

    int main() {
      call_alert();
      return 0;
    }

EM_JS’s implementation is essentially a shorthand for implementing a JavaScript library.

EM_ASM is used in a similar manner to inline assembly code. The “alert” example might be written with 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`, for example:

    EM_ASM({
      console.log('I received: ' + $0);
    }, 100);

This will show `I received: 100`.

You can also receive values back, for example the following will print out `I received: 100` and then `101`:

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

See the `emscripten.h docs` for more details.

Note

- You need to specify if the return value is an `int`, `double` or pointer type using the appropriate macro `EM_ASM_INT`, `EM_ASM_DOUBLE` or `EM_ASM_PTR`. (`EM_ASM_PTR` is the same as `EM_ASM_INT` unless `MEMORY64` is used, so is mostly needed in code that wants to be compatible with `MEMORY64`).

- The input values appear as `$0`, `$1`, etc.

- `return` is used to provide the value sent from JavaScript back to C.

- See how `{` and `}` are used here to enclose the code. This is necessary to differentiate the code from the arguments passed later, which are the input values (this is how C macros work).

- When using the `EM_ASM` macro, ensure that you only use single quotes(‘). Double quotes(”) will cause a syntax error that is not detected by the compiler and is only shown when looking at a JavaScript console while running the offending code.

- clang-format may clobber Javascript constructions, such as `=>` turning to `= >`. To avoid this, add to your `.clang-format`: `WhitespaceSensitiveMacros: ['EM_ASM', 'EM_JS', 'EM_ASM_INT', 'EM_ASM_DOUBLE', 'EM_ASM_PTR', 'MAIN_THREAD_EM_ASM', 'MAIN_THREAD_EM_ASM_INT', 'MAIN_THREAD_EM_ASM_DOUBLE', 'MAIN_THREAD_EM_ASM_DOUBLE', 'MAIN_THREAD_ASYNC_EM_ASM']`. Or, turn clang-format off by writing `// clang-format off` before the `EM_ASM` section and `// clang-format on` after it.

## Implement a C API in JavaScript

It is possible to implement a C API in JavaScript! This is the approach used in many of Emscripten’s libraries, like SDL1 and OpenGL.

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 can then implement the symbols in JavaScript by simply adding their definition to one of the core JS library files. Undefined native symbols will be resolved by looking for them in JavaScript library files.

The core JS library files are where you will find Emscripten internals. For example, parts of Emscripten’s libc are implemented there. You can also put the JavaScript implementation in your own library file and add it using the emcc option `--js-library`. See test_jslib in test/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(void);

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

Note

When using C++ you should encapsulate `extern void my_js();` in an `extern "C" {}` block to prevent C++ name mangling:

    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

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

`addToLibrary` copies the properties of the input object into `LibraryManager.library` (the global object where all JavaScript library code lives). In this case its adds a function called `my_js` onto this object.

### JavaScript limits in library files

If you’re not familiar with JavaScript, say if you’re a C/C++ programmer and just using emscripten, then the following issues probably won’t come up, but if you’re an experienced JavaScript programmer you need to be aware some common JavaScript practices can not be used in certain ways in emscripten library files.

To save space, by default, emscripten only includes library properties referenced from C/C++. It does this by calling `toString` on each used property on the JavaScript libraries that are linked in. That means that you can’t use a closure directly, for example, as `toString` isn’t compatible with that - just like when using a string to create a Web Worker, where you also can’t pass a closure. (Note that this limitation is just for the values for the keys of the object passed to `addToLibrary` in the JS library, that is, the toplevel key-value pairs are special. Interior code inside a function can have arbitrary JS, of course).

To avoid this limitation of JS libraries, you can put code in another file using the `--pre-js` or `--post-js` options, which allow arbitrary normal JS, and it is included and optimized with the rest of the output. That is the recommended approach for most cases. Another option is another `<script>` tag.

Alternatively, if you prefer to use a JS library file, you can have a function replace itself and have it called during initialization.

    addToLibrary({

      // Solution for bind or referencing other functions directly
      good_02__postset: '_good_02();',
      good_02: function() {
        _good_02 = document.querySelector.bind(document);
      },

      // Solution for closures
      good_03__postset: '_good_03();',
      good_03: function() {
        var callCount = 0;
        _good_03 = function() {
          console.log("times called: ", ++callCount);
        };
      },

      // Solution for curry/transform
      good_05__postset: '_good_05();',
      good_05: function() {
        _good_05 = curry(scrollTo, 0);
     },

    });

A __postset is a string the compiler will emit directly to the output file. For the example above this code will be emitted.

     function _good_02() {
       _good_o2 = document.querySelector.bind(document);
     }

     function _good_03() {
       var callCount = 0;
       _good_03 = function() {
         console.log("times called: ", ++callCount);
       };
     }

     function _good_05() {
       _good_05 = curry(scrollTo, 0);
    };

    // Call each function once so it will replace itself
    _good_02();
    _good_03();
    _good_05();

You can also put most of your code in the `xxx__postset` strings. The example below each method declares a dependency on `$method_support` and are otherwise dummy functions. `$method_support` itself has a corresponding `__postset` property with all the code to set the various methods to the functions we actually want.

    addToLibrary({
      $method_support: {},
      $method_support__postset: [
        '(function() {                                  ',
        '  var SomeLib = function() {                   ',
        '    this.callCount = 0;                        ',
        '  };                                           ',
        '                                               ',
        '  SomeLib.prototype.getCallCount = function() {',
        '    return this.callCount;                     ',
        '  };                                           ',
        '                                               ',
        '  SomeLib.prototype.process = function() {     ',
        '    ++this.callCount;                          ',
        '  };                                           ',
        '                                               ',
        '  SomeLib.prototype.reset = function() {       ',
        '    this.callCount = 0;                        ',
        '  };                                           ',
        '                                               ',
        '  var inst = new SomeLib();                    ',
        '  _method_01 = inst.getCallCount.bind(inst);   ',
        '  _method_02 = inst.process.bind(inst);        ',
        '  _method_03 = inst.reset.bind(inst);          ',
        '}());                                          ',
      ].join('\n'),
      method_01: function() {},
      method_01__deps: ['$method_support'],
      method_02: function() {},
      method_01__deps: ['$method_support'],
      method_03: function() {},
      method_01__deps: ['$method_support'],
     });

Note: If you are using node 4.1 or newer you can use multi-line strings. They are only used at compile time not runtime so output will still run in ES5 based environments.

Another option is to put most of your code in an object, not a function,

    addToLibrary({
      $method_support__postset: 'method_support();',
      $method_support: function() {
        var SomeLib = function() {
          this.callCount = 0;
        };

        SomeLib.prototype.getCallCount = function() {
          return this.callCount;
        };

        SomeLib.prototype.process = function() {
          ++this.callCount;
        };

        SomeLib.prototype.reset = function() {
          this.callCount = 0;
        };

        var inst = new SomeLib();
        _method_01 = inst.getCallCount.bind(inst);
        _method_02 = inst.process.bind(inst);
        _method_03 = inst.reset.bind(inst);
      },
      method_01: function() {},
      method_01__deps: ['$method_support'],
      method_02: function() {},
      method_01__deps: ['$method_support'],
      method_03: function() {},
      method_01__deps: ['$method_support'],
     });

See the library_*.js files for other examples.

Note

- JavaScript libraries can declare dependencies (`__deps`), however those are only for other JavaScript libraries. See examples in /src with the name format library_*.js

- You can add dependencies for all your methods using `autoAddDeps(myLibrary, name)` where myLibrary is the object with all your methods, and `name` is the thing they all depend upon. This is useful when all the implemented methods use a JavaScript singleton containing helper methods. See `library_webgl.js` for an example.

- The keys passed into addToLibrary generate functions that are prefixed by `_`. In other words `my_func: function() {},` becomes `function _my_func() {}`, as all C methods in emscripten have a `_` prefix. Keys starting with `$` have the `$` stripped and no underscore added.

## Calling JavaScript functions as function pointers from C

You can use `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 `addFunction` will be called.

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

You should build with `-sALLOW_TABLE_GROWTH` to allow new functions to be added to the table. Otherwise by default the table has a fixed size.

When using `addFunction` with a JavaScript function, you need to provide an additional second argument, a Wasm function signature string, explained below. See test/interop/test_add_function_post.js for an example.

## Function Signatures

The LLVM Wasm backend requires a Wasm function signature string when using `addFunction` and in JavaScript libraries. Each character within a signature string represents a type. The first character represents the return type of a function, and remaining characters are for parameter types.

> - `'v'`: void type

> - `'i'`: 32-bit integer type

> - `'j'`: 64-bit integer type (see note below)

> - `'f'`: 32-bit float type

> - `'d'`: 64-bit float type

> - `'p'`: 32-bit or 64-bit pointer (MEMORY64)

For example, if you add a function that takes an integer and does not return anything, the signature is `'vi'`.

When `'j'` is used there are several ways in which the parameter value will be passed to JavaScript. By default, the value will either be passed as a single BigInt or a pair of JavaScript numbers (double) depending on whether the `WASM_BIGINT` settings is enabled. In addition, if you only require 53 bits of precision you can add the `__i53abi` decorator, which will ignore the upper bits and the value will be received as a single JavaScript number (double). It cannot be used with `addFunction`. Here is an example of a library function that sets the size of a file using a 64-bit value passed as a 53 bit (double) and returns an integer error code:

    extern "C" int _set_file_size(int handle, uint64_t size);

    _set_file_size__i53abi: true,  // Handle 64-bit
    _set_file_size__sig: 'iij',    // Function signature
    _set_file_size: function(handle, size) { ... return error; }

Using `-sWASM_BIGINT` when linking is an alternative method of handling 64-bit types in libraries. ``Number()`` may be needed on the JavaScript side to convert it to a usable value. See settings reference.

## 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 test/core/test_utf.in and test/test_core.py.

Note

This is a lower-level operation than `ccall()` and `cwrap()` — we do need to care what specific type (e.g. integer) is being used.

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)`.

The converse case of exporting allocated memory into JavaScript can be tricky when Wasm-based memory is allowed to grow, by compiling with `-sALLOW_MEMORY_GROWTH`. Increasing the size of memory changes to a new buffer and existing array views essentially become invalid, so you cannot simply do this:

    function func() {
      let someView = HEAPU8.subarray(x, y);
      compute(someView);

      // This may grow memory, which would invalidate all views.
      maybeGrow();

      // If we grew, this use of an invalidated view will fail. Failure in this
      // case will return undefined, the same as reading out of bounds from a
      // typed array. If the operation were someView.subarray(), however, then it
      // would throw an error.
      return someView[z];
    }

Emscripten will refresh the canonical views like `HEAPU8` for you, which you can use to refresh your own views:

    function func() {
      let someView = HEAPU8.subarray(x, y);
      compute(someView);

      // This may grow memory, which would invalidate all views.
      maybeGrow();

      // Create a new, fresh view after the possible growth.
      someView = HEAPU8.subarray(x, y);
      return someView[z];
    }

Another option to avoid such problems is to copy the data, when that makes sense.

## 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 = () => {ENV.MY_FILE_ROOT = "/usr/lib/test"};

Note that Emscripten will set default values for some environment variables (e.g. LANG) after you have configured `ENV`, if you have not set your own values. If you want such variables to remain unset, you can explicitly set their value to undefined. For example:

    Module.preRun = () => {ENV.LANG = undefined};

## 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.

Note

There is no strong evidence that one tool is “better” than the other in terms of performance (no comparative benchmarks exist), and both have been used successfully in a number of projects. The selection of one tool over the other will usually be based on which is the most natural fit for the project and its build system.

## Binding C/C++ and JavaScript - Node-API

Emnapi is an unofficial Node-API implementation which can be used on Emscripten. If you would like to port existing Node-API addon to WebAssembly or compile the same binding code to both Node.js native addon and WebAssembly, you can give it a try. See Emnapi documentation for more details.