Friday, May 22, 2009

How to write Android native service by C contntinue

Here is some thing I did; first, I copied biner.c to my private directory and used it as a library to access kernel binder driver. And then, I wrote my service to use the functions in binder.c to talk to service manager.
In order to talk with service manager, we need following things:
1. an unique id for the service we are going to build. It is a string16
2. add permission for our service to service manager.

For the point 1, I create an uint16 array like
uint16_t svcmgr_id16[] = {

for the point 2, I'm kind of lazy, I add a new item to the allowd[] in servicemanager. It looks like this:
{10001, "Binder.Test.IBindertest"}

Ok, lets get something real, we need to do following steps to make everything to work:
1. connect to the binder by calling binder_open function. The binder_open function creates a shared memory with /dev/binder. I copied service manager code to create a 128K shared memory.
2.create an io block through bio_init function to talk with service manager through binder. You need to provide a buff to bio_init. I use an integer array with size of 128
3. fill in the target (servicemanager) and source(Binder.Test.IBindertest) by calling bio_put_string16_x
4. call binder_call with command SVC_MGR_ADD_SERVICE. It is a blocking call. You need to provide a binder_io object for the result.
5. if binder_call returns 0, call binder_loop with a message handler callback function to process messages sent from service client.

The message processing callback function:
The prototype of the message processing callback functions is:
static int binder_handler(struct binder_state *bs,
struct binder_txn *txn,
struct binder_io *msg,
struct binder_io *reply)

In the callback function, we at least need to handle INTERFACE_TRANSACTION so that our service client can find our service.
switch(txn->code) {
case :
error handling.


  1. I'm trying to do something similar. I have a C++ service which builds nicely and all but I can't get it to hook to android so that it show up when I do a listing of services.

    Could you supply the code?

  2. Same here, how do you actually access the Android context from your service?

    Can you post the code? Thanks!