Binder communication process and servicemanager creation process

Binder Communication process

1. First , A process uses BINDER_SET_CONTEXT_MGR Command to pass Binder The driver registers itself as ServiceManager
2.Server Drive to ServiceManager Register in Binder（Server Medium Binder Entity ）, Indicates that services can be provided externally . The driver is this Binder Create entity nodes located in the kernel and ServiceManager References to entities , Package and pass the name and the new reference to ServiceManager,ServiceManger Fill it in the lookup table .
3.Client By name , stay Binder With the help of drive from ServiceManager Get a pair of Binder References to entities , Through this reference, and Server Process communication .

Binder Agent mode of communication
When A The process wants to get B In process object when , Driving doesn't really put object Return to A, But back to a heel object It looks as like as two peas objectProxy, This objectProxy With and object The same way , But these methods do not B In progress object The ability to target those methods , These methods only need to give the request parameters to the driver . about A Process and direct call object The method in is the same .
When Binder The driver receives A After the process message , Find out this is a objectProxy Just check the form you maintain , A search revealed that it was B process object Proxy object of . So I'll call B Process call object Methods , And ask the B The process sends the returned result to itself . When the driver gets B The return result of the process will be forwarded to A process , One communication is done .

ServiceManager The process starts

1.ServiceManager Become Binder managers
First, in the android When the system starts up, it passes init.rc Launched the servicemanager(Native process ) Executable process , The system really ServieManager Management finally passed Native process servicemanager To complete . Next, the analysis is in main That's what's done in the function ：

1.binder_open(128*1024) By opening the /dev/binder The device node returns fd File descriptor , adopt mmap Final realization binder drive 128K Memory mapping of size
2.binder_become_context_manager(bs) adopt ioctl Send to the device node BINDER_SET_CONTEXT_MGR Order notice Binder Drive as servicemanager Create a special invariant 0 Handle entity binder_node:binder_context_mgr_node

3.Binder_loop(bs,svcmgr_handler) adopt iotcl send out BINDER_WRITE_READ Command and carry BC_ENTER_LOOPER Request an order to inform Binder The driver created binder Main thread and make binder The driver creates the corresponding binder_thread The structure is stored in binder_proc in , And pass for Cycle into cycle read binder Drive the circular flow of returned information

ServiceManager（native process ） Finally, get binder Drive device node fd Address , And binder Drive memory mapping （ Virtual memory and physical memory mapping ）, Turn on binder The main thread reads circularly binder Drive the returned message and become the server ;
After becoming a server, it can continuously receive information from the client （ Server side And clients are collectively referred to as clients ） Of binder Request communication ,ServiceManager（native process ） Complete the registration of other services and save binder Driver created binder Handle value and service name string , Ensure that the client obtains other service reference handles through the string ; thus ServiceManager（native process ） by binder managers （ A bridge between service registration and acquisition ）.
2. Client pass ServiceManager Access to services
User processes need and ServiceManager（native process ） Process of communication ,ServiceManager After receiving the request, the process responds
1. The first step of the user process is to instantiate ServiceManager Binder Proxy agent

adopt BinderInternal.getContextObject() The handle value obtained by the call is 0 Of BpBinder native object , Finally through javaObjectForIbinder() Functional jni Convert to BinderProxy Class object is actually filled with this class mObject The object is ServiceManager Binder The reference of the object encapsulates the object , therefore SMN Can pass asInterface Complete the agency Proxy Instantiation