Creating a Symbolic Macro

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IMPORTANT: This tutorial is for symbolic macros – the new macro system introduced in Bazel 8. If you need to support older Bazel versions, you will want to write a legacy macro instead; take a look at Creating a Legacy Macro.

Imagine that you need to run a tool as part of your build. For example, you may want to generate or preprocess a source file, or compress a binary. In this tutorial, you are going to create a symbolic macro that resizes an image.

Macros are suitable for simple tasks. If you want to do anything more complicated, for example add support for a new programming language, consider creating a rule. Rules give you more control and flexibility.

The easiest way to create a macro that resizes an image is to use a genrule:

genrule(
    name = "logo_miniature",
    srcs = ["logo.png"],
    outs = ["small_logo.png"],
    cmd = "convert $< -resize 100x100 $@",
)

cc_binary(
    name = "my_app",
    srcs = ["my_app.cc"],
    data = [":logo_miniature"],
)

If you need to resize more images, you may want to reuse the code. To do that, define an implementation function and a macro declaration in a separate .bzl file, and call the file miniature.bzl:

# Implementation function
def _miniature_impl(name, visibility, src, size, **kwargs):
    native.genrule(
        name = name,
        visibility = visibility,
        srcs = [src],
        outs = [name + "_small_" + src.name],
        cmd = "convert $< -resize " + size + " $@",
        **kwargs,
    )

# Macro declaration
miniature = macro(
    doc = """Create a miniature of the src image.

    The generated file name will be prefixed with `name + "_small_"`.
    """,
    implementation = _miniature_impl,
    # Inherit most of genrule's attributes (such as tags and testonly)
    inherit_attrs = native.genrule,
    attrs = {
        "src": attr.label(
            doc = "Image file",
            allow_single_file = True,
            # Non-configurable because our genrule's output filename is
            # suffixed with src's name. (We want to suffix the output file with
            # srcs's name because some tools that operate on image files expect
            # the files to have the right file extension.)
            configurable = False,
        ),
        "size": attr.string(
            doc = "Output size in WxH format",
            default = "100x100",
        ),
        # Do not allow callers of miniature() to set srcs, cmd, or outs -
        # _miniature_impl overrides their values when calling native.genrule()
        "srcs": None,
        "cmd": None,
        "outs": None,
    },
)

A few remarks:

• Symbolic macro implementation functions must have name and visibility parameters. They should used for the macro's main target.

• To document the behavior of a symbolic macro, use doc parameters for macro() and its attributes.

• To call a genrule, or any other native rule, use native..

• Use **kwargs to forward the extra inherited arguments to the underlying genrule (it works just like in Python). This is useful so that a user can set standard attributes like tags or testonly.

Now, use the macro from the BUILD file:

load("//path/to:miniature.bzl", "miniature")

miniature(
    name = "logo_miniature",
    src = "image.png",
)

cc_binary(
    name = "my_app",
    srcs = ["my_app.cc"],
    data = [":logo_miniature"],
)