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WaWrapper is a wrapper script intended for running WebAssembly source code. Most of the actual work is done via delegating to a WebAssembly virtual machine, which must be provided by the user. (Currently, WaWrapper is compatible only with WAVM.)


WaWrapper is intended for use as a backend for sites which allow the user to enter the source code for a program, and which compile and run it. As such, the usual source format is the textual .wast / .wat format.

Users may not want to write out an entire WebAssembly file; some parts, such as the imports and exports, can be determined automatically. As such, WaWrapper supports a situation in which the user enters only the data, globals, and functions that make up their code; imports will automatically be added for any functions in module env that WAVM supports and that are used in the user's code, and for 256 MiB of memory; and an export will be added for a function main that takes no arguments and returns a 32-bit integer exit code (which will be interpreted as an 8-bit integer and used as WaWrapper's exit code), in addition to a module wrapper for the module itself. The file extension .waw is used for this sort of ".wast missing imports" file.

WaWrapper also allows users to enter their own compiled .wasm files. However, as such files contain many nonprintable characters (especially NUL), a different encoding needs to be used to make this possible. As such, WaWrapper accepts a format consisting of a .wasm file interpreted as characters in codepage 437 and encoded as UTF-8, but with three substitutions:

  • NUL (codepoint 0): encoded as , not NUL
  • Space (codepoint 32): encoded as , not
  • Non-breaking space (codepoint 255): encoded as , not  

Note that the full version of codepage 437 is used, e.g. codepoint 127 is encoded as not DEL, and codepoint 10 as not NL.

Because this encoding uses no whitespace characters, whitespace can freely be added to avoid issues with systems that use long lines. The filename extension .wae is used for this "encoded" form of .wasm.

WaWrapper accepts only one command-line option (besides the name of the file to execute), -f. If this is given, in addition to executing the file, WaWrapper will translate the file to both .wasm (encoded using the format above), and a pretty-printed form of .wast (including all imports, elidable type information, etc.); these will be output on standard error.


WaWrapper is a standalone Perl script, that can be executed directly from an unpacked version of its installation tarball (it looks at some files relative to itself, so the directory structure should be left the same).

The only potentially tricky part is that WaWrapper will need to be given the path to WAVM. The current version of the script assumes a parallel install, with WAVM's binaries in ../prefix/bin relative to; you can easily change this via editing the path_to_WAVM constant in


WaWrapper is licensed under the Apache license version 2. See the COPYING file for more information.

The file lib/WaWrapper/ is a listing of which imports are supported by WAVM. As such, it is licensed under the same terms as WAVM, a 3-clause BSD-style license; see its copyright header for more information.