Extension Overview

This page describes how to extend the BUILD language using macros and rules.

Bazel extensions are files ending in .bzl. Use a load statement to import a symbol from an extension.

Before learning the more advanced concepts, first:

• Read about the Starlark language, used in both the BUILD and .bzl files.

• Learn how you can share variables between two BUILD files.

Macros and rules

A macro is a function that instantiates rules. Macros come in two flavors: symbolic macros (new in Bazel 8) and legacy macros. The two flavors of macros are defined differently, but behave almost the same from the point of view of a user. A macro is useful when a BUILD file is getting too repetitive or too complex, as it lets you reuse some code. The function is evaluated as soon as the BUILD file is read. After the evaluation of the BUILD file, Bazel has little information about macros. If your macro generates a genrule, Bazel will behave almost as if you declared that genrule in the BUILD file. (The one exception is that targets declared in a symbolic macro have special visibility semantics: a symbolic macro can hide its internal targets from the rest of the package.)

A rule is more powerful than a macro. It can access Bazel internals and have full control over what is going on. It may for example pass information to other rules.

If you want to reuse simple logic, start with a macro; we recommend a symbolic macro, unless you need to support older Bazel versions. If a macro becomes complex, it is often a good idea to make it a rule. Support for a new language is typically done with a rule. Rules are for advanced users, and most users will never have to write one; they will only load and call existing rules.

Evaluation model

A build consists of three phases.

• Loading phase. First, load and evaluate all extensions and all BUILD files that are needed for the build. The execution of the BUILD files simply instantiates rules (each time a rule is called, it gets added to a graph). This is where macros are evaluated.

• Analysis phase. The code of the rules is executed (their implementation function), and actions are instantiated. An action describes how to generate a set of outputs from a set of inputs, such as “run gcc on hello.c and get hello.o”. You must list explicitly which files will be generated before executing the actual commands. In other words, the analysis phase takes the graph generated by the loading phase and generates an action graph.

• Execution phase. Actions are executed, when at least one of their outputs is required. If a file is missing or if a command fails to generate one output, the build fails. Tests are also run during this phase.

Bazel uses parallelism to read, parse and evaluate the .bzl files and BUILD files. A file is read at most once per build and the result of the evaluation is cached and reused. A file is evaluated only once all its dependencies (load() statements) have been resolved. By design, loading a .bzl file has no visible side-effect, it only defines values and functions.

Bazel tries to be clever: it uses dependency analysis to know which files must be loaded, which rules must be analyzed, and which actions must be executed. For example, if a rule generates actions that you don’t need for the current build, they will not be executed.

Creating extensions

• Create your first macro in order to reuse some code. Then learn more about macros and using them to create “custom verbs”.

• Follow the rules tutorial to get started with rules. Next, you can read more about the rules concepts.

The two links below will be very useful when writing your own extensions. Keep them within reach:

• The API reference

• Examples

Going further

In addition to macros and rules, you may want to write aspects and repository rules.

• Use Buildifier consistently to format and lint your code.

• Follow the .bzl style guide.

• Test your code.

• Generate documentation to help your users.

• Optimize the performance of your code.

• Deploy your extensions to other people.