---
title: 'Code coverage with Bazel'
---


Bazel features a `coverage` sub-command to produce code coverage
reports on repositories that can be tested with `bazel coverage`. Due
to the idiosyncrasies of the various language ecosystems, it is not
always trivial to make this work for a given project.

This page documents the general process for creating and viewing
coverage reports, and also features some language-specific notes for
languages whose configuration is well-known. It is best read by first
reading [the general section](#creating-a-coverage-report), and then
reading about the requirements for a specific language.

While a lot of customization is possible, this document focuses on
producing and consuming [`lcov`][lcov] reports, which is currently the
most well-supported route.

## Creating a coverage report {#creating-a-coverage-report}

### Preparation

The basic workflow for creating coverage reports requires the
following:

- A basic repository with test targets
- A toolchain with the language-specific code coverage tools installed
- A correct "instrumentation" configuration

The former two are language-specific and mostly straightforward,
however the latter can be more difficult for complex projects.

"Instrumentation" in this case refers to the coverage tools that are
used for a specific target. Bazel allows turning this on for a
specific subset of files using the
[`--instrumentation_filter`](/reference/command-line-reference#flag--instrumentation_filter)
flag, which specifies a filter for targets that are tested with the
instrumentation enabled. To enable instrumentation for tests, the
[`--instrument_test_targets`](/reference/command-line-reference#flag--instrument_test_targets)
flag is required.

By default, bazel tries to match the target package(s), and prints the
relevant filter as an `INFO` message.

### Running coverage

To produce a coverage report, use [`bazel coverage
--combined_report=lcov
[target]`](/reference/command-line-reference#coverage). This runs the
tests for the target, generating coverage reports in the lcov format
for each file.

Once finished, bazel runs an action that collects all the produced
coverage files, and merges them into one, which is then finally
created under `$(bazel info
output_path)/_coverage/_coverage_report.dat`.

Coverage reports are also produced if tests fail, though note that
this does not extend to the failed tests - only passing tests are
reported.

### Viewing coverage

The coverage report is only output in the non-human-readable `lcov`
format. From this, we can use the `genhtml` utility (part of [the lcov
project][lcov]) to produce a report that can be viewed in a web
browser:

```console
genhtml --branch-coverage --output genhtml "$(bazel info output_path)/_coverage/_coverage_report.dat"
```

Note that `genhtml` reads the source code as well, to annotate missing
coverage in these files. For this to work, it is expected that
`genhtml` is executed in the root of the bazel project.

To view the result, simply open the `index.html` file produced in the
`genhtml` directory in any web browser.

For further help and information around the `genhtml` tool, or the
`lcov` coverage format, see [the lcov project][lcov].

## Language-specific configuration

### C++

#### Linux

C++ coverage should work out-of-the-box with the default configuration.

#### MacOS

The default value of `GCOV_PREFIX_STRIP` is almost certainly incorrect and needs
adjusting manually because the correct value depends on your setup.

When the value is incorrect, no coverage data will be found.

Example to set `GCOV_PREFIX_STRIP=10`
```
bazel coverage //foo:foo_test --test_env=GCOV_PREFIX_STRIP=10`
```

### Java

Java should work out-of-the-box with the default configuration. The
[bazel toolchains][bazel_toolchains] contain everything necessary for
remote execution, as well, including JUnit.

### Python

See the [`rules_python` coverage docs](https://rules-python.readthedocs.io/en/latest/coverage.html)
for additional steps needed to enable coverage support in Python.

[lcov]: https://github.com/linux-test-project/lcov
[bazel_toolchains]: https://github.com/bazelbuild/bazel-toolchains