【android学习笔记】init.rc中声明的守护进程启动的流程

在Init.rc中，用service关键字声明了一系列服务.

init.rc对service的说明如下：（详见system/core/init/readme.txt）

Services--------Services are programs which init launches and (optionally) restartswhen they exit.  

Services take the form of:service <name> <pathname> [ <argument> ]*   <option>   <option>   ...

Options-------Options are modifiers to services.  

They affect how and when initruns the service.

critical   

This is a device-critical service. If it exits more than four times in   four minutes, the device will reboot into recovery mode.

disabled  

 This service will not automatically start with its class.   It must be explicitly started by name.

setenv <name> <value>   

Set the environment variable <name> to <value> in the launched process.

socket <name> <type> <perm> [ <user> [ <group> ] ]   

Create a unix domain socket named /dev/socket/<name> and pass   its fd to the launched process.  <type> must be "dgram", "stream" or "seqpacket".   

User and group default to 0.

user <username>   

Change to username before exec'ing this service.   Currently defaults to root.  (??? probably should default to nobody)   Currently, if your process requires linux capabilities then you cannot use   this command. You must instead request the capabilities in-process while   still root, and then drop to your desired uid.

group <groupname> [ <groupname> ]*   

Change to groupname before exec'ing this service.  Additional   groupnames beyond the (required) first one are used to set the   supplemental groups of the process (via setgroups()).   Currently defaults to root.  (??? probably should default to nobody)oneshot   Do not restart the service when it exits.

class <name>   

Specify a class name for the service.  All services in a   named class may be started or stopped together.  A service   is in the class "default" if one is not specified via the   class option.

onrestart    Execute a Command (see below) when service restarts.

可以看出，service中比较关键的几个选项是类别（class），启动选项（oneshot disabled,critical）,用户组（group,user）。

以surfaceflinger为例：

service surfaceflinger /system/bin/surfaceflinger    

class main    

user system    

group graphics    

onrestart restart zygote

它的类别是main，用户是system，属于graphics ，且没有声明disabled，所以在启动main这个类别的时候，surfaceflinger就会被启动。

main类别是在哪里启动的呢？

搜索class_start关键字，在On boot的时候，就会启动了

on boot

  #省略无关.....

 class_start core    

 class_start main

readme中对class_start有如下说明：

class_start <serviceclass> 

Start all services of the specified class if they are   not already running.

class_stop <serviceclass> 

Stop all services of the specified class if they are   currently runn