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Creating a Macro

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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 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 a function in a separate .bzl file, and call the file miniature.bzl:

def miniature(name, src, size="100x100", **kwargs):
  """Create a miniature of the src image.

  The generated file is prefixed with 'small_'.
  """
  native.genrule(
    name = name,
    srcs = [src],
    outs = ["small_" + src],
    cmd = "convert $< -resize " + size + " $@",
    **kwargs
  )

A few remarks:

  • By convention, macros have a name argument, just like rules.

  • To document the behavior of a macro, use docstring like in Python.

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

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

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"],
)