Android如何生成设备节点

在Android中，由于没有mdev和udev，所以它没有办法动态的生成设备节点，那么它是如何做的呢？

我们可以在system/core/init/下的init.c和devices.c中找到答案：

init.c中

                    intmain(intargc,char**argv)
            {
            ...
            
            /*Getthebasicfilesystemsetupweneedput
            *togetherintheinitramdiskon/andthenwe'll
            *letthercfilefigureouttherest.
            */
            mkdir("/dev",0755);
            mkdir("/proc",0755);
            mkdir("/sys",0755);
            
            mount("tmpfs","/dev","tmpfs",0,"mode=0755");
            mkdir("/dev/pts",0755);
            mkdir("/dev/socket",0755);
            mount("devpts","/dev/pts","devpts",0,NULL);
            mount("proc","/proc","proc",0,NULL);
            mount("sysfs","/sys","sysfs",0,NULL);
            
            for(;;){
            ...
            if(ufds[0].revents==POLLIN)
            handle_device_fd(device_fd);
            
            if(ufds[1].revents==POLLIN)
            handle_property_set_fd(property_set_fd);
            if(ufds[3].revents==POLLIN)
            handle_keychord(keychord_fd);
            }
            
            return0;
            }

我们再来看看handle_device_fd()，该函数定义在devices.c中

                    voidhandle_device_fd(intfd)
            {
            ...
            handle_device_event(&uevent);
            handle_firmware_event(&uevent);
            }
            }

而handle_device_event定义如下：

                    staticvoidhandle_device_event(structuevent*uevent)
            {
            ...
            if(!strcmp(uevent->action,"add")){
            make_device(devpath,block,uevent->major,uevent->minor);
            return;
            }
            ...
            }

make_device定义如下：

                    staticvoidmake_device(constchar*path,intblock,intmajor,intminor)
            {
            ...
            mode=get_device_perm(path,&uid,&gid)|(block?S_IFBLK:S_IFCHR);
            dev=(major<<8)|minor;
            ...
            setegid(gid);
            mknod(path,mode,dev);
            chown(path,uid,-1);
            setegid(AID_ROOT);
            }

我们看看get_device_perm如下实现：

                    staticmode_tget_device_perm(constchar*path,unsigned*uid,unsigned*gid)
            {
            mode_tperm;
            
            if(get_device_perm_inner(qemu_perms,path,uid,gid,&perm)==0){
            returnperm;
            }elseif(get_device_perm_inner(devperms,path,uid,gid,&perm)==0){
            returnperm;
            }else{
            structlistnode*node;
            structperm_node*perm_node;
            structperms_*dp;
            
            /*Checkpartnerslist.*/
            list_for_each(node,&devperms_partners){
            perm_node=node_to_item(node,structperm_node,plist);
            dp=&perm_node->dp;
            
            if(dp->prefix){
            if(strncmp(path,dp->name,strlen(dp->name)))
            continue;
            }else{
            if(strcmp(path,dp->name))
            continue;
            }
            /*Foundperminpartnerlist.*/
            *uid=dp->uid;
            *gid=dp->gid;
            returndp->perm;
            }
            /*Defaultifnothingfound.*/
            *uid=0;
            *gid=0;
            return0600;
            }
            }

我们最后可以看到在devperms中定义了要生成的设备节点：

                    staticstructperms_devperms[]={
            {"/dev/null",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/zero",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/full",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/ptmx",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/tty",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/random",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/urandom",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/ashmem",0666,AID_ROOT,AID_ROOT,0},
            {"/dev/binder",0666,AID_ROOT,AID_ROOT,0},
            
            /*loggershouldbeworldwritable(forlogging)butnotreadable*/
            {"/dev/log/",0662,AID_ROOT,AID_LOG,1},
            
            /*themsmhw3dclientdevicenodeisworldwritable/readable.*/
            {"/dev/msm_hw3dc",0666,AID_ROOT,AID_ROOT,0},
            
            /*gpudriverforadreno200isgloballyaccessible*/
            {"/dev/kgsl",0666,AID_ROOT,AID_ROOT,0},
            
            /*theseshouldnotbeworldwritable*/
            {"/dev/diag",0660,AID_RADIO,AID_RADIO,0},
            {"/dev/diag_arm9",0660,AID_RADIO,AID_RADIO,0},
            {"/dev/android_adb",0660,AID_ADB,AID_ADB,0},
            {"/dev/android_adb_enable",0660,AID_ADB,AID_ADB,0},
            {"/dev/ttyMSM0",0600,AID_BLUETOOTH,AID_BLUETOOTH,0},
            {"/dev/ttyHS0",0600,AID_BLUETOOTH,AID_BLUETOOTH,0},
            {"/dev/uinput",0660,AID_SYSTEM,AID_BLUETOOTH,0},
            {"/dev/alarm",0664,AID_SYSTEM,AID_RADIO,0},
            {"/dev/tty0",0660,AID_ROOT,AID_SYSTEM,0},
            {"/dev/graphics/",0660,AID_ROOT,AID_GRAPHICS,1},
            {"/dev/msm_hw3dm",0660,AID_SYSTEM,AID_GRAPHICS,0},
            {"/dev/input/",0660,AID_ROOT,AID_INPUT,1},
            {"/dev/eac",0660,AID_ROOT,AID_AUDIO,0},
            {"/dev/cam",0660,AID_ROOT,AID_CAMERA,0},
            {"/dev/pmem",0660,AID_SYSTEM,AID_GRAPHICS,0},
            {"/dev/pmem_adsp",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/pmem_camera",0660,AID_SYSTEM,AID_CAMERA,1},
            {"/dev/oncrpc/",0660,AID_ROOT,AID_SYSTEM,1},
            {"/dev/adsp/",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/snd/",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/mt9t013",0660,AID_SYSTEM,AID_SYSTEM,0},
            {"/dev/msm_camera/",0660,AID_SYSTEM,AID_SYSTEM,1},
            {"/dev/akm8976_daemon",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/akm8976_aot",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/akm8973_daemon",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/akm8973_aot",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/bma150",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/cm3602",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/akm8976_pffd",0640,AID_COMPASS,AID_SYSTEM,0},
            {"/dev/lightsensor",0640,AID_SYSTEM,AID_SYSTEM,0},
            {"/dev/msm_pcm_out",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/msm_pcm_in",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/msm_pcm_ctl",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/msm_snd",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/msm_mp3",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/audience_a1026",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/tpa2018d1",0660,AID_SYSTEM,AID_AUDIO,1},
            {"/dev/msm_audpre",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/msm_audio_ctl",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/htc-acoustic",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/vdec",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/q6venc",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/snd/dsp",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/snd/dsp1",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/snd/mixer",0660,AID_SYSTEM,AID_AUDIO,0},
            {"/dev/smd0",0640,AID_RADIO,AID_RADIO,0},
            {"/dev/qemu_trace",0666,AID_SYSTEM,AID_SYSTEM,0},
            {"/dev/qmi",0640,AID_RADIO,AID_RADIO,0},
            {"/dev/qmi0",0640,AID_RADIO,AID_RADIO,0},
            {"/dev/qmi1",0640,AID_RADIO,AID_RADIO,0},
            {"/dev/qmi2",0640,AID_RADIO,AID_RADIO,0},
            /*CDMAradiointerfaceMUX*/
            {"/dev/ts0710mux",0640,AID_RADIO,AID_RADIO,1},
            {"/dev/ppp",0660,AID_RADIO,AID_VPN,0},
            {"/dev/tun",0640,AID_VPN,AID_VPN,0},
            {NULL,0,0,0,0},
            };

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