Introduction to the Android system startup process

 1. Introduction to the System Startup Process

After the Linux kernel is started, init 1 (process 1) will serve as the first user space (the Linux virtual memory size is 232 (on a 32-bit x86 machine ), the kernel divides the space of 4G bytes into two parts. The maximum 1 GB is used by the kernel, which is called "kernel space ". The lower 3G bytes are used by each process, which is called "user space ".) Android system. The startup process can be divided into the following five stages, for example:

 

(1) Start preparation: this stage includes creating the basic directory of the file system, enabling the basic input and output devices, and initializing the log function;

(2) parsing the init. RC file: This phase parses the init. RC Script file, mainly for service and action parsing. The Service is composed of a command and a series of additional services (options). For example, "service vold/system/bin/vold" is a service, "socket vold stream 0660 root Mount" is the option used with the service; action is composed of a series of commands, such: "On init mkdir/System" is the action for creating the System Folder during system initialization;

(3) trigger the action to be executed: The action needs to be called in triggers (triggers condition). In this stage, the action to be executed is triggered, add the action to be executed to the action Queue according to the trigger condition;

(4) execute the command in the Action queue: Execute the action and service triggered in the previous stage. In this process, two very important processes, zygote and Service Manager, are derived;

(5) Cyclic processing events: the INIT process enters an infinite loop to handle device insertion/removal, service attribute status changes, and signal events.

Ii. source code analysis results

Analyze the following source code files in Android-2.1:

(1) Init. C: the path is system/CORE/init. C.

(2) parser. C: Path: System/CORE/init/parser. c

(3) builtins. C: the path is system/CORE/init/builtins. C.

(4) property_service.c: the path is system/CORE/init/property_service.c.

(5) keycords. C: the path is system/CORE/init/keycords. C.

(6) signal_handler.c: Path: System/CORE/init/signal_handler.c

The source code files and functions corresponding to the system startup process are summarized as follows:

2.1 Phase 1 (preparation) system/CORE/init. c

The specific function execution process is as follows:

Mkdir-> Mount-> open_devnull_stdio-> log_init

2.2 Phase 2 (parsing the init. RC file)

The specific function call process is as follows:

Init_parse_config_file-> parse_config-> parse_new_section-> parse_service (or parse_action)-> Parse_line_service (or parce_line_action)

2.3 Stage 3 (trigger the action to be executed)

The specific call process is as follows:

Action_for_each_trigger ("early-init", action_add_queue_tail);-> action_add_queue_tail (class_start default)-> drain_action_queue ();-> action_remove_queue_head-> Do_class_start

2.4 Stage 4 (execute commands in the Action Queue)

The specific call process is as follows:

Execute_one_command-> action_remove_queue_head-> do_class_start-> service_for_each_class-> Service_start_if_not_disabled-> service_start

2.5 Stage 5 (Cyclic processing)

The specific cyclic processing process is as follows:

 
 

  
  
for (; ;) { 
  
  
poll > handle_property_set_fd > handle_keychord > handle_signal 
  
  
} 
 
 

2.6 Main Functions

Function Name	File	Function Overview
Main	System/CORE/init. c	The entry function of process init 1. This module mainly analyzes the init. RC configuration file, executes basic actions and starts necessary native services, and then enters an infinite loop to process events from property and signal.
Mkdir	System/CORE/init. c	Create a basic directory for the File System
Mount	System/CORE/init. c	Mount a File System
Open_devnull_stdio	System/CORE/init. c	Enable basic input and output devices
Log_init	System/CORE/init. c	Initialize the log feature
Init_parse_config_file	System/CORE/init/parser. c	Read the content of the init. RC file to the memory data zone.
Parse_config	System/CORE/init/parser. c	Identify section (Service and action series) and text in the init. RC file
Parse_new_section	System/CORE/init/parser. c	Recognize section categories
Parse_service	System/CORE/init/parser. c	Analyze the first line of Service Section
Parse_line_service	System/CORE/init/parser. c	Analyze the option options of Service Section
Parse_action	System/CORE/init/parser. c	Analyze the first line of the Action Section
Parse_line_action	System/CORE/init/parser. c	Analyze each line of independent commands in the Action Section
Action_for_each_trigger	System/CORE/init/parser. c	Trigger the execution of an action
Action_add_queue_tail	System/CORE/init/parser. c	Add an action from action_list to action_queue
Execute_one_command	System/CORE/init. c	Execute a command for the current action
Action_remove_queue_head	System/CORE/init/parser. c	Remove the header node (Action) from the action_queue linked list)
Do_class_start	System/CORE/init/builtins. c	The entry function corresponding to class_start default, which is mainly used to start native service.
Service_for_each_class	System/CORE/init/parser. c	Traverse all nodes on the service_list linked list
Service_start_if_not_disabled	System/CORE/init/builtins. c	Determine whether the flag of the service is disabled. If not, call the relevant function to start the service.
Service_start	System/CORE/init. c	Start the main entry function of the service, set the relevant data structure of the Service, call fork to create a new operation, and then call execve to execute the new service
Fork	Lib function (ulibc)	Process Creation Function
Execve	Lib function (ulibc)	Call to execute the new service
Poll	Lib function (ulibc)	Queries whether the property_set_fd, signal_fd, and keychord_fd file handles have service requests.
Handle_property_set_fd	System/CORE/init/property_service.c	Process System property service requests, such as service, WLAN, and DHCP.
Handle_keychord	System/CORE/init/keycords. c	Process keychord registered on the service structure, usually starting the service
Handle_signal	System/CORE/init/signal_handler.c	Processing sigchld Signal