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---
layout: documentation
title: Creating a Macro
---
# Creating a Macro
Let's suppose 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, we are going to resize an image.
The easiest way is to use a `genrule`:
``` python
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,
we are going to define a function in a separate `.bzl` file. Let's call the file
`miniature.bzl`:
``` python
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 100x100 $@",
**kwargs
)
```
A few remarks:
* By convention, macros have a `name` argument, just like rules.
* We document the behavior of a macro by using a
[docstring](https://www.python.org/dev/peps/pep-0257/) like in Python.
* To call a `genrule`, or any other native rule, use `native.`.
* `**kwargs` is used to forward the extra arguments to the underlying `genrule`
(it works just like in [Python](https://docs.python.org/3/tutorial/controlflow.html#keyword-arguments)).
This is useful, so that a user can use standard attributes like `visibility`,
or `tags`.
Now, you can use the macro from the `BUILD` file:
``` python
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"],
)
```
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.md). Rules will give you more control and flexibility.
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