Emscripten Test Suite

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 pass on the master branch. In addition to correctness tests, there are also benchmarks that you can run.

This article explains how to run the test and benchmark suite, and provides an overview of what tests are available.

Setting up

To run the tests, you need an emscripten setup, as it will run emcc and other commands. See the developer’s guide for how best to do that.

Running tests

Run the test suite runner (tests/runner.py) with no arguments to see the help message:

python tests/runner.py

The tests are divided into modes. You can run either an entire mode or an individual test, or use wildcards to run some tests in some modes. For example:

# run one test (in the default mode)
python tests/runner.py test_loop

# run one test in a specific mode (here, asm.js -O2)
python tests/runner.py asm2.test_loop

# run a test in a bunch of modes (here, all asm.js modes)
python tests/runner.py asm*.test_loop

# run a bunch of tests in one mode (here, all i64 tests in wasm -O3)
python tests/runner.py wasm3.test_*i64*

# run all tests in a specific mode (here, asm.js -O1)
python tests/runner.py asm1

The core test modes (defined at the bottom of tests/test_core.py) let you run a specific test in either asm.js or wasm, and with different optimization flags. There are also non-core test modes, that run tests in more special manner (in particular, in those tests it is not possible to say “run the test with a different optimization flag” - that is what the core tests are for). The non-core test modes include

  • other: Non-core tests running in the shell.

  • browser: Tests that run in a browser.

  • sockets: Networking tests that run in a browser.

  • interactive: Browser tests that are not fully automated, and require user interaction (these should be automated eventually).

  • sanity: Tests for emscripten setting itself up. This modifies your .emscripten file temporarily.

  • benchmark: Runs benchmarks, measuring speed and code size.

The wildcards we mentioned above work for non-core test modes too, for example:

# run one browser test
python tests/runner.py browser.test_sdl_image

# run all SDL2 browser tests
python tests/runner.py browser.test_sdl2*

# run all browser tests
python tests/runner.py browser

Skipping Tests

An individual test can be skipped by passing the “skip:” prefix. E.g.

python tests/runner.py other skip:other.test_cmake

Wildcards can also be passed in skip, so

python tests/runner.py browser skip:browser.test_pthread_*

will run the whole browser suite except for all the pthread tests in it.

Running a bunch of random tests

You can run a random subset of the test suite, using something like

python tests/runner.py random100

Replace 100 with another number as you prefer. This will run that number of random tests, and tell you the statistical likelihood of almost all the test suite passing assuming those tests do. This works just like election surveys do - given a small sample, we can predict fairly well that so-and-so percent of the public will vote for candidate A. In our case, the “candidates” are pass or fail, and we can predict how much of the test suite will pass given that sample. Assuming the sample tests all pass, we can say with high likelihood that most of the test suite will in fact pass. (Of course, this is no guarantee, and even a single test failure is serious, however, this gives a quick estimate that your patch does not cause significant and obvious breakage.)

Important Tests

Please see the bottom the file tests/test_core.py for the current test modes, as they may change slowly over time. When you want to run the entire test suite locally, these are currently the important commands:

# Run all core wasm tests
python tests/runner.py wasm*

# Run "other" test suite
python tests/runner.py other

# Run "browser" test suite - this requires a web browser
python tests/runner.py browser

# Run "sockets" test suite - this requires a web browser too
python tests/runner.py sockets

# Run "sanity" test suite - this tests setting up emscripten during
# first run, etc., and so it modifies your .emscripten file temporarily.
python tests/runner.py sanity

# Optionally, also run benchmarks to check for regressions
python tests/runner.py benchmark


Emscripten has a benchmark suite that measures both speed and code size, which includes several interesting real-world codebases, from physics engines to compression libraries to virtual machines. It also includes some existing benchmarks such as CoreMark and LINPACK. See for example this post’s section on speed which gives an overview.

To run the benchmark suite, do:

# Run all benchmarks
python tests/runner.py benchmark

As with all the test suites, you can also run a specific benchmark:

# Run one specific benchmark
python tests/runner.py benchmark.test_skinning

Usually you will want to customize the python in tests/test_benchmark.py to run the benchmarks you want (there is currently no external config file). Things you may want to modify include:

  • benchmarkers is the list of VMs to run the benchmarks on.

  • DEFAULT_ARG is how long the benchmark should run (they all try to run for a similar amount of time for consistency).

  • TEST_REPS is how many times to repeat each run (more will take longer, but should have less noise).

  • PROFILING controls whether the builds are set up for profiling (which can increase code size, so it’s not done by default).

Debugging test failures

Setting the Debug mode (EMCC_DEBUG) is useful for debugging tests, as it emits debug output and intermediate files (the files go in /tmp/emscripten_temp/):

# On Windows, use "set" to set and un-set the EMCC_DEBUG environment variable:
python tests/runner.py test_hello_world

# On Linux, you can do this all in one line
EMCC_DEBUG=1 python tests/runner.py test_hello_world

# EMCC_DEBUG=2 generates additional debug information.
EMCC_DEBUG=2 python tests/runner.py test_hello_world

You can also specify EMTEST_SAVE_DIR=1 in the environment to save the temporary directory that the test runner uses into /tmp/emscripten_test/. This is a test suite-specific feature, and is useful for inspecting test outputs as well as temporary files generated by the test. By default, the temporary directory will be cleaned between each test run, but setting EMTEST_SAVE_DIR=2 will preserve the directory even when a new test is started.

The Debugging topic provides more guidance on how to debug Emscripten-generated code.