android ctl属性的实现过程
对于android开机动画的启动和停止，在代码中是通过调用property_service.c中的property_set实现的，代码如下：
property_set(“ctl.start”, “bootanim”);
property_set(“ctl.stop”, “bootanim”);
当然也可以通过命令行来控制，命令如下：
#setprop ctl.start bootanim
#setprop ctl.stop bootanim
其实通过控制ctl.start和ctl.stop这两个属性，不仅仅可以控制开机动画，还可以控制init.rc文件中定义的各种service。
那ctl.xxx是怎么样实现start或者stop一个service的呢？
Android的init进程在启动的过程当中会通过start_property_service函数创建一个属性服务的socket,这个socket描述符为/dev/socket/
property_service,系统中对属性的操作都会以请求的形式发给这个socket, start_property_service函数代码如下：
void start_property_service(void)
{
int fd;
load_properties_from_file(PROP_PATH_SYSTEM_BUILD);
load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT);
load_override_properties();
/* Read persistent properties after all default values have been loaded. */
load_persistent_properties();
fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0);
if(fd < 0) return;
fcntl(fd, F_SETFD, FD_CLOEXEC);
fcntl(fd, F_SETFL, O_NONBLOCK);
listen(fd, 8);
property_set_fd = fd;
}
那这些请求是在哪里被处理呢？也是在init进程当中。
Init进程启动的最后，会进入一个循环结构,代码如下:
for(;;) {
int nr, i, timeout = -1;
execute_one_command();
restart_processes();
if (!property_set_fd_init && get_property_set_fd() > 0) {
ufds[fd_count].fd = get_property_set_fd();
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
property_set_fd_init = 1;
}
if (!signal_fd_init && get_signal_fd() > 0) {
ufds[fd_count].fd = get_signal_fd();
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
signal_fd_init = 1;
}
if (!keychord_fd_init && get_keychord_fd() > 0) {
ufds[fd_count].fd = get_keychord_fd();
ufds[fd_count].events = POLLIN;
ufds[fd_count].revents = 0;
fd_count++;
keychord_fd_init = 1;
}
if (process_needs_restart) {
timeout = (process_needs_restart - gettime()) * 1000;
if (timeout < 0)
timeout = 0;
}
if (!action_queue_empty() || cur_action)
timeout = 0;
#if BOOTCHART
if (bootchart_count > 0) {
if (timeout < 0 || timeout > BOOTCHART_POLLING_MS)
timeout = BOOTCHART_POLLING_MS;
if (bootchart_step() < 0 || --bootchart_count == 0) {
bootchart_finish();
bootchart_count = 0;
}
}
#endif
nr = poll(ufds, fd_count, timeout);
if (nr <= 0)
continue;
for (i = 0; i < fd_count; i++) {
if (ufds[i].revents == POLLIN) {
if (ufds[i].fd == get_property_set_fd())
handle_property_set_fd(); //处理对属性的控制请求
else if (ufds[i].fd == get_keychord_fd())
handle_keychord();
else if (ufds[i].fd == get_signal_fd())
handle_signal();
}
}
}
这个循环体就一直在poll几个fd,其中有一个就是上面讲的属性服务的socket, handle_property_set_fd的代码如下:
void handle_property_set_fd()
{
prop_msg msg;
int s;
int r;
int res;
struct ucred cr;
struct sockaddr_un addr;
socklen_t addr_size = sizeof(addr);
socklen_t cr_size = sizeof(cr);
char * source_ctx = NULL;
if ((s = accept(property_set_fd, (struct sockaddr *) &addr, &addr_size)) < 0) {
return;
}
/* Check socket options here */
if (getsockopt(s, SOL_SOCKET, SO_PEERCRED, &cr, &cr_size) < 0) {
close(s);
ERROR("Unable to recieve socket options\n");
return;
}
r = TEMP_FAILURE_RETRY(recv(s, &msg, sizeof(msg), 0));
if(r != sizeof(prop_msg)) {
ERROR("sys_prop: mis-match msg size recieved: %d expected: %d errno: %d\n",
r, sizeof(prop_msg), errno);
close(s);
return;
}
switch(msg.cmd) {
case PROP_MSG_SETPROP:
msg.name[PROP_NAME_MAX-1] = 0;
msg.value[PROP_VALUE_MAX-1] = 0;
#ifdef HAVE_SELINUX
getpeercon(s, &source_ctx);
#endif
//如果是对ctl.xxx属性的请求，则最后会调用init.c文件定义的service_start或者service_stop函数来运行或者停止一个service
if(memcmp(msg.name,"ctl.",4) == 0) {
// Keep the old close-socket-early behavior when handling
// ctl.* properties.
close(s);
if (check_control_perms(msg.value, cr.uid, cr.gid, source_ctx)) {
handle_control_message((char*) msg.name + 4, (char*) msg.value);
} else {
ERROR("sys_prop: Unable to %s service ctl [%s] uid:%d gid:%d pid:%d\n",
msg.name + 4, msg.value, cr.uid, cr.gid, cr.pid);
}
} else {
if (check_perms(msg.name, cr.uid, cr.gid, source_ctx)) {
property_set((char*) msg.name, (char*) msg.value);
} else {
ERROR("sys_prop: permission denied uid:%d name:%s\n",
cr.uid, msg.name);
}
// Note: bionic's property client code assumes that the
// property server will not close the socket until *AFTER*
// the property is written to memory.
close(s);
}
#ifdef HAVE_SELINUX
freecon(source_ctx);
#endif
break;
default:
close(s);
break;
}
}
这样通过设置ctl.start或者ctl.stop属性就可以达到启动或者停止service的目的。