Packaging Files

This topic shows how to package the files that will be used to populate Emscripten’s virtual file system when the page is loaded.

There are two alternatives for how files are packaged: preloading and embedding. Embedding puts the specified files inside the generated JavaScript, while preloading packages the files separately. Embedding files is much less efficient than preloading and should only be used when packaging small numbers of small files. Preloading also enables the option to separately host the data.

Emcc uses the file packager to package the files and generate the File System API calls that create and load the file system at run time. While Emcc is the recommended tool for packaging, there are cases where it can make sense to run the file packager manually.

Packaging using emcc

The easiest way to package files is to use emcc at compile time. The preload and embed commands select their respective packaging methods.

The command below shows how to package files for preloading:

./emcc file.cpp -o file.html --preload-file asset_dir

The command generates file.html, file.js and The .data file contains all the files in asset_dir/, and is loaded by file.js.


The Tutorial demonstrates preloading using the hello_world_file.cpp test code.

The command for embedding is shown below. In this case emcc generates file.html and file.js — the contents of asset_dir/ are embedded directly into the file.js:

./emcc file.cpp -o file.html --embed-file asset_dir

By default, the files to be packaged should be nested in or below the compile-time command prompt directory. At runtime the same nested file structure is mapped to the virtual file system, with the root corresponding to the command prompt directory.

For example, consider a file structure dir1/dir2/dir3/asset_dir/ where the project is compiled from dir2. When we package asset_dir, we specify its relative location dir3/asset_dir/:

./emcc file.cpp -o file.html --preload-file dir3/asset_dir

The folder is available at this same location dir3/asset_dir in the virtual file system at runtime. Similarly, if we packaged a file in dir2, it would be available in the root of the virtual file system at runtime.

The @ symbol can be used to map packaged files from any location in the local file system to any location in the virtual file system. This is discussed below in Modifying file locations in the virtual file system.

Packaging using the file packager tool

You can also run the file packager manually using the instructions at the top of

The file packager generates a .data file and .js file. The .js file contains the code to use the data file, and must be loaded before loading your main compiled code.


  • Using the file packager allows you to run file packaging separately from compiling the code.
  • You can load multiple datafiles by running the file packager on each and loading the .js outputs. See BananaBread for an example of this (cube2/js/game-setup.js).

Changing the data file location

By default, the .data file containing all the preloaded files is loaded from the same URL as your .js file. In some cases it may be useful to have the data file in a different location from the other files — for example if your .html and .js change a lot you may want to keep the data file on a fast CDN somewhere else.

This model is supported by changing the Module.filePackagePrefixURL to be the URL where the data file is stored (this is a prefix, so should include the full path before the data’s file name). The attribute must be specified in a <script> element before the one that loads the data file.

Modifying file locations in the virtual file system

The default approach for packaging is to directly map the nested file structure at compile time — relative to the compile-time command prompt directory — to the root of the virtual file system. The @ symbol can be used in a path at build time to explicitly specify where the resource will be located in the virtual file system at runtime.


The @ symbol is needed because sometimes it is useful to package files that are not nested below the compile-time directory, and for which there is therefore no default mapping to a location in the virtual file system.

For example, we can map the preloaded folder ../../asset_dir to the root of the virtual file system (/) using:

./emcc file.cpp -o file.html --preload-file ../../asset_dir@/

We can also map a new path and filename. For example, to make the embedded file ../res/gen123.png available as /main.png we might do:

./emcc file.cpp -o file.html --embed-file ../res/gen123.png@main.png

Valid Character Set

The following characters may be used in filenames: A-Z, a-z, 0-9, the space character and any of the characters !#$%&'()+,-.;=@[]^_`{}~. Additionally, the following characters may be used if your host filesystem supports them: "*<>?| (Windows does not allow using these in filenames). When specifying the character @ on the command line, it must be escaped to the form @@ to avoid triggering the src@dst mapping notation (see above). The characters /, \ and : cannot be used.

Monitoring file usage


Only package the files your app actually needs, in order to reduce download size and improve startup speed.

There is an option to log which files are actually used at runtime. To use it, define the Module.logReadFiles object. The Module.printErr function will be called on each file that is read (this function must also be defined, and should log to a convenient place).

An alternative approach is to look at FS.readFiles() in your compiled JavaScript. This is an object with keys for all the files that were read from. You may find it easier to use than logging as it records files rather than potentially multiple file accesses.


You can also modify the FS.readFiles() object or remove it entirely. This can be useful, say, in order to see which files are read between two points in time in your app.

Test code

The test suite contains many file packaging examples, and is a good place to search for working code.