Android系统中prop详解
16lz
2021-01-24
文章出处：http://blog.csdn.net/shift_wwx/article/details/9788451
请转载的朋友表明出处~~
Android 中很多时候会用到prop的操作，这样的话在系统操作的时候可能会更方便一点。这篇文章就是关于prop的详细使用，讲解的可能还是不全，但是还是希望给大家带来一点帮助。也希望能得到一些建议。
主要分两部分：
1. SystemProperties 类
2. 终端prop命令
首先来看一下SystemProperties类的source code。
package android.os;import java.util.ArrayList;import android.util.Log;/** * Gives access to the system properties store. The system properties * store contains a list of string key-value pairs. * * {@hide} */public class SystemProperties{ public static final int PROP_NAME_MAX = 31; public static final int PROP_VALUE_MAX = 91; private static final ArrayList sChangeCallbacks = new ArrayList(); private static native String native_get(String key); private static native String native_get(String key, String def); private static native int native_get_int(String key, int def); private static native long native_get_long(String key, long def); private static native boolean native_get_boolean(String key, boolean def); private static native void native_set(String key, String def); private static native void native_add_change_callback(); /** * Get the value for the given key. * @return an empty string if the key isn't found * @throws IllegalArgumentException if the key exceeds 32 characters */ public static String get(String key) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } return native_get(key); } /** * Get the value for the given key. * @return if the key isn't found, return def if it isn't null, or an empty string otherwise * @throws IllegalArgumentException if the key exceeds 32 characters */ public static String get(String key, String def) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } return native_get(key, def); } /** * Get the value for the given key, and return as an integer. * @param key the key to lookup * @param def a default value to return * @return the key parsed as an integer, or def if the key isn't found or * cannot be parsed * @throws IllegalArgumentException if the key exceeds 32 characters */ public static int getInt(String key, int def) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } return native_get_int(key, def); } /** * Get the value for the given key, and return as a long. * @param key the key to lookup * @param def a default value to return * @return the key parsed as a long, or def if the key isn't found or * cannot be parsed * @throws IllegalArgumentException if the key exceeds 32 characters */ public static long getLong(String key, long def) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } return native_get_long(key, def); } /** * Get the value for the given key, returned as a boolean. * Values 'n', 'no', '0', 'false' or 'off' are considered false. * Values 'y', 'yes', '1', 'true' or 'on' are considered true. * (case sensitive). * If the key does not exist, or has any other value, then the default * result is returned. * @param key the key to lookup * @param def a default value to return * @return the key parsed as a boolean, or def if the key isn't found or is * not able to be parsed as a boolean. * @throws IllegalArgumentException if the key exceeds 32 characters */ public static boolean getBoolean(String key, boolean def) { if (key.length() > PROP_NAME_MAX) { throw new IllegalArgumentException("key.length > " + PROP_NAME_MAX); } return native_get_boolean(key, def); } /** * 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); } public static void addChangeCallback(Runnable callback) { synchronized (sChangeCallbacks) { if (sChangeCallbacks.size() == 0) { native_add_change_callback(); } sChangeCallbacks.add(callback); } } static void callChangeCallbacks() { synchronized (sChangeCallbacks) { //Log.i("foo", "Calling " + sChangeCallbacks.size() + " change callbacks!"); if (sChangeCallbacks.size() == 0) { return; } ArrayList callbacks = new ArrayList(sChangeCallbacks); for (int i=0; i 因此，要用到SystemProperties类的时候首先用import android.os.SystemProperties，其他的接口很明了。
至于native的接口，当然是通过jni调用的底层的呀，详细可以查看android_os_SystemProperties.cpp就知道了，例如prop set就调用了jni的static void SystemProperties_set函数，最终通过err = property_set(key, val);调用底层的接口函数，这个接口从哪里调用的，需要注意头文件了：
#include "cutils/properties.h"@/system/core/include/cutils/properties.h
/* property_set: returns 0 on success, < 0 on failure*/int property_set(const char *key, const char *value);int property_set(const char *key, const char *value){ return __system_property_set(key, value);}@/bionic/libc/bionic/system_properties.c
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;} 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(); } }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; }}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(strcmp(name, "ro.ubootenv.varible.prefix") == 0) { int vlen = (valuelen < 30) ? valuelen : 30; memcpy(uboot_var_prefix, value, vlen); uboot_var_prefix[vlen] = '.'; } } /* 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;} if (!is_legal_property_name(name, namelen)) return -1; if (valuelen >= PROP_VALUE_MAX) return -1;#define PROP_NAME_MAX 32#define PROP_VALUE_MAX 92map_prop_area_rw会定义：
pa_size = PA_SIZE; pa_data_size = pa_size - sizeof(prop_area); compat_mode = false;到这里就基本介绍了从setprop到处理prop的过程，可是有一点奇怪，sendmsg是通过什么连接的呢？
还是从init.c入手：
queue_builtin_action(property_service_init_action, "property_service_init");@/syste/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;}总结一下过程：
1、init 起来的时候会做property_init，对prop存储空间做了限定
2、创建了一个socket，然后对这个fd做了监听，init里面会有个for一直在listen知道handle_property_set_fd触发
3、应用通过SystemProperties这个类做prop的相关动作，例如setprop触发了handle_property_set_fd
到这里就可以知道android 上层通过SystemProperties类调用setprop的整个过程。
接下来，看一下终端命令setprop、getprop、watchprops。
在init.c文件main函数中会调用start_property_service()，而它分别调用load_properties_from_file函数读取PROP_PATH_SYSTEM_BUILD（/system/build.prop）、PROP_PATH_SYSTEM_DEFAULT（/system/default.prop）和PROP_PATH_LOCAL_OVERRIDE（/data/local.prop）存放系统属性的文件并设置到系统属性。
编译时由build/tool/buildinfo.sh文件写到文件build.prop，修改系统默认属性一般是改build/tool/buildinfo.sh文件
在android系统中，有一些初始化的配置文件，例如:
/init.rc
/default.prop
/system/build.prop
文件里面里面配置了开机设置的系统属性值，
这些属性值，可以通过getprop获取，setprop设置，
它的格式如下：
getprop [key]
获取指定key的配置值，如果不带参数，只是getprop则是显示系统所有的配置值。
[dalvik.vm.heapsize]: [24m]
[curlockscreen]: [1]
[ro.sf.hwrotation]: [0]
[ro.config.notification_sound]: [OnTheHunt.ogg]
[ro.config.alarm_alert]: [Alarm_Classic.ogg]
setprop [key] [value] 设置指定key的属性值， watchprops 监听系统属性的变化，如果期间系统的属性发生变化则把变化的值显示出来 /system # watchprops 1307501833 sys.settings_system_version = '37' 1307501836 sys.settings_system_version = '38' 1307501862 persist.sys.timezone = 'Asia/Hong_Kong' 其实这三个命令都是toolbox的子命令，如果有兴趣的可以看在android源码中看到其对应的源码：system/core/toolbox/toolbox命令主要完成扩展命令的执行，每一个扩展命令对应一个name_main函数，如ls命令，对应ls_main函数。同时，每一个扩展命令都由一个system/core/toolbox/目录下面的.c文件实现。toolbox.c会根据这个目录下面的.c文件生成tools.h头文件，并在system/core/toolbox/Android.mk文件中为每个命令生成指向toolbox的连接。toolbox的实现结构使它扩展一个命令很容易。
假设现在我们自己想手工添加一个shell命令mycommand，只要在system/core/toolbox/目录下面新建一个mycommand.c文件，并在里面实现一个mycommand_main函数，然后在system/core/toolbox/Android.mk中添加mycommand.c即可。Android.mk会自动把它编译进toolbox程序，并在编译生成的Android系统/system/bin目录下为这个命令生成一个指向toolbox的连接。
以setprop这个命令为例：在system/core/toolbox目录下会有个setprop.c这个文件， source code如下：
#include #include int setprop_main(int argc, char *argv[]){ if(argc != 3) { fprintf(stderr,"usage: setprop \n"); return 1; } if(property_set(argv[1], argv[2])){ fprintf(stderr,"could not set property\n"); return 1; } return 0;} 不难分析出来，setprop这个命令是需要3个参数，一个命令本身，一个是prop name，一个是prop value，如果不是三个就会报错usage: setprop
而如果参数正确的话就会调用property_set这个函数，传入的值分别是prop name 和prop value。
同样的，根据code可以看出会调用system/core/libcutils/properties.c下面的接口，与之前说的是一样的。
最后还是会通过init.c对prop 的fd做轮询。
至于prop的存储，从bionic/libc/include/sys/_system_properties.h就可以看出来：
#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"
而通过上面code中write_persistent_property函数就可以知道一般persist.xx等prop是存储在/data/property下，具体code如下：
static void write_persistent_property(const char *name, const char *value){ const char *tempPath = PERSISTENT_PROPERTY_DIR "/.temp"; char path[PATH_MAX]; int fd, length; snprintf(path, sizeof(path), "%s/%s", PERSISTENT_PROPERTY_DIR, name); fd = open(tempPath, O_WRONLY|O_CREAT|O_TRUNC, 0600); if (fd < 0) { ERROR("Unable to write persistent property to temp file %s errno: %d\n", tempPath, errno); return; } write(fd, value, strlen(value)); close(fd); if (rename(tempPath, path)) { unlink(tempPath); ERROR("Unable to rename persistent property file %s to %s\n", tempPath, path); }}过程感觉还是不够详细，后期碰到再补充吧。
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