.. _Building-Projects:

=================
Building Projects
=================

Building large projects with Emscripten is very easy. Emscripten provides two simple scripts that configure your makefiles to use :ref:`emcc <emccdoc>` as a drop-in replacement for *gcc* — in most cases the rest of your project’s current build system remains unchanged.


.. _building-projects-build-system:

Integrating with a build system
==================================

To build using Emscripten you need to replace *gcc* with *emcc* in your makefiles. This is done using *emconfigure*, which sets the appropriate environment variables like CXX (C++ compiler) and CC (the compiler).

Consider the case where you normally build with the following commands:

.. code-block:: bash

	./configure
	make

To build with Emscripten, you would instead use the following commands:

.. code-block:: bash
		
	# Run emconfigure with the normal configure command as an argument.
	./emconfigure ./configure
		
	# Run emmake with the normal make to generate linked LLVM bitcode.
	./emmake make
		
	# Compile the linked bitcode generated by make (project.bc) to JavaScript.
	#  'project.bc' should be replaced with the make output for your project (e.g. 'yourproject.so')
	#  [-Ox] represents build optimisations (discussed in the next section).
	./emcc [-Ox] project.bc -o project.js


		
*emconfigure* is called with the normal *configure* as an argument (in *configure*-based build systems), and *emmake* with *make* as an argument. If your build system doesn't use configure, then you can omit the first step.

.. tip:: We recommend you call both *emconfigure* and *emmake* scripts in *configure*-based build systems. Whether you actually **need** to call both tools depends on the build system (some systems will store the environment variables in the configure step, and others will not).

*Make* generates linked LLVM bitcode. It does not automatically generate JavaScript during linking because all the files must be compiled using the :ref:`same optimizations and compiler options <building-projects-optimizations>` — and it makes sense to do this in the final conversion from bitcode to JavaScript. 

.. note:: 

	The file output from *make* might have a different suffix: **.a** for a static library archive, **.so** for a shared library, **.o** or **.bc** for object files (these file extensions are the same as *gcc* would use for the different types). Irrespective of the file extension, these files contain linked LLVM bitcode that *emcc* can compile into JavaScript in the final step.

The last step is to compile the linked bitcode into JavaScript. We do this by calling *emcc* again, specifying the linked LLVM bitcode file as an input, and a JavaScript file as the output.


.. _building-projects-optimizations:

Building projects with optimizations
====================================

Emscripten performs :ref:`compiler optimization <Optimizing-Code>` at two levels: each source file is optimized by LLVM as it is compiled into an object file, and then JavaScript-specific optimizations are applied when converting object files into JavaScript.

In order to properly optimize code, it is important to use the **same** :ref:`optimization flags <emcc-compiler-optimization-options>` and other :ref:`compiler options <emcc-s-option-value>` when compiling source to object code, and object code to JavaScript (or HTML).

Consider the examples below:

.. code-block:: bash

	# Sub-optimal - JavaScript optimizations are omitted
	./emcc -O2 a.cpp -o a.bc
	./emcc -O2 b.cpp -o b.bc
	./emcc a.bc b.bc -o project.js

	# Sub-optimal - LLVM optimizations omitted
	./emcc a.cpp -o a.bc
	./emcc b.cpp -o b.bc
	./emcc -O2 a.bc b.bc -o project.js

	# Broken! Different JavaScript and LLVM optimisations used.
	./emcc -O1 a.cpp -o a.bc
	./emcc -O2 b.cpp -o b.bc
	./emcc -O3 a.bc b.bc -o project.js
	
	# Correct. The SAME LLVM and JavaScript options are provided at both levels.
	./emcc -O2 a.cpp -o a.bc
	./emcc -O2 b.cpp -o b.bc
	./emcc -O2 a.bc b.bc -o project.js


The same rule applies when :ref:`building Emscripten using a build system <building-projects-build-system>` — both LLVM and JavaScript must be optimized using the same settings. 

.. note:: Unfortunately each build-system defines its own mechanisms for setting compiler and optimization methods. **You will need to work out the correct approach to set the LLVM optimization flags for your system**.

	- Some build systems have a flag like ``./configure --enable-optimize``. 
	- You can control whether LLVM optimizations are run using ``--llvm-opts N`` where N is an integer in the range 0-3. Sending ``-O2 --llvm-opts 0`` to *emcc* during all compilation stages will disable LLVM optimizations but utilize JavaScript optimizations. This can be useful when debugging a build failure.


JavaScript optimizations are specified in the final step, when you compile the linked LLVM bitcode to JavaScript. For example, to compile with :ref:`-O1 <emcc-O1>`:
	
.. code-block:: bash

	# Compile the linked bitcode to JavaScript with -O1 optimizations.
	./emcc -O1 project.bc -o project.js


.. _building-projects-debug:

Building projects with debug information
========================================

Building a project containing debug information requires that debug flags are specified for both the LLVM and JavaScript compilation phases.

To make *Clang* and LLVM emit debug information in the bitcode files you need to compile the sources with :ref:`-g <emcc-g>` (exactly the same as with :term:`clang` or *gcc* normally). To get *emcc* to include the debug information when compiling the bitcode to JavaScript, specify :ref:`-g <emcc-g>` or one of the ``-gN`` :ref:`debug level options <emcc-gN>`.

.. note:: Each build-system defines its own mechanisms for setting debug flags. **To get Clang to emit LLVM debug information, you will need to work out the correct approach for your system**.

	- Some build systems have a flag like ``./configure --enable-debug``. 

The flags for emitting debug information when compiling from bitcode to JavaScript are specified as an *emcc* option in the final step:
	
.. code-block:: bash

	# Compile the linked bitcode to JavaScript.
	# -g or -gN can be used to set the debug level (N)
	./emcc -g project.bc -o project.js	

For more general information, see the topic :ref:`Debugging`.


Using libraries
===============

Built-in support is available for a number of standard libraries: *libc*, *libc++* and *SDL*. These will automatically be linked when you compile code that uses them (you do not even need to add ``-lSDL``, but see below for more SDL-specific details).

If your project uses other libraries, for example `zlib <https://github.com/kripken/emscripten/tree/master/tests/zlib>`_ or *glib*, you will need to build and link them. The normal approach is to build the libraries to bitcode and then compile library and main program bitcode together to JavaScript. 

For example, consider the case where a project "project" uses a library "libstuff":

.. code-block:: bash
		
	# Compile libstuff to bitcode
	./emconfigure ./configure
	./emmake make
	
	# Compile project to bitcode
	./emconfigure ./configure
	./emmake make
		
	# Compile the library and code together to HTML
	emcc project.bc libstuff.bc -o final.html

	
It is also possible to link the bitcode libraries first, and then compile the combined **.bc** file to JavaScript:

.. code-block:: bash

	# Generate bitcode files project.bc and libstuff.bc
	...

	# Link together the bitcode files
	emcc project.bc libstuff.bc -o allproject.bc
	
	# Compile the combined bitcode to HTML
	emcc allproject.bc -o final.html



Emscripten Ports
================

Emscripten Ports is a collection of useful libraries, ported to Emscripten. They reside `on github <https://github.com/emscripten-ports>`_, and have integration support in *emcc*. When you request that a port be used, emcc will fetch it from the remote server, set it up and build it locally, then link it with your project, add necessary include to your build commands, etc. For example, SDL2 is in ports, and you can request that it be used with ``-s USE_SDL=2``. For example,

.. code-block:: bash

	./emcc tests/sdl2glshader.c -s USE_SDL=2 -s LEGACY_GL_EMULATION=1 -o sdl2.html

You should see some notifications about SDL2 being used, and built if it wasn't previously. You can then view ``sdl2.html`` in your browser.

.. note:: *SDL_image* has also been added to ports, use it with ``-s USE_SDL_IMAGE=2``. To see a list of all available ports, run ``emcc --show-ports``.

.. note:: Emscripten also has support for older SDL1, which is built-in. If you do not specify SDL2 as in the command above, then SDL1 is linked in and the SDL1 include paths are used. SDL1 has support for *sdl-config*, which is present in `system/bin <https://github.com/kripken/emscripten/blob/master/system/bin/sdl-config>`_. Using the native *sdl-config* may result in compilation or missing-symbol errors. You will need to modify the build system to look for files in **emscripten/system** or **emscripten/system/bin** in order to use the Emscripten *sdl-config*.

Adding more ports
-----------------

Adding more ports is fairly easy. Basically, the steps are

 * Make sure the port is open source and has a suitable license.
 * Add it to emscripten-ports on github. The ports maintainers can create the repo and add the relevant developers to a team for that repo, so they have write access.
 * Add a script to handle it under ``tools/ports/`` (see existing code for examples) and use it in ``tools/ports/__init__.py``.
 * Add testing in the test suite.


Build system issues
===================

Build system self-execution
---------------------------

Some large projects generate executables and run them in order to generate input for later parts of the build process (for example, a parser may be built and then run on a grammar, which then generates C/C++ code that implements that grammar). This sort of build process causes problems when using Emscripten because you cannot directly run the code you are generating.

The simplest solution is usually to build the project twice: once natively, and once to JavaScript. When the JavaScript build procedure fails because a generated executable is not present, you can then copy that executable from the native build, and continue to build normally. This approach was successfully used for compiling Python (see `tests/python/readme.md <https://github.com/kripken/emscripten/blob/master/tests/python/readme.md>`_ for more details).

In some cases it makes sense to modify the build scripts so that they build the generated executable natively. For example, this can be done by specifying two compilers in the build scripts, *emcc* and *gcc*, and using *gcc* just for generated executables. However, this can be more complicated than the previous solution because you need to modify the project build scripts, and you may have to work around cases where code is compiled and used both for the final result and for a generated executable.


Dynamic linking
---------------

Emscripten's goal is to generate the fastest and smallest possible code, and for that reason it focuses on generating a single JavaScript file for an entire project. For that reason, dynamic linking should be avoided when possible.

By default, Emscripten ``.so`` files are the same as ``.bc`` or ``.o`` files, that is, they contain LLVM bitcode. Dynamic libraries that you specify in the final build stage (when generating JavaScript or HTML) are linked in as static libraries. *Emcc* ignores commands to dynamically link libraries when linking together bitcode (i.e., not in the final build stage). This is to ensure that the same dynamic library is not linked multiple times in intermediate build stages, which would result in duplicate symbol errors.

There is `experimental support <https://github.com/kripken/emscripten/wiki/Linking>`_ for true dynamic libraries, loaded as runtime, either via dlopen or as a shared library. See that link for the details and limitations.


Configure may run checks that appear to fail
--------------------------------------------

Projects that use *configure*, *cmake*, or some other portable configuration method may run checks during the configure phase to verify that the toolchain and paths are set up properly. *Emcc* tries to get checks to pass where possible, but you may need to disable tests that fail due to a "false negative" (for example, tests that would pass in the final execution environment, but not in the shell during *configure*).

.. tip:: Ensure that if a check is disabled, the tested functionality does work. This might involve manually adding commands to the make files using a build system-specific method.

.. note:: In general *configure* is not a good match for a cross-compiler like Emscripten. *configure* is designed to build natively for the local setup, and works hard to find the native build system and the local system headers. With a cross-compiler, you are targeting a different system, and ignoring these headers etc.


Archive (.a) files
------------------

Emscripten supports **.a** archive files, which are bundles of object files. This is an old format for libraries, and it has special semantics - for example, the order of linking matters with **.a** files, but not with plain object files (in **.bc**, **.o** or **.so**). For the most part those special semantics should work in Emscripten, however, we support **.a** files using llvm's tools, which have a few limitations.

The main limitation is that if you have multiple files in a single **.a** archive that have the same basename (for example, ``dir1/a.o, dir2/a.o``), then llvm-ar cannot access both of those files. Emscripten will attempt to work around this by adding a hash to the basename, but collisions are still possible in principle.

Where possible it is better to generate shared library files (**.so**) rather than archives (**.a**) — this is generally a simple change in your project's build system. Shared libraries are simpler, and are more predictable with respect to linking.


Manually using emcc
===================

The :ref:`Tutorial` showed how :ref:`emcc <emccdoc>` can be used to compile single files into JavaScript. *Emcc* can also be used in all the other ways you would expect of *gcc*:

::

	# Generate a.out.js from C++. Can also take .ll (LLVM assembly) or .bc (LLVM bitcode) as input 
	./emcc src.cpp

	# Generate src.o containing LLVM bitcode.
	./emcc src.cpp -c

	# Generate result.js containing JavaScript.
	./emcc src.cpp -o result.js

	# Generate result.bc containing LLVM bitcode (the suffix matters).
	./emcc src.cpp -o result.bc

	# Generate a.out.js from two C++ sources.
	./emcc src1.cpp src2.cpp

	# Generate src1.o and src2.o, containing LLVM bitcode
	./emcc src1.cpp src2.cpp -c

	# Combine two LLVM bitcode files into a.out.js
	./emcc src1.o src2.o

	# Combine two LLVM bitcode files into another LLVM bitcode file
	./emcc src1.o src2.o -o combined.o

In addition to the capabilities it shares with *gcc*, *emcc* supports options to optimize code, control what debug information is emitted, generate HTML and other output formats, etc. These options are documented in the :ref:`emcc tool reference <emccdoc>` (``./emcc --help`` on the command line).


Alternatives to emcc
====================

.. tip:: Do not attempt to bypass *emcc* and call the Emscripten tools directly from your build system. 

You can in theory call *clang*, *llvm-ld*, and the other tools yourself. This is however considered dangerous because by default:

- *Clang* does not use the Emscripten-bundled headers, which can lead to various errors. 
- *llvm-ld* uses unsafe/unportable LLVM optimizations. 

*Emcc* automatically ensures the tools are configured and used properly.


Examples / test code
====================

The Emscripten test suite (`tests/runner.py <https://github.com/kripken/emscripten/blob/master/tests/runner.py>`_) contains a number of good examples — large C/C++ projects that are built using their normal build systems as described above: `freetype <https://github.com/kripken/emscripten/tree/master/tests/freetype>`_, `openjpeg <https://github.com/kripken/emscripten/tree/master/tests/openjpeg>`_, `zlib <https://github.com/kripken/emscripten/tree/master/tests/zlib>`_, `bullet <https://github.com/kripken/emscripten/tree/master/tests/bullet>`_ and `poppler <https://github.com/kripken/emscripten/tree/master/tests/poppler>`_.

It is also worth looking at the build scripts in the `ammo.js <https://github.com/kripken/ammo.js/blob/master/make.py>`_ project.




Troubleshooting
===============

- Make sure to use bitcode-aware *llvm-ar* instead of *ar* (which may discard code). *emmake* and *emconfigure* set the AR environment variable correctly, but a build system might incorrectly hardcode *ar*.
- 
	The compilation error ``multiply defined symbol`` indicates that the project has linked a particular static library multiple times. The project will need to be changed so that the problem library is linked only once.

	.. note:: You can use ``llvm-nm`` to see which symbols are defined in each bitcode file.
	
	One solution is to use the :ref:`building-projects-dynamic-linking-workaround` approach described above. This ensures that libraries are linked only once, in the final build stage. 



