Binder进程通信中的DNS

Binder进程通信中的DNS -----ServiceManager

如同互联网中的"DNS"需要在浏览各种网页之前就必须准备就绪，同样ServiceManager也需要有人使用Binder之前就必须处于正常状态。

接下来我们简单介绍ServiceManager。

1.何时启动

既然需要有人在使用Binder之前就需要准备就绪，那么很自然的就会想到Android系统上电init进程起来之后解析init.rc时候启动的，事实证明的确如此。 system/core/rootdir/init.rc

service servicemanager /system/bin/servicemanager class core user system group system critical onrestart restart healthd onrestart restart zygote onrestart restart media onrestart restart surfaceflinger onrestart restart drm

从上面的代码可以知道，如果servicemanager遇到问题重启后，healthd、zygote、media、surfaceflinger、drm也会跟着重启。

2.ServiceManager源码在哪儿

frameworks\native\cmds\servicemanager

3.启动之后主要负责什么工作

int main(int argc, char **argv) { struct binder_state *bs; bs = binder_open(128*1024);//打开Binder设备 if (!bs) { ALOGE("failed to open binder driver\n"); return -1; } if (binder_become_context_manager(bs)) {//将自己设置成Binder大管家，整个Android只允许一个servicemanager存在 ALOGE("cannot become context manager (%s)\n", strerror(errno)); return -1; } selinux_enabled = is_selinux_enabled(); sehandle = selinux_android_service_context_handle(); selinux_status_open(true); if (selinux_enabled > 0) { if (sehandle == NULL) { ALOGE("SELinux: Failed to acquire sehandle. Aborting.\n"); abort(); } if (getcon(&service_manager_context) != 0) { 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); binder_loop(bs, svcmgr_handler);//进入loop循环，等待用户请求 return 0; }

从上述代码知道，main函数主要做了如下几件事：

1>打开Binder设备

frameworks\native\cmds\servicemanager\binder.c

struct binder_state *binder_open(size_t mapsize) { struct binder_state *bs; struct binder_version vers; bs = malloc(sizeof(*bs)); if (!bs) { errno = ENOMEM; return NULL; } bs->fd = open("/dev/binder", O_RDWR);//打开Binder驱动节点 if (bs->fd < 0) { fprintf(stderr,"binder: cannot open device (%s)\n", strerror(errno)); goto fail_open; } if ((ioctl(bs->fd, BINDER_VERSION, &vers) == -1) || (vers.protocol_version != BINDER_CURRENT_PROTOCOL_VERSION)) { fprintf(stderr, "binder: kernel driver version (%d) differs from user space version (%d)\n", vers.protocol_version, BINDER_CURRENT_PROTOCOL_VERSION); goto fail_open; } bs->mapsize = mapsize; bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0); if (bs->mapped == MAP_FAILED) { fprintf(stderr,"binder: cannot map device (%s)\n", strerror(errno)); goto fail_map; } return bs; fail_map: close(bs->fd); fail_open: free(bs); return NULL; }

2>将自己设置成Binder大管家

frameworks\native\cmds\servicemanager\binder.c

int binder_become_context_manager(struct binder_state *bs) { return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0); }

3>进入主循环

frameworks\native\cmds\servicemanager\binder.c

void binder_loop(struct binder_state *bs, binder_handler func) { int res; struct binder_write_read bwr; uint32_t readbuf[32]; bwr.write_size = 0; bwr.write_consumed = 0; bwr.write_buffer = 0; readbuf[0] = BC_ENTER_LOOPER; binder_write(bs, readbuf, sizeof(uint32_t));//往Binder写入信息 for (;;) { bwr.read_size = sizeof(readbuf); bwr.read_consumed = 0; bwr.read_buffer = (uintptr_t) readbuf; res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);//从Binder读取消息 if (res < 0) { ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno)); break; } res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);//处理这条消息 if (res == 0) { ALOGE("binder_loop: unexpected reply?!\n"); break; } if (res < 0) { ALOGE("binder_loop: io error %d %s\n", res, strerror(errno)); break; } } }