Generating C++ Binder Interfaces with aidl-cpp

Background

“aidl” refers to several related but distinct concepts:

• the AIDL interface definition language
• .aidl files (which contain AIDL)
• the aidl generator which transforms AIDL into client/server IPC interfaces

The aidl generator is a command line tool that generates client and server stubs for Binder interfaces from a specification in a file with the .aidl extension. For Java interfaces, the executable is called aidl while for C++ the binary is called aidl-cpp. In this document, we’ll use AIDL to describe the language of .aidl files and aidl generator to refer to the code generation tool that takes an .aidl file, parses the AIDL, and outputs code.

Previously, the aidl generator only generated Java interface/stub/proxy objects. C++ Binder interfaces were handcrafted with various degrees of compatibility with the Java equivalents. The Brillo project added support for generating C++ with the aidl generator. This generated C++ is cross-language compatible (e.g. Java clients are tested to interoperate with native services).

Overview

This document describes how C++ generation works with attention to:

• build interface
• cross-language type mapping
• implementing a generated interface
• C++ parcelables
• cross-language error reporting
• cross-language null reference handling

Detailed Design

Build Interface

Write AIDL in .aidl files and add them to LOCAL_SRC_FILES in your Android.mk. If your build target is a binary (e.g. you include $(BUILD_SHARED_LIBRARY)), then the generated code will be C++, not Java.

AIDL definitions should be hosted from the same repository as the implementation. Any system that needs the definition will also need the implementation (for both parcelables and interface). If there are multiple implementations (i.e. one in Java and one in C++), keep the definition with the native implementation. Android now has systems that run the native components of the system without the Java.

If you use an import statement in your AIDL, even from the same package, you need to add a path to LOCAL_AIDL_INCLUDES. This path should be relative to the root of the Android tree. For instance, a file IFoo.aidl defining com.example.IFoo might sit in a folder hierarchy something/something-else/com/example/IFoo.aidl. Then we would write:

LOCAL_AIDL_INCLUDES := something/something-else

Generated C++ ends up in nested namespaces corresponding to the interface’s package. The generated header also corresponds to the interface package. So com.example.IFoo becomes ::com::example::IFoo in header “com/example/IFoo.h”.

Similar to how Java works, the suffix of the path to a .aidl file must match the package. So if IFoo.aidl declares itself to be in package com.example, the folder structure (as given to LOCAL_SRC_FILES) must look like: some/prefix/com/example/IFoo.aidl.

To generate code from .aidl files from another build target (e.g. another binary or java), just add a relative path to the .aidl files to LOCAL_SRC_FILES. Remember that importing AIDL works the same, even for code in other directory hierarchies: add the include root path relative to the checkout root to LOCAL_AIDL_INCLUDES.

Type Mapping

The following table summarizes the equivalent C++ types for common Java types and whether those types may be used as in/out/inout parameters in AIDL interfaces.

Note that annotations may be placed at the interface level, as well as on a type by type basis. Interface level annotations will be applied opportunistically and be overridden by per type annotations. For instance, an interface marked @nullable will still not allow null int parameters.

Implementing a generated interface

Given an interface declaration like:

package foo;

import bar.IAnotherInterface;

interface IFoo {
  IAnotherInterface DoSomething(int count, out List<String> output);
}

aidl-cpp will generate a C++ interface:

namespace foo {

// Some headers have been omitted for clarity.
#include <android/String16.h>
#include <cstdint>
#include <vector>
#include <bar/IAnotherInterface.h>

// Some class members have been omitted for clarity.
class IFoo : public android::IInterface {
 public:
  virtual android::binder::Status DoSomething(
      int32_t count,
      std::vector<android::String16>* output,
      android::sp<bar::IAnotherInterface>* returned_value) = 0;
};

Note that aidl-cpp will import headers for types used in the interface. For imported types (e.g. parcelables and interfaces), it will import a header corresponding to the package/class name of the import. For instance, import bar.IAnotherInterface causes aidl-cpp to generate #include <bar/IAnotherInterface.h>.

When writing a service that implements this interface, write:

#include "foo/BnFoo.h"

namespace unrelated_namespace {

class MyFoo : public foo::BnFoo {
 public:
  android::binder::Status DoSomething(
      int32_t count,
      std::vector<android::String16>* output,
      android::sp<bar::IAnotherInterface>* returned_value) override {
    for (int32_t i = 0; i < count; ++i) {
      output->push_back(String16("..."));
    }
    *returned_value = new InstanceOfAnotherInterface;
    return Status::ok();
  }
};  // class MyFoo

}  // namespace unrelated_namespace

Note that the output values, output and returned_value are passed by pointer, and that this pointer is always valid.

Dependencies

The generated C++ code will use symbols from libbinder as well as libutils. AIDL files using the FileDescriptor type will also explicitly require libnativehelper, although this is likely a transitive dependency of the other two, and should be included automatically within the Android build tree regardless.

C++ Parcelables

In Java, a parcelable should extend android.os.Parcelable and provide a static final CREATOR field that acts as a factory for new instances/arrays of instances of the parcelable. In addition, in order to be used as an out parameter, a parcelable class must define a readFromParcel method.

In C++, parcelables must implement android::Parcelable from binder/Parcelable.h in libbinder. Parcelables must define a constructor that takes no arguments. In order to be used in arrays, a parcelable must implement a copy or move constructor (called implicitly in vector).

The C++ generator needs to know what header defines the C++ parcelable. It learns this from the cpp_header directive shown below. The generator takes this string and uses it as the literal include statement in generated code. The idea here is that you generate your code once, link it into a library along with parcelable implementations, and export appropriate header paths. This header include must make sense in the context of the Android.mk that compiles this generated code.

// ExampleParcelable.aidl
package com.example.android;

// Native types must be aliased at their declaration in the appropriate .aidl
// file.  This allows multiple interfaces to use a parcelable and its C++
// equivalent without duplicating the mapping between the C++ and Java types.
// Generator will assume bar/foo.h declares class
// com::example::android::ExampleParcelable
parcelable ExampleParcelable cpp_header "bar/foo.h";

Null Reference Handling

The aidl generator for both C++ and Java languages has been expanded to understand nullable annotations.

Given an interface definition like:

interface IExample {
  void ReadStrings(String neverNull, in @nullable String maybeNull);
};

the generated C++ header code looks like:

class IExample {
  android::binder::Status ReadStrings(
      const android::String16& in_neverNull,
      const std::unique_ptr<android::String16>& in_maybeNull);
};

Note that by default, the generated C++ passes a const reference to the value of a parameter and rejects null references with a NullPointerException sent back the caller. Parameters marked with @nullable are passed by pointer, allowing native services to explicitly control whether they allow method overloading via null parameters. Java stubs and proxies currently do nothing with the @nullable annotation.

Consider an AIDL type in @nullable List<String> bar. This type indicates that the remote caller may pass in a list of strings, and that both the list and any string in the list may be null. This type will map to a C++ type unique_ptr<vector<unique_ptr<String16>>>* bar. In this case:

• bar is never null
• *bar might be null
• (*bar)->empty() could be true
• (**bar)[0] could be null (and so on)

Exception Reporting

C++ methods generated by the aidl generator return android::binder::Status objects, rather than android::status_t. This Status object allows generated C++ code to send and receive exceptions (an exception type and a String16 error message) since we do not use real exceptions in C++. More background on Status objects can be found here.

For legacy support and migration ease, the Status object includes a mechanism to report a android::status_t. However, that return code is interpreted by a different code path and does not include a helpful String message.

For situations where your native service needs to throw an error code specific to the service, use Status::fromServiceSpecificError(). This kind of exception comes with a helpful message and an integer error code. Make your error codes consistent across services by using interface constants (see below).