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Android中的属性主要用来保存一些全局性的信息，这里可以理解为Android中的“注册表”。Android中的属性服务只针对系统开发者使用，并不对应用开发者开发，这通过SystemProperties是hide的可以看出。下面让我们一起来剖析属性服务。
初始化属性空间
在init进程启动一文中我们讲到，init的其中一个作用就是启动系统属性服务。在init进程的main()函数中有这样一句：
@system/core/init/init.c
property_init();//属性服务初始化
property_init（）的实现如下：
@system/core/init/property_service.c
void property_init(void)
{
init_property_area();
}
从字面意思来看init_property_area是用来初始化属性存储区域，让我们来看一下：
@system/core/init/property_service.c
static int init_property_area(void)
{
if (property_area_inited)//属性区域已初始化则直接返回，不再初始化
return -1;
if(__system_property_area_init())//通过mmap创建共享内存
return -1;
if(init_workspace(&pa_workspace, 0))//初始化pa_workspace
return -1;
fcntl(pa_workspace.fd, F_SETFD, FD_CLOEXEC);
property_area_inited = 1;
return 0;
}
通过上面的代码我们可以知道，属性服务是创建在共享内存上的，通过共享内存实现跨进程的访问。那共享内存是如何创建的呢？来看一下__system_property_area_init的实现：
@/bionic/libc/bionic/system_properties.c
int __system_property_area_init()
{
return map_prop_area_rw();
}
static int map_prop_area_rw()
{
prop_area *pa;
int fd;
int ret;
/* dev is a tmpfs that we can use to carve a shared workspace
* out of, so let's do that...
*/
fd = open(property_filename, O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC |
O_EXCL, 0444);//打开property_filename文件，即：/dev/__properties__
if (fd < 0) {
if (errno == EACCES) {
/* for consistency with the case where the process has already
* mapped the page in and segfaults when trying to write to it
*/
abort();
}
return -1;
}
ret = fcntl(fd, F_SETFD, FD_CLOEXEC);//这里设置为FD_CLOEXEC表示当程序执行exec函数时本fd将被系统自动关闭,表示不传递给exec创建的新进程
if (ret < 0)
goto out;
if (ftruncate(fd, PA_SIZE) < 0)//更改fd指向文件的大小为PA_SIZE(128 * 1024)大小
goto out;
pa_size = PA_SIZE;//设置属性空间的大小为PA_SIZE(128 * 1024)
pa_data_size = pa_size - sizeof(prop_area);//属性空间中可以用来保存属性的区域的大小，prop_area用来保存属性空间自身的一些信息
compat_mode = false;
pa = mmap(NULL, pa_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);//映射属性文件到进程
if(pa == MAP_FAILED)
goto out;
memset(pa, 0, pa_size);//初始化属性区域
pa->magic = PROP_AREA_MAGIC;
pa->version = PROP_AREA_VERSION;
/* reserve root node */
pa->bytes_used = sizeof(prop_bt);
/* plug into the lib property services */
__system_property_area__ = pa;
close(fd);
return 0;
out:
close(fd);
return -1;
}
上面的内容比较简单，不过最后的赋值语句可是大有来头。_system_property_area_是bionic libc库中输出的一个变量，为什么这里要给她赋值呢？
原来，虽然属性区域是由init进程创建的，但是Android系统希望其他进程也能够读取这块内存的东西。为了做到这一点，它便做了一下两项工作：
- 把属性区域创建在共享内存上，而共享内存是可以跨进程的。
- 如何让其他进程知道这个共享内存呢？Android利用gcc的constructor属性，这个属性指明了一个_libc_prenit函数，当bionic libc库被加载时，将自动调用这个_libc_prenit，这个函数内部完成共享内存到本地进程的映射工作。（这一段说明转自：《深入理解Android：卷1》）
关于上面的内容来看下面的代码：
@/bionic/libc/bionic/libc_init_dynamic.cpp
// We flag the __libc_preinit function as a constructor to ensure
// that its address is listed in libc.so's .init_array section.
// This ensures that the function is called by the dynamic linker
// as soon as the shared library is loaded.
__attribute__((constructor)) static void __libc_preinit() {
// Read the kernel argument block pointer from TLS.
void* tls = const_cast<void*>(__get_tls());
KernelArgumentBlock** args_slot = &reinterpret_cast<KernelArgumentBlock**>(tls)[TLS_SLOT_BIONIC_PREINIT];
KernelArgumentBlock* args = *args_slot;
// Clear the slot so no other initializer sees its value.
// __libc_init_common() will change the TLS area so the old one won't be accessible anyway.
*args_slot = NULL;
__libc_init_common(*args);
// Hooks for the debug malloc and pthread libraries to let them know that we're starting up.
pthread_debug_init();
malloc_debug_init();
}
@/bionic/libc/bionic/libc_init_common.cpp
void __libc_init_common(KernelArgumentBlock& args) {
// Initialize various globals.
environ = args.envp;
errno = 0;
__libc_auxv = args.auxv;
__progname = args.argv[0] ? args.argv[0] : "<unknown>";
__abort_message_ptr = args.abort_message_ptr;
// AT_RANDOM is a pointer to 16 bytes of randomness on the stack.
__stack_chk_guard = *reinterpret_cast<uintptr_t*>(getauxval(AT_RANDOM));
// Get the main thread from TLS and add it to the thread list.
pthread_internal_t* main_thread = __get_thread();
main_thread->allocated_on_heap = false;
_pthread_internal_add(main_thread);
__system_properties_init(); // Requires 'environ'.
}
@/bionic/libc/bionic/system_properties.c
int __system_properties_init()
{
return map_prop_area();
}
static int map_prop_area()
{
bool fromFile = true;
int result = -1;
int fd;
int ret;
fd = open(property_filename, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
if (fd >= 0) {
/* For old kernels that don't support O_CLOEXEC */
ret = fcntl(fd, F_SETFD, FD_CLOEXEC);
if (ret < 0)
goto cleanup;
}
if ((fd < 0) && (errno == ENOENT)) {
/*
* For backwards compatibility, if the file doesn't
* exist, we use the environment to get the file descriptor.
* For security reasons, we only use this backup if the kernel
* returns ENOENT. We don't want to use the backup if the kernel
* returns other errors such as ENOMEM or ENFILE, since it
* might be possible for an external program to trigger this
* condition.
*/
fd = get_fd_from_env();
fromFile = false;
}
if (fd < 0) {
return -1;
}
struct stat fd_stat;
if (fstat(fd, &fd_stat) < 0) {
goto cleanup;
}
if ((fd_stat.st_uid != 0)
|| (fd_stat.st_gid != 0)
|| ((fd_stat.st_mode & (S_IWGRP | S_IWOTH)) != 0)
|| (fd_stat.st_size < sizeof(prop_area)) ) {
goto cleanup;
}
pa_size = fd_stat.st_size;
pa_data_size = pa_size - sizeof(prop_area);
prop_area *pa = mmap(NULL, pa_size, PROT_READ, MAP_SHARED, fd, 0);
if (pa == MAP_FAILED) {
goto cleanup;
}
if((pa->magic != PROP_AREA_MAGIC) || (pa->version != PROP_AREA_VERSION &&
pa->version != PROP_AREA_VERSION_COMPAT)) {
munmap(pa, pa_size);
goto cleanup;
}
if (pa->version == PROP_AREA_VERSION_COMPAT) {
compat_mode = true;
}
result = 0;
__system_property_area__ = pa;
cleanup:
if (fromFile) {
close(fd);
}
return result;
}
对比上面map_prop_area_rw()和map_prop_area()这两个方法，我们可以发现他们在打开文件和进行映射时的不同。在map_prop_area_rw（）中：
static int map_prop_area_rw()
{
......
fd = open(property_filename, O_RDWR | O_CREAT | O_NOFOLLOW | O_CLOEXEC |
O_EXCL, 0444);
......
pa = mmap(NULL, pa_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);//映射属性文件到进程
......
}
在map_prop_area（）中：
static int map_prop_area()
{
......
fd = open(property_filename, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
......
prop_area *pa = mmap(NULL, pa_size, PROT_READ, MAP_SHARED, fd, 0);
......
}
map_prop_area_rw()是init进程初始化属性服务时调用到的；map_prop_area（）则是其他进程通过bionic初始化时调用的。通过对比，也说明只有init进程才拥有属性的写权限，其他进程只能读。好了，到这里又引出来另外一个问题，其他进程要想设置属性，应该怎么做呢？
属性服务
启动属性服务
在init进程的main()中，我们可以看见如下语句：
@system/core/init/init.c
queue_builtin_action(property_service_init_action, "property_service_init");
这句话的意思是启动action链表中的property服务：
static int property_service_init_action(int nargs, char **args)
{
/* read any property files on system or data and
* fire up the property service. This must happen
* after the ro.foo properties are set above so
* that /data/local.prop cannot interfere with them.
*/
start_property_service();
return 0;
}
start_property_service()的实现如下：
@system/core/init/property_service.c
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;
}
关于这段代码做一些说明：
在Android中定义了5个存储属性的文件，它们分别是：
@/bionic/libc/includes/sys/_system_properties.h
#define PROP_PATH_RAMDISK_DEFAULT "/default.prop" #define PROP_PATH_SYSTEM_BUILD "/system/build.prop" #define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop" #define PROP_PATH_LOCAL_OVERRIDE "/data/local.prop" #define PROP_PATH_FACTORY "/factory/factory.prop"
load_persistent_properties()用来加载persist开头的属性文件，这些属性文件是需要保存到永久介质上的，这些属性文件存储在/data/property目录下，并且文件名必须以“psesist."开头，下面是我的手机上的persist属性文件：
在start_property_service（）的最后创建了一个名为"property_service"的socket，启动监听，并将socket的句柄保存在property_set_fd中。
处理设置属性请求
接收属性设置请求的地方在init进程中：
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();
else if (ufds[i].fd == get_keychord_fd())
handle_keychord();
else if (ufds[i].fd == get_signal_fd())
handle_signal();
}
}
}
可以看到，在init接收到其他进程设置属性的请求时，会调用handle_property_set_fd()函数进程处理：
@system/core/init/property_service.c
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 receive 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 received: %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;
if (!is_legal_property_name(msg.name, strlen(msg.name))) {
ERROR("sys_prop: illegal property name. Got: \"%s\"\n", msg.name);
close(s);
return;
}
getpeercon(s, &source_ctx);
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);
}
freecon(source_ctx);
break;
default:
close(s);
break;
}
}
从上面的代码可以看出在进行一系列检查和判断后，最后悔调用property_set()函数设置属性，其实现如下：
int property_set(const char *name, const char *value)
{
prop_info *pi;
int ret;
size_t namelen = strlen(name);
size_t valuelen = strlen(value);
if (!is_legal_property_name(name, namelen)) return -1;
if (valuelen >= PROP_VALUE_MAX) return -1;
pi = (prop_info*) __system_property_find(name);
if(pi != 0) {
/* ro.* properties may NEVER be modified once set */
if(!strncmp(name, "ro.", 3)) return -1;
__system_property_update(pi, value, valuelen);
} else {
ret = __system_property_add(name, namelen, value, valuelen);
if (ret < 0) {
ERROR("Failed to set '%s'='%s'\n", name, value);
return ret;
}
}
/* If name starts with "net." treat as a DNS property. */
if (strncmp("net.", name, strlen("net.")) == 0) {
if (strcmp("net.change", name) == 0) {
return 0;
}
/*
* The 'net.change' property is a special property used track when any
* 'net.*' property name is updated. It is _ONLY_ updated here. Its value
* contains the last updated 'net.*' property.
*/
property_set("net.change", name);
} else if (persistent_properties_loaded &&
strncmp("persist.", name, strlen("persist.")) == 0) {
/*
* Don't write properties to disk until after we have read all default properties
* to prevent them from being overwritten by default values.
*/
write_persistent_property(name, value);
} else if (strcmp("selinux.reload_policy", name) == 0 &&
strcmp("1", value) == 0) {
selinux_reload_policy();
}
property_changed(name, value);
return 0;
}
从上面的代码可以看出：首先会判断所设置属性的属性名和属性值是否超过限制，这个限制定义如下：
@bionic/libc/includes/sys/system _properties.h
#define PROP_NAME_MAX 32 #define PROP_VALUE_MAX 92
可以看出属性名最长不能超过32个字符；属性值最长不能超过92个字符，到这里我们也可以根据之前map_prop_area_rw（）中的信息推断出，系统总共可以支持多少个属性：
pa_size = PA_SIZE;//设置属性空间的大小为PA_SIZE(128 * 1024)
pa_data_size = pa_size - sizeof(prop_area);//属性空间中可以用来保存属性的区域的大小，prop_area用来保存属性空间自身的一些信息
compat_mode = false;
按照这里的推断，系统支持1000多个属性的样子。这里与之前比较老的Android版本有所不同，在老的Android版本中系统最多支持的属性个数为247个。
在判断文新添加属性的合法性以后接下来会判断该属性是否以存在，如果已存在则更新属性的值即可，如果不存在则增加新的属性。
然后还有剩下的一些其他判断，这里就不再赘述。到时最后一句property_changed()引起了我的注意：
property_changed(name, value);
看到这里，你有没有想起些什么？是的，init.rc中类似于下面这样的句子就是在这里通过property_changed（）触发的。
on property:vold.decrypt=trigger_restart_min_framework
class_start main
on property:vold.decrypt=trigger_restart_framework
class_start main
class_start late_start
OK，到这里属性服务相关的东东基本介绍完了。接下来简单介绍一下如何设置系统属性。
设置系统属性
C代码中属性的设置是由libcutils库提供的，代码如下：
@system/core/libutils/properties.c
int property_set(const char *key, const char *value)
{
return __system_property_set(key, value);
}
@/bionic/libc/bionic/system_property
int __system_property_set(const char *key, const char *value)
{
int err;
prop_msg msg;
if(key == 0) return -1;
if(value == 0) value = "";
if(strlen(key) >= PROP_NAME_MAX) return -1;
if(strlen(value) >= PROP_VALUE_MAX) return -1;
memset(&msg, 0, sizeof msg);
msg.cmd = PROP_MSG_SETPROP;
strlcpy(msg.name, key, sizeof msg.name);
strlcpy(msg.value, value, sizeof msg.value);
err = send_prop_msg(&msg);
if(err < 0) {
return err;
}
return 0;
}
注意这里的msg.cmd = PROP_MSG_SETPROP，send_prop_msg（）的实现如下：
static int send_prop_msg(prop_msg *msg)
{
struct pollfd pollfds[1];
struct sockaddr_un addr;
socklen_t alen;
size_t namelen;
int s;
int r;
int result = -1;
s = socket(AF_LOCAL, SOCK_STREAM, 0);
if(s < 0) {
return result;
}
memset(&addr, 0, sizeof(addr));
namelen = strlen(property_service_socket);
strlcpy(addr.sun_path, property_service_socket, sizeof addr.sun_path);
addr.sun_family = AF_LOCAL;
alen = namelen + offsetof(struct sockaddr_un, sun_path) + 1;
if(TEMP_FAILURE_RETRY(connect(s, (struct sockaddr *) &addr, alen)) < 0) {
close(s);
return result;
}
r = TEMP_FAILURE_RETRY(send(s, msg, sizeof(prop_msg), 0));
if(r == sizeof(prop_msg)) {
// We successfully wrote to the property server but now we
// wait for the property server to finish its work. It
// acknowledges its completion by closing the socket so we
// poll here (on nothing), waiting for the socket to close.
// If you 'adb shell setprop foo bar' you'll see the POLLHUP
// once the socket closes. Out of paranoia we cap our poll
// at 250 ms.
pollfds[0].fd = s;
pollfds[0].events = 0;
r = TEMP_FAILURE_RETRY(poll(pollfds, 1, 250 /* ms */));
if (r == 1 && (pollfds[0].revents & POLLHUP) != 0) {
result = 0;
} else {
// Ignore the timeout and treat it like a success anyway.
// The init process is single-threaded and its property
// service is sometimes slow to respond (perhaps it's off
// starting a child process or something) and thus this
// times out and the caller thinks it failed, even though
// it's still getting around to it. So we fake it here,
// mostly for ctl.* properties, but we do try and wait 250
// ms so callers who do read-after-write can reliably see
// what they've written. Most of the time.
// TODO: fix the system properties design.
result = 0;
}
}
close(s);
return result;
}
设置属性的请求在这里被通过socket发送出去。
Java层设置属性的方法位于SystemProperties类中，在该类中实现了一系列set/get方法，Java代码中就是通过这些set/get方法设置和读取系统属性的。
@/framework/base/core/ java/android/os/SystemProperties.java
/**
* Set the value for the given key.
* @throws IllegalArgumentException if the key exceeds 32 characters
* @throws IllegalArgumentException if the value exceeds 92 characters
*/
public static void set(String key, String val) {
if (key.length() > PROP_NAME_MAX) {
throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX);
}
if (val != null && val.length() > PROP_VALUE_MAX) {
throw new IllegalArgumentException("val.length > " +
PROP_VALUE_MAX);
}
native_set(key, val);
}
注意到这里最终会调用到native的set方法。
private static native void native_set(String key, String def);
看到这里，接下来会调到那里去，相信大家应该都清楚了。。。