I want to implement a simple IPC mechanism using Binders in android. For that, I searched on the Internet and found this. I compiled it and it runs fine on my Android device. I tried to take an overall understanding of the program, by searching for each class on AOSP, but everything got more difficult and messed up. Can anyone please explain (just high level), maybe by adding more comments, so that it also helps some future visitors. Here's the code is taken from there:
#define LOG_TAG "binder_demo"
#include <stdlib.h>
#include "utils/RefBase.h"
#include "utils/Log.h"
#include "utils/TextOutput.h"
#include <binder/IInterface.h>
#include <binder/IBinder.h>
#include <binder/ProcessState.h>
#include <binder/IServiceManager.h>
#include <binder/IPCThreadState.h>
using namespace android;
#define INFO(...) \
do { \
printf(__VA_ARGS__); \
printf("\n"); \
LOGD(__VA_ARGS__); \
} while(0)
void assert_fail(const char *file, int line, const char *func, const char *expr) {
INFO("assertion failed at file %s, line %d, function %s:",
file, line, func);
INFO("%s", expr);
abort();
}
#define ASSERT(e) \
do { \
if (!(e)) \
assert_fail(__FILE__, __LINE__, __func__, #e); \
} while(0)
// Where to print the parcel contents: aout, alog, aerr. alog doesn't seem to work.
#define PLOG aout
// Interface (our AIDL) - Shared by server and client
class IDemo : public IInterface {
public:
enum {
ALERT = IBinder::FIRST_CALL_TRANSACTION,
PUSH,
ADD
};
// Sends a user-provided value to the service
virtual void push(int32_t data) = 0;
// Sends a fixed alert string to the service
virtual void alert() = 0;
// Requests the service to perform an addition and return the result
virtual int32_t add(int32_t v1, int32_t v2) = 0;
DECLARE_META_INTERFACE(Demo); // Expands to 5 lines below:
//static const android::String16 descriptor;
//static android::sp<IDemo> asInterface(const android::sp<android::IBinder>& obj);
//virtual const android::String16& getInterfaceDescriptor() const;
//IDemo();
//virtual ~IDemo();
};
// Client
class BpDemo : public BpInterface<IDemo> {
public:
BpDemo(const sp<IBinder>& impl) : BpInterface<IDemo>(impl) {
LOGD("BpDemo::BpDemo()");
}
virtual void push(int32_t push_data) {
Parcel data, reply;
data.writeInterfaceToken(IDemo::getInterfaceDescriptor());
data.writeInt32(push_data);
aout << "BpDemo::push parcel to be sent:\n";
data.print(PLOG); endl(PLOG);
remote()->transact(PUSH, data, &reply);
aout << "BpDemo::push parcel reply:\n";
reply.print(PLOG); endl(PLOG);
LOGD("BpDemo::push(%i)", push_data);
}
virtual void alert() {
Parcel data, reply;
data.writeInterfaceToken(IDemo::getInterfaceDescriptor());
data.writeString16(String16("The alert string"));
remote()->transact(ALERT, data, &reply, IBinder::FLAG_ONEWAY); // asynchronous call
LOGD("BpDemo::alert()");
}
virtual int32_t add(int32_t v1, int32_t v2) {
Parcel data, reply;
data.writeInterfaceToken(IDemo::getInterfaceDescriptor());
data.writeInt32(v1);
data.writeInt32(v2);
aout << "BpDemo::add parcel to be sent:\n";
data.print(PLOG); endl(PLOG);
remote()->transact(ADD, data, &reply);
LOGD("BpDemo::add transact reply");
reply.print(PLOG); endl(PLOG);
int32_t res;
status_t status = reply.readInt32(&res);
LOGD("BpDemo::add(%i, %i) = %i (status: %i)", v1, v2, res, status);
return res;
}
};
//IMPLEMENT_META_INTERFACE(Demo, "Demo");
// Macro above expands to code below. Doing it by hand so we can log ctor and destructor calls.
const android::String16 IDemo::descriptor("Demo");
const android::String16& IDemo::getInterfaceDescriptor() const {
return IDemo::descriptor;
}
android::sp<IDemo> IDemo::asInterface(const android::sp<android::IBinder>& obj) {
android::sp<IDemo> intr;
if (obj != NULL) {
intr = static_cast<IDemo*>(obj->queryLocalInterface(IDemo::descriptor).get());
if (intr == NULL) {
intr = new BpDemo(obj);
}
}
return intr;
}
IDemo::IDemo() { LOGD("IDemo::IDemo()"); }
IDemo::~IDemo() { LOGD("IDemo::~IDemo()"); }
// End of macro expansion
// Server
class BnDemo : public BnInterface<IDemo> {
virtual status_t onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags = 0);
};
status_t BnDemo::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) {
LOGD("BnDemo::onTransact(%i) %i", code, flags);
data.checkInterface(this);
data.print(PLOG); endl(PLOG);
switch(code) {
case ALERT: {
alert(); // Ignoring the fixed alert string
return NO_ERROR;
} break;
case PUSH: {
int32_t inData = data.readInt32();
LOGD("BnDemo::onTransact got %i", inData);
push(inData);
ASSERT(reply != 0);
reply->print(PLOG); endl(PLOG);
return NO_ERROR;
} break;
case ADD: {
int32_t inV1 = data.readInt32();
int32_t inV2 = data.readInt32();
int32_t sum = add(inV1, inV2);
LOGD("BnDemo::onTransact add(%i, %i) = %i", inV1, inV2, sum);
ASSERT(reply != 0);
reply->print(PLOG); endl(PLOG);
reply->writeInt32(sum);
return NO_ERROR;
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}
class Demo : public BnDemo {
virtual void push(int32_t data) {
INFO("Demo::push(%i)", data);
}
virtual void alert() {
INFO("Demo::alert()");
}
virtual int32_t add(int32_t v1, int32_t v2) {
INFO("Demo::add(%i, %i)", v1, v2);
return v1 + v2;
}
};
// Helper function to get a hold of the "Demo" service.
sp<IDemo> getDemoServ() {
sp<IServiceManager> sm = defaultServiceManager();
ASSERT(sm != 0);
sp<IBinder> binder = sm->getService(String16("Demo"));
// TODO: If the "Demo" service is not running, getService times out and binder == 0.
ASSERT(binder != 0);
sp<IDemo> demo = interface_cast<IDemo>(binder);
ASSERT(demo != 0);
return demo;
}
int main(int argc, char **argv) {
if (argc == 1) {
LOGD("We're the service");
defaultServiceManager()->addService(String16("Demo"), new Demo());
android::ProcessState::self()->startThreadPool();
LOGD("Demo service is now ready");
IPCThreadState::self()->joinThreadPool();
LOGD("Demo service thread joined");
} else if (argc == 2) {
INFO("We're the client: %s", argv[1]);
int v = atoi(argv[1]);
sp<IDemo> demo = getDemoServ();
demo->alert();
demo->push(v);
const int32_t adder = 5;
int32_t sum = demo->add(v, adder);
LOGD("Addition result: %i + %i = %i", v, adder, sum);
}
return 0;
}
