Android源码(10) --- Binder(4) ServiceManager 启动

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纵观Binder通讯过程，无不在跟ServiceManager打交道，了解ServiceManager 交互流程就显得很有必要了。ServiceManager分为启动和、获取、添加、注册服务。首先从启动过程来了解,ServiceManager如何成为Binder守护进程。

源码路径
android/system/core/rootdir/init.rc
android/frameworks/native/cmds/servicemanager/|-- service_manager.c|-- binder.c
Binder内核

1 启动ServiceManager

1.Android在init进程启动以后，通过脚本init.rc,启动ServiceManager:

|– init.rc

service servicemanager /system/bin/servicemanagerclass coreuser systemgroup systemcriticalonrestart restart healthdonrestart restart zygoteonrestart restart mediaonrestart restart surfaceflingeronrestart restart drm

对应执行程序/system/bin/servicemanager，源码service_manager.c。下一步追踪一下service_manager.c的main()入口程序。

2.1 main

|– service_manager.c

int main(int argc, char **argv)
{struct binder_state *bs;// 打开Binder驱动..并申请128字节内存bs = binder_open(128*1024);if (!bs) {ALOGE("failed to open binder driver\n");return -1;}// 变成 Context Managerif (binder_become_context_manager(bs)) {ALOGE("cannot become context manager (%s)\n", strerror(errno));return -1;}// 判断 selinux 权限能否使用selinux_enabled = is_selinux_enabled();sehandle = selinux_android_service_context_handle();selinux_status_open(true);if (selinux_enabled > 0) {if (sehandle == NULL) {// 获取sehandle 权限失败ALOGE("SELinux: Failed to acquire sehandle. Aborting.\n");abort();}if (getcon(&service_manager_context) != 0) {// 获取service_manager 上下文失败ALOGE("SELinux: Failed to acquire service_manager context. Aborting.\n");abort();}}union selinux_callback cb;cb.func_audit = audit_callback;selinux_set_callback(SELINUX_CB_AUDIT, cb);cb.func_log = selinux_log_callback;selinux_set_callback(SELINUX_CB_LOG, cb);// 进入 binder 循环binder_loop(bs, svcmgr_handler);return 0;
}

mian 入口函数一共处理了三件事：
- 1.打开Binder驱动并申请内存
- 2.告诉Binder驱动成为Binder的上下文管理者（ServiceManager成为守护进程）
- 3.开启循环，处理IPC请求（等待Client请求）

2.2 binder_open

在ServiceManaer 的main入口第一步看到打开Binder驱动，调用了binder_open。binder_open又做了哪些具体的工作呢？

|– binder.c

struct binder_state *binder_open(size_t mapsize)
{struct binder_state *bs; // 初始化状态参数struct binder_version vers;bs = malloc(sizeof(*bs));if (!bs) {errno = ENOMEM;return NULL;}//打开 Binder 设备驱动bs->fd = open("/dev/binder", O_RDWR);if (bs->fd < 0) {fprintf(stderr,"binder: cannot open device (%s)\n",strerror(errno));goto fail_open; //打开失败}//系统调用 ioctl获取binder版本信息if ((ioctl(bs->