本章看看客户端是如何去操作属性的，这里主要分为 2 个部分：
- 服务端中属性的初始化过程由init完成，那客户端属性初始化是哪里做的呢
- 客户端如何发起属性操作
客户端属性初始化
客户端对属性初始化是由__system_properties_init()函数完成，其调用路径如下：
c
// 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.
// We give this constructor priority 1 because we want libc's constructor
// to run before any others (such as the jemalloc constructor), and lower
// is better (http://b/68046352).
__attribute__((constructor(1))) static void __libc_preinit() {
// The linker has initialized its copy of the global stack_chk_guard, and filled in the main
// thread's TLS slot with that value. Initialize the local global stack guard with its value.
__stack_chk_guard = reinterpret_cast<uintptr_t>(__get_tls()[TLS_SLOT_STACK_GUARD]);
__libc_preinit_impl();
}
__attribute__((noinline))
static void __libc_preinit_impl() {
...省略代码
__libc_init_common();
...省略代码
}
// bionic/libc/bionic/libc_init_common.cpp
void __libc_init_common() {
...省略代码
__system_properties_init(); // Requires 'environ'.
...省略代码
}我看看重点看看__libc_preinit函数的注释
c
// 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.
// We give this constructor priority 1 because we want libc's constructor
// to run before any others (such as the jemalloc constructor), and lower
// is better (http://b/68046352).__libc_preinit函数会被放到libc.so的init_array段中,这样就确保了共享库被加载时能尽可能快的调用__libc_preinit；constructor被设置为 1，这就意味着__libc_preinit被执行的优先级会非常高（值越低执行优先级越高）
总结：
- __system_properties_init在加载libc.so时被执行
- 所有应用程序无论c、c++、java，系统都会自动加载libc.so
这样应用程序就完成了属性的初始化。接下来看看__system_properties_init
c
// bionic/libc/bionic/system_property_api.cpp
__BIONIC_WEAK_FOR_NATIVE_BRIDGE
int __system_properties_init() {
return system_properties.Init(PROP_FILENAME) ? 0 : -1;
}看看又调用了system_property_api.cpp这个API文件中，里面最后还是调用我们前面说的大管家类SystemProperties。
c
bool SystemProperties::Init(const char* filename) {
// This is called from __libc_init_common, and should leave errno at 0 (http://b/37248982).
ErrnoRestorer errno_restorer;
if (initialized_) {
contexts_->ResetAccess();
return true;
}
if (strlen(filename) >= PROP_FILENAME_MAX) {
return false;
}
strcpy(property_filename_, filename);
if (is_dir(property_filename_)) {
if (access("/dev/__properties__/property_info", R_OK) == 0) {
contexts_ = new (contexts_data_) ContextsSerialized();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
} else {
contexts_ = new (contexts_data_) ContextsSplit();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
}
} else {
contexts_ = new (contexts_data_) ContextsPreSplit();
if (!contexts_->Initialize(false, property_filename_, nullptr)) {
return false;
}
}
initialized_ = true;
return true;
}这个contexts_->Initialize方法我们在"服务端属性文件创建和mmap映射"这篇文章中详细分析过，它完成如下功能：
- 加载"/dev/properties/property_info"文件的内容（前一章属性安全上下文序列化的内容）
- 创建ContextNode数组
- 创建属性文件，并mmap映射属性文件，地址保存到ContextNode中
这个过程其实就是在客户端进程初始化构建了橙色框中的数据结构，后续可以通过属性名查找到prop_area对象，进而读取到属性值。唯一需要注意的是，客户端进程只能读属性不能修改，所以第一个参数设置的false，标识prop_area映射的内存控件只读。
c
contexts_->Initialize(false, property_filename_, nullptr)而init进程设置的是true，是可以写的
c
bool SystemProperties::AreaInit(const char* filename, bool* fsetxattr_failed) {
...省略代码
contexts_ = new (contexts_data_) ContextsSerialized();
if (!contexts_->Initialize(true, property_filename_, fsetxattr_failed)) {
return false;
}
...省略代码
return true;
}属性操作
客户端可以通过如下代码操作属性
c
// system/core/libcutils/include/cutils/properties.h
// system/core/libcutils/properties.cpp
#include <cutils/properties.h>
int property_set(const char *key, const char *value)
int property_get(const char *key, char *value, const char *default_value)读属性
c
int property_get(const char *key, char *value, const char *default_value) {
int len = __system_property_get(key, value);
if (len > 0) {
return len;
}
if (default_value) {
len = strnlen(default_value, PROPERTY_VALUE_MAX - 1);
memcpy(value, default_value, len);
value[len] = '\0';
}
return len;
}
// bionic/libc/bionic/system_property_api.cpp
__BIONIC_WEAK_FOR_NATIVE_BRIDGE
int __system_property_get(const char* name, char* value) {
return system_properties.Get(name, value);
}property_get最后还是调用到大管家SystemProperties中的Get方法
c
int SystemProperties::Get(const char* name, char* value) {
const prop_info* pi = Find(name);
if (pi != nullptr) {
return Read(pi, nullptr, value);
} else {
value[0] = 0;
return 0;
}
}Get方法还是先通过属性名找到prop_info，然后读取里面的属性。
写属性
c
int property_set(const char *key, const char *value) {
return __system_property_set(key, value);
}前面看到调用的很多方法都是在system_property_api.cpp中，property_set有所不同，它调用的是system_property_set.cpp中方法
c
// bionic/libc/bionic/system_property_set.cpp
__BIONIC_WEAK_FOR_NATIVE_BRIDGE
int __system_property_set(const char* key, const char* value) {
if (key == nullptr) return -1;
if (value == nullptr) value = "";
if (g_propservice_protocol_version == 0) {
detect_protocol_version();
}
if (g_propservice_protocol_version == kProtocolVersion1) {//旧版本
....省略代码
} else {//走这里新版本
// New protocol only allows long values for ro. properties only.
if (strlen(value) >= PROP_VALUE_MAX && strncmp(key, "ro.", 3) != 0) return -1;
// Use proper protocol
PropertyServiceConnection connection;
if (!connection.IsValid()) {
errno = connection.GetLastError();
async_safe_format_log(
ANDROID_LOG_WARN, "libc",
"Unable to set property \"%s\" to \"%s\": connection failed; errno=%d (%s)", key, value,
errno, strerror(errno));
return -1;
}
SocketWriter writer(&connection);
if (!writer.WriteUint32(PROP_MSG_SETPROP2).WriteString(key).WriteString(value).Send()) {
errno = connection.GetLastError();
async_safe_format_log(ANDROID_LOG_WARN, "libc",
"Unable to set property \"%s\" to \"%s\": write failed; errno=%d (%s)",
key, value, errno, strerror(errno));
return -1;
}
int result = -1;
if (!connection.RecvInt32(&result)) {
errno = connection.GetLastError();
async_safe_format_log(ANDROID_LOG_WARN, "libc",
"Unable to set property \"%s\" to \"%s\": recv failed; errno=%d (%s)",
key, value, errno, strerror(errno));
return -1;
}
if (result != PROP_SUCCESS) {
async_safe_format_log(ANDROID_LOG_WARN, "libc",
"Unable to set property \"%s\" to \"%s\": error code: 0x%x", key, value,
result);
return -1;
}
return 0;
}
}- PropertyServiceConnection构造函数创建socket，并连接"/dev/socket/property_service"
- 然后调用如下代码发送请求
c
writer.WriteUint32(PROP_MSG_SETPROP2).WriteString(key).WriteString(value).Send()到这里客户端的写操作也就玩成了。