Skip to content

Latest commit

 

History

History
105 lines (53 loc) · 5.26 KB

module.rst

File metadata and controls

105 lines (53 loc) · 5.26 KB

Module object

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

Developers can provide an implementation of Module to control the execution of code. For example, to define how notification messages from Emscripten are displayed, developers implement the :js:attr:`Module.print` attribute.

Note

Module is also used to provide access to all Emscripten API functions (for example :js:func:`ccall`) in a way that avoids issues with function name minification at higher optimisation levels. These functions are documented as part of their own APIs.

Table of Contents

Creating the Module object

Use emcc's :ref:`pre-js option<emcc-pre-js>` to add JavaScript code that defines (or extends) the Module object with the behaviour you need.

When generating only JavaScript (as opposed to HTML), no Module object is created by default, and the behaviour is entirely defined by the developer. For example, creating a Module object with the following code will cause all notifications from the program to be calls to alert().

var Module = {
  'print': function(text) { alert('stdout: ' + text) }
  'printErr': function(text) { alert('stderr: ' + text) }
};

Important

If you run the :term:`Closure Compiler` on your code (which is optional, and can be done by --closure 1), you will need quotation marks around the properties of Module as in the example above. In addition, you need to run closure on the compiled code together with the declaration of Module — this is done automatically for a -pre-js file.

When generating HTML, Emscripten creates a Module object with default methods (see src/shell.html). In this case you should again use --pre-js, but this time you add properties to the existing Module object, for example

Module['print'] = function(text) { alert('stdout: ' + text) };

Affecting execution

The following Module attributes affect code execution.

.. js:attribute:: Module.print

        Called when something is printed to standard output (stdout)


.. js:attribute:: Module.printErr

        Called when something is printed to standard error (stderr)



.. js:attribute:: Module.arguments

        The commandline arguments. The value of ``arguments`` contains the values returned if compiled code checks ``argc`` and ``argv``.



.. js:attribute:: Module.preInit

        A function (or array of functions) that must be called before global initializers run, but after basic initialization of the JavaScript runtime. This is typically used for :ref:`File System operations <Filesystem-API>`.



.. js:attribute:: Module.preRun

        An array of functions to call right before calling ``run()``, but after defining and setting up the environment, including global initializers. This is useful, for example, to set up directories and files using the :ref:`Filesystem-API` — as this needs to happen after the FileSystem API has been loaded, but before the program starts to run.

        .. note:: If code needs to affect global initializers, it should instead be run using :js:attr:`preInit`.



.. js:attribute:: Module.noInitialRun

        If ``noInitialRun`` is set to ``true``, ``main()`` will not be automatically called (you can do so yourself later). The program will still call global initializers, set up memory initialization, and so forth.



.. js:attribute:: Module.noExitRuntime

        If ``noExitRuntime`` is set to ``true``, the runtime is not shut down after ``run`` completes. Shutting down the runtime calls shutdown callbacks, for example ``atexit`` calls. If you want to continue using the code after ``run()`` finishes, it is necessary to set this. This is automatically set for you if you use an API command that implies that you want the runtime to not be shut down, for example ``emscripten_set_main_loop``.


.. js:attribute:: Module.filePackagePrefixURL

        This is the "prefix" URL for a preloaded data file that is hosted separately from its JavaScript and HTML files (it includes the full path up to, but not including, the data file). See :ref:`packaging-files-data-file-location` for more information.


.. js:attribute:: Module.locateFile

        If set, this method will be called when the runtime needs to load either a file generated by the file packager (this is a generalization of ``Module.filePackagePrefixURL``), or the ``.mem`` memory init file. In both cases the function receives the URL, and should return the actual URL. This lets you host file packages or the ``.mem`` file on a different location than the current directory (which is the default expectation), for example if you want to host them on a CDN.


.. js:attribute:: Module.logReadFiles

        If set, :js:attr:`Module.printErr` will log when any file is read.

Other methods

.. js:function:: Module.destroy(obj)

        This method should be called to destroy C++ objects created in JavaScript using :ref:`WebIDL bindings <WebIDL-Binder>`. If this method is not called, an object may be garbage collected, but its destructor will not be called.

        :param obj: The JavaScript-wrapped C++ object to be destroyed.