site/source/docs/getting_started/Tutorial.rst
.. _Tutorial:
Using Emscripten is, at a base level, fairly simple. This tutorial takes you through the steps needed to compile your first Emscripten examples from the command line. It also shows how to work with files and set the main compiler optimization flags.
Make sure you have :ref:downloaded and installed <sdk-download-and-install>
Emscripten (the exact approach for doing this will depend on your operating system:
Linux, Windows, or Mac).
Emscripten is accessed using the :ref:emccdoc. This script invokes all the
other tools needed to build your code, and can act as a drop-in replacement for
a standard compiler like gcc or clang. It is called on the command line
using ./emcc or ./em++.
.. note:: On Windows the tool is called using the slightly different syntax:
emcc or em++. The remainder of this tutorial uses the Linux approach
(./emcc).
For the next section you will need to open a command prompt:
Emscripten Command Prompt <emcmdprompt>, a command
prompt that has been pre-configured with the correct system paths and settings
to point to the :term:active <Active Tool/SDK> Emscripten tools. To access
this prompt, type Emscripten in the Windows start menu, and then
select the Emscripten Command Prompt option.Navigate with the command prompt to the emscripten directory under the SDK. This
is a folder below the :term:emsdk root directory, typically
<emsdk root directory>/upstream/emscripten/.
The examples below will depend on finding files relative to that location.
.. note:: In older emscripten versions, the directory structure was different: the version number appeared, and the backend (fastcomp/upstream) did not, so you would use something like <emsdk root directory>/emscripten/1.20.0/.
If you haven't run Emscripten before, run it now with: ::
./emcc -v
If the output contains warnings about missing tools, see
:ref:verifying-the-emscripten-environment for debugging help. Otherwise,
continue to the next sections where we'll build some code.
You can now compile your first C/C++ file to JavaScript.
First, let's have a look at the file to be compiled: hello_world.c. This is the simplest test code in the SDK, and as you can see, all it does is print "hello, world!" to the console and then exit.
.. include:: ../../../../test/hello_world.c :literal:
To build the JavaScript version of this code, simply specify the C/C++ file after emcc (use em++ to force compilation as C++): ::
./emcc test/hello_world.c
You should see two files generated by that command: a.out.js and
a.out.wasm. The second is a WebAssembly file containing the compiled code,
and the first is a JavaScript file containing the runtime support to load and
execute it. You can run them using :term:node.js:
::
node a.out.js
This prints "hello, world!" to the console, as expected.
.. tip:: If an error occurs when calling emcc, run it with the -v option
to print out a lot of useful debug information.
.. note:: In this section, and later on, we run some files from the test/
folder. That folder contains files for the Emscripten test suite. Some can be
run standalone, but others must be run through the test harness itself, see
:ref:emscripten-test-suite for more information.
Emscripten can also generate HTML for testing embedded JavaScript. To generate
HTML, use the -o (:ref:output <emcc-o-target>) command and specify an html
file as the target file: ::
./emcc test/hello_world.c -o hello.html
You can now open hello.html in a web browser.
.. note:: Unfortunately, several browsers (including Chrome and Safari) do
not support file:// :term:XHR requests, and can't load extra files
needed by the HTML (like a .wasm file, or packaged file data as mentioned
lower down). For these browsers, you'll need to serve the files using a
:ref:local webserver <faq-local-webserver> and then open
http://localhost:8000/hello.html).
Once you have the HTML loaded in your browser, you'll see a text area for
displaying the output of the printf() calls in the native code.
The HTML output isn't limited just to just displaying text. You can also use the
SDL API to show a colored cube in a <canvas> element (on browsers that
support it). For an example, build the hello_world_sdl.c <https://github.com/emscripten-core/emscripten/blob/main/test/hello_world_sdl.c>_
test code and then refresh the browser: ::
./emcc test/hello_world_sdl.c -o hello.html
The source code for the second example is given below:
.. include:: ../../../../test/hello_world_sdl.c :literal:
.. _tutorial-files:
.. note:: Your C/C++ code can access files using the normal libc stdio API
(fopen, fclose, etc.)
JavaScript is usually run in the sandboxed environment of a web browser, without direct access to the local file system. Emscripten simulates a file system that you can access from your compiled C/C++ code using the normal libc stdio API.
Files that you want to access should be :ref:preloaded <emcc-preload-file> or
:ref:embedded <emcc-embed-file> into the virtual file system. Preloading (or
embedding) generates a virtual file system that corresponds to the file system
structure at compile time, relative to the current directory.
The hello_world_file.cpp <https://github.com/emscripten-core/emscripten/blob/main/test/hello_world_file.cpp>_
example shows how to load a file (both the test code and the file to be loaded
shown below):
.. include:: ../../../../test/hello_world_file.cpp :literal:
.. include:: ../../../../test/hello_world_file.txt :literal:
.. note:: The example expects to be able to load a file located at test/hello_world_file.txt: ::
FILE *file = fopen("test/hello_world_file.txt", "rb");
We compile the example from the directory "above" test to ensure the virtual filesystem is created with the correct structure relative to the compile-time directory.
The following command is used to specify a data file to :ref:preload <emcc-preload-file> into Emscripten's virtual file system — before running any
compiled code. This approach is useful because Browsers can only load data from
the network asynchronously (except in Web Workers) while a lot of native code
uses synchronous file system access. Preloading ensures that the asynchronous
download of data files is complete (and the file is available) before compiled
code has the opportunity to access the Emscripten file system.
::
./emcc test/hello_world_file.cpp -o hello.html --preload-file test/hello_world_file.txt
Run the above command, then open hello.html in a web browser to see the data from hello_world_file.txt being displayed.
For more information about working with the file system see the
:ref:file-system-overview, :ref:Filesystem-API and
:ref:Synchronous-virtual-XHR-backed-file-system-usage.
Emscripten, like gcc and clang, generates unoptimized code by default. You
can generate :ref:slightly-optimized <emcc-O1> code with the -O1 command
line argument: ::
./emcc -O1 test/hello_world.cpp
The "hello world" code created in a.out.js doesn't really need to be optimized, so you won't see a difference in speed when compared to the unoptimized version.
However, you can compare the generated code to see the differences. -O1
applies several minor optimizations and removes some runtime assertions. For
example, printf will have been replaced by puts in the generated code.
The optimizations provided by -O2 (see :ref:here <emcc-O2>) are much more
aggressive. If you run the following command and inspect the generated code
(a.out.js) you will see that it looks very different: ::
./emcc -O2 test/hello_world.cpp
For more information about compiler optimization options see
:ref:Optimizing-Code and the :ref:emcc tool reference <emcc-compiler-optimization-options>.
.. _running-emscripten-tests:
Emscripten has a comprehensive test suite, which covers virtually all Emscripten
functionality. These tests are an excellent resource for developers as they
provide practical examples of most features, and are known to build successfully
on the main branch.
See :ref:emscripten-test-suite for more information.
This tutorial walked you through your first steps in calling Emscripten from the command line. There is, of course, far more you can do with the tool. Below are other general tips for using Emscripten:
compiling and building projects <compiling-and-running-projects-index>, :ref:integrating your native code with the web environment <integrating-porting-index>,
:ref:packaging your code <packaging-code-index> and publishing.--pre-js option works, search for --pre-js in the test suite: the test
suite is extensive and there are likely to be at least some examples.src/settings.js <settings-js> and :ref:emcc <emccdoc> which describe the compiler options,
and :ref:emscripten-h for details on JavaScript-specific C APIs that your
C/C++ programs can use when compiled with Emscripten.FAQ.get in touch <contact>!