platform_build_soong/README.md

# Soong

Soong is the replacement for the old Android make-based build system.  It
replaces Android.mk files with Android.bp files, which are JSON-like simple
declarative descriptions of modules to build.

## Android.bp file format

By design, Android.bp files are very simple.  There are no conditionals or
control flow statements - any complexity is handled in build logic written in
Go.  The syntax and semantics of Android.bp files are intentionally similar
to [Bazel BUILD files](https://www.bazel.io/versions/master/docs/be/overview.html)
when possible.

### Modules

A module in an Android.bp file starts with a module type, followed by a set of
properties in `name: value,` format:

```
cc_binary {
    name: "gzip",
    srcs: ["src/test/minigzip.c"],
    shared_libs: ["libz"],
    stl: "none",
}
```

Every module must have a `name` property, and the value must be unique across
all Android.bp files.

For a list of valid module types and their properties see
[$OUT_DIR/soong/.bootstrap/docs/soong_build.html](https://go/Android.bp).

### Variables

An Android.bp file may contain top-level variable assignments:
```
gzip_srcs = ["src/test/minigzip.c"],

cc_binary {
    name: "gzip",
    srcs: gzip_srcs,
    shared_libs: ["libz"],
    stl: "none",
}
```

Variables are scoped to the remainder of the file they are declared in, as well
as any child blueprint files.  Variables are immutable with one exception - they
can be appended to with a += assignment, but only before they have been
referenced.

### Comments
Android.bp files can contain C-style multiline `/* */` and C++ style single-line
`//` comments.

### Types

Variables and properties are strongly typed, variables dynamically based on the
first assignment, and properties statically by the module type.  The supported
types are:
* Bool (`true` or `false`)
* Strings (`"string"`)
* Lists of strings (`["string1", "string2"]`)
* Maps (`{key1: "value1", key2: ["value2"]}`)

Maps may values of any type, including nested maps.  Lists and maps may have
trailing commas after the last value.

### Operators

Strings, lists of strings, and maps can be appended using the `+` operator.
Appending a map produces the union of keys in both maps, appending the values
of any keys that are present in both maps.

### Defaults modules

A defaults module can be used to repeat the same properties in multiple modules.
For example:

```
cc_defaults {
    name: "gzip_defaults",
    shared_libs: ["libz"],
    stl: "none",
}

cc_binary {
    name: "gzip",
    defaults: ["gzip_defaults"],
    srcs: ["src/test/minigzip.c"],
}
```

### Formatter

Soong includes a canonical formatter for blueprint files, similar to
[gofmt](https://golang.org/cmd/gofmt/).  To recursively reformat all Android.bp files
in the current directory:
```
bpfmt -w .
```

The canonical format includes 4 space indents, newlines after every element of a
multi-element list, and always includes a trailing comma in lists and maps.

### Convert Android.mk files

Soong includes a tool perform a first pass at converting Android.mk files
to Android.bp files:

```
androidmk Android.mk > Android.bp
```

The tool converts variables, modules, comments, and some conditionals, but any
custom Makefile rules, complex conditionals or extra includes must be converted
by hand.

#### Differences between Android.mk and Android.bp

* Android.mk files often have multiple modules with the same name (for example
for static and shared version of a library, or for host and device versions).
Android.bp files require unique names for every module, but a single module can
be built in multiple variants, for example by adding `host_supported: true`.
The androidmk converter will produce multiple conflicting modules, which must
be resolved by hand to a single module with any differences inside
`target: { android: { }, host: { } }` blocks.

## Build logic

The build logic is written in Go using the
[blueprint](http://godoc.org/github.com/google/blueprint) framework.  Build
logic receives module definitions parsed into Go structures using reflection
and produces build rules.  The build rules are collected by blueprint and
written to a [ninja](http://ninja-build.org) build file.

## FAQ

### How do I write conditionals?

Soong deliberately does not support conditionals in Android.bp files.
Instead, complexity in build rules that would require conditionals are handled
in Go, where high level language features can be used and implicit dependencies
introduced by conditionals can be tracked.  Most conditionals are converted
to a map property, where one of the values in the map will be selected and
appended to the top level properties.

For example, to support architecture specific files:
```
cc_library {
    ...
    srcs: ["generic.cpp"],
    arch: {
        arm: {
            srcs: ["arm.cpp"],
        },
        x86: {
            srcs: ["x86.cpp"],
        },
    },
}
```

## Contact

Email android-building@googlegroups.com (external) for any questions, or see
[go/soong](http://go/soong) (internal).