How to write tests

A primary goal of the ns-3 project is to help users to improve the validity and credibility of their results. There are many elements to obtaining valid models and simulations, and testing is a major component. If you contribute models or examples to ns-3, you may be asked to contribute test code. Models that you contribute will be used for many years by other people, who probably have no idea upon first glance whether the model is correct. The test code that you write for your model will help to avoid future regressions in the output and will aid future users in understanding the verification and bounds of applicability of your models.

There are many ways to verify the correctness of a model’s implementation. In this section, we hope to cover some common cases that can be used as a guide to writing new tests.

Sample TestSuite skeleton

When starting from scratch (i.e. not adding a TestCase to an existing TestSuite), these things need to be decided up front:

  • What the test suite will be called
  • What type of test it will be (Build Verification Test, Unit Test, System Test, or Performance Test)
  • Where the test code will live (either in an existing ns-3 module or separately in src/test/ directory). You will have to edit the wscript file in that directory to compile your new code, if it is a new file.

A program called utils/ is a good starting point. This program can be invoked such as router for a hypothetical new module called router. Once you do this, you will see a router directory, and a test/ test suite. This file can be a starting point for your initial test. This is a working test suite, although the actual tests performed are trivial. Copy it over to your module’s test directory, and do a global substitution of “Router” in that file for something pertaining to the model that you want to test. You can also edit things such as a more descriptive test case name.

You also need to add a block into your wscript to get this test to compile:

module_test.source = [

Before you actually start making this do useful things, it may help to try to run the skeleton. Make sure that ns-3 has been configured with the “–enable-tests” option. Let’s assume that your new test suite is called “router” such as here:

RouterTestSuite::RouterTestSuite ()
  : TestSuite ("router", UNIT)

Try this command:

$ ./ -s router

Output such as below should be produced:

PASS: TestSuite router
1 of 1 tests passed (1 passed, 0 skipped, 0 failed, 0 crashed, 0 valgrind errors)

See src/lte/test/ for a worked example.

Test macros

There are a number of macros available for checking test program output with expected output. These macros are defined in src/core/model/test.h.

The main set of macros that are used include the following:

NS_TEST_ASSERT_MSG_EQ(actual, limit, msg)
NS_TEST_ASSERT_MSG_NE(actual, limit, msg)
NS_TEST_ASSERT_MSG_LT(actual, limit, msg)
NS_TEST_ASSERT_MSG_GT(actual, limit, msg)
NS_TEST_ASSERT_MSG_EQ_TOL(actual, limit, tol, msg)

The first argument actual is the value under test, the second value limit is the expected value (or the value to test against), and the last argument msg is the error message to print out if the test fails.

The first four macros above test for equality, inequality, less than, or greater than, respectively. The fifth macro above tests for equality, but within a certain tolerance. This variant is useful when testing floating point numbers for equality against a limit, where you want to avoid a test failure due to rounding errors.

Finally, there are variants of the above where the keyword ASSERT is replaced by EXPECT. These variants are designed specially for use in methods (especially callbacks) returning void. Reserve their use for callbacks that you use in your test programs; otherwise, use the ASSERT variants.

How to add an example program to the test suite

There are two methods for adding an example program to the the test suite. Normally an example is added using only one of these methods to avoid running the example twice.

First, you can “smoke test” that examples compile and run successfully to completion (without memory leaks) using the script located in your module’s test directory. Briefly, by including an instance of this file in your test directory, you can cause the test runner to execute the examples listed. It is usually best to make sure that you select examples that have reasonably short run times so as to not bog down the tests. See the example in src/lte/test/ directory. The exit status of the example will be checked when run and a non-zero exit status can be used to indicate that the example has failed. This is the easiest way to add an example to the test suite but has limited checks.

The second method you can use to add an example to the test suite is more complicated but enables checking of the example output (std::out and std::err). This approach uses the test suite framework with a specialized TestSuite or TestCase class designed to run an example and compare the output with a specified known “good” reference file. To use an example program as a test you need to create a test suite file and add it to the appropriate list in your module wscript file. The “good” output reference file needs to be generated for detecting regressions.

If you are thinking about using this class, strongly consider using a standard test instead. The TestSuite class has better checking using the NS_TEST_* macros and in almost all cases is the better approach. If your test can be done with a TestSuite class you will be asked by the reviewers to rewrite the test when you do a pull request.

Let’s assume your module is called mymodule, and the example program is mymodule/examples/ First you should create a test file mymodule/test/ which looks like this:

#include "ns3/example-as-test.h"
static ns3::ExampleAsTestSuite g_modExampleOne ("mymodule-example-mod-example-one", "mod-example", NS_TEST_SOURCEDIR, "--arg-one");
static ns3::ExampleAsTestSuite g_modExampleTwo ("mymodule-example-mod-example-two", "mod-example", NS_TEST_SOURCEDIR, "--arg-two");

The arguments to the constructor are the name of the test suite, the example to run, the directory that contains the “good” reference file (the macro NS_TEST_SOURCEDIR is normally the correct directory), and command line arguments for the example. In the preceding code the same example is run twice with different arguments.

You then need to add that newly created test suite file to the list of test sources in mymodule/wscript. Building of examples is an option so you need to guard the inclusion of the test suite:

if (bld.env['ENABLE_EXAMPLES']):

Since you modified a wscript file you need to reconfigure and rebuild everything.

You just added new tests so you will need to generate the “good” output reference files that will be used to verify the example:

./ --suite="mymodule-example-*" --update

This will run all tests starting with “mymodule-example-” and save new “good” reference files. Updating the reference files should be done when you create the test and whenever output changes. When updating the reference output you should inspect it to ensure that it is valid. The reference files should be committed with the new test.

This completes the process of adding a new example.

You can now run the test with the standard script. For example to run the suites you just added:

./ --suite="mymodule-example-*"

This will run all mymodule-example-... tests and report whether they produce output matching the reference files.

You can also add multiple examples as test cases to a TestSuite using ExampleAsTestCase. See src/core/test/ for examples of setting examples as tests.

When setting up an example for use by this class you should be very careful about what output the example generates. For example, writing output which includes simulation time (especially high resolution time) makes the test sensitive to potentially minor changes in event times. This makes the reference output hard to verify and hard to keep up-to-date. Output as little as needed for the example and include only behavioral state that is important for determining if the example has run correctly.

Testing for boolean outcomes

Testing outcomes when randomness is involved

Testing output data against a known distribution

Providing non-trivial input vectors of data

Storing and referencing non-trivial output data

Presenting your output test data