Quick Start

To build Zircon and run unit tests, run one of the following commands:

# Build and run x64.
./scripts/build-zircon-x64 && ./scripts/run-zircon-x64

# Build and run arm64.
./scripts/build-zircon-arm64 && ./scripts/run-zircon-arm64

Once the scripts finish running, you should see the Zircon shell. To run userspace tests, use the Zircon shell to run:


To run in-kernel tests, use the Zircon shell to run:

k ut all

The Notes for hacking on Zircon page has more details about how to use the Zircon shell and how to automatically build all supported architectures.

Userspace Tests

The test harness, runtests, picks up and runs all of the executables from the /boot/test and /system/test directories. If you provide a command-line argument, such as runtests -S -m widget_test, runtests will only run the single test requested -- in this case, widget_test.

“runtests” takes command-line arguments to toggle classes of tests to execute.

These classes are the following:

  • Small: Isolated tests for functions and classes. These must be totally synchronous and single-threaded. These tests should be parallelizable; there shouldn't be any shared resources between them.
  • Medium: Single-process integration tests. Ideally these are also synchronous and single-threaded but they might run through a large chunk of code in each test case, or they might use disk, making them a bit slower.
  • Large: Slow, multi-process, or particularly incomprehensible single-process integration tests. These tests are often too slow / flaky to run in a CQ, and we should try to limit how many we have.
  • Performance: Tests which are expected to pass, but which are measured using other metrics (thresholds, statistical techniques) to identify regressions.

Since runtests doesn't really know what “class” is executing when it launches a test, it encodes this information in the environment variable RUNTESTS_TEST_CLASS, which is detailed in the unittest header , and lets the executable itself decide what to run / not run. This environment variable is a bitmask indicating which tests to run.

For example, if a a test executable is run with “small” and “medium” tests, it will be executed ONCE with RUNTESTS_TEST_CLASS set to 00000003 (the hex bitwise OR of “TEST_SMALL” and “TEST_MEDIUM” -- though this information should be parsed using the unittest header, as it may be updated in the future).

Zircon Tests (ulib/test, and/or using ulib/unittest)

The following macros can be used to filter tests into these categories:


The legacy RUN_TEST(widget_test) is aliased to mean the same thing as RUN_TEST_SMALL.

Fuchsia Tests (not using ulib/unittest)

The environment variable RUNTESTS_TEST_CLASS will still be available to all executables launched by runtests. The unittest header can be used to parse different categories of tests which the runtests harness attempted to run.

Runtests CLI

By default, runtests will run both small and medium tests.

To determine how to run a custom set of test categories, run runtests -h, which includes usage information.

Kernel-mode Tests

The kernel contains unit tests and diagnostics, which can be run using the k command. The output of the k command will only be shown on the console. Depending on your configuration, this might be the serial console, or the debuglog virtual terminal.

Unit tests

Many parts of the kernel have unit tests, which report success/failure automatically. These unit tests are built using the primitives provided by the kernel unit-test library. You can find these statically by searching for UNITTEST_START_TESTCASE.

These tests can be run from the shell with k ut. k ut all will run all tests or you can use k ut $TEST_NAME to run a specific test.


Many parts of the kernel provide diagnostics, whose output requires manual inspection. Some of these diagnostics are used to verify correctness (e.g. timer_diag), while others simply stress test a part of the system (e.g. timer_stress).

To run a diagnostic, simply pass its name to the k command. For example, to run the kernel's builtin benchmarks, run k bench. To find the full set of kernel diagnostics statically, search for STATIC_COMMAND. To enumerate them dynamically, run k help.

Diagnostic tests are intended to be run via serial console, or with physical access to the system. Some diagnostics may be destructive, and leave the system in a broken state.