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我知道 pid(int) 在创建时由 os 系统分配给进程，这是进程的唯一标识符。uid(int) 是用户（或 android 中的应用程序）的唯一标识符。

  • Q1：什么时候创建一个uid并分配给一个应用程序？
  • Q2：它与AndroidManifest.xml中的'属性或其他属性有关系吗？我知道shareUerId通常是一个系统userId，例如。显然，sharedUserId是一个字符串。
  • Q3：系统uid(int)和shareUserId(string)的映射在哪里？frameworks\base\core\java\android\os 中有一个 UserId.java。可以看到，getUid(int userId, int appId) 方法可以从 userId 和 appId 中获取一个 uid。

  • Q4：这里的userId和appId是什么？

    public static final int getCallingUserId() { return getUserId(Binder.getCallingUid()); 从上面的代码中，我可以得出结论，当 IPC 时，Binder 使用 uid 而不是 UserId。如果是，那么 UserId 的功能是什么。

这里有这么多问题！我认为所有这些对于理解android的IPC都很重要。有人可以回答我的问题吗？最好将所有答案一一列出。提前非常感谢！

公共最终类用户 ID {

     /**
     * Range of IDs allocated for a user.
     *
     * @hide
     */
    public static final int PER_USER_RANGE = 100000;

    public static final int USER_ALL = -1;

    /**
     * Enable multi-user related side effects. Set this to false if there are problems with single
     * user usecases.
     * */
    public static final boolean MU_ENABLED = true;

    /**
     * Checks to see if the user id is the same for the two uids, i.e., they belong to the same
     * user.
     * @hide
     */
    public static final boolean isSameUser(int uid1, int uid2) {
        return getUserId(uid1) == getUserId(uid2);
    }

    /**
     * Checks to see if both uids are referring to the same app id, ignoring the user id part of the
     * uids.
     * @param uid1 uid to compare
     * @param uid2 other uid to compare
     * @return whether the appId is the same for both uids
     * @hide
     */
    public static final boolean isSameApp(int uid1, int uid2) {
        return getAppId(uid1) == getAppId(uid2);
    }

    public static final boolean isIsolated(int uid) {
        uid = getAppId(uid);
        return uid >= Process.FIRST_ISOLATED_UID && uid <= Process.LAST_ISOLATED_UID;
    }

    public static boolean isApp(int uid) {
        if (uid > 0) {
            uid = UserId.getAppId(uid);
            return uid >= Process.FIRST_APPLICATION_UID && uid <= Process.LAST_APPLICATION_UID;
        } else {
            return false;
        }
    }

    /**
     * Returns the user id for a given uid.
     * @hide
     */
    public static final int getUserId(int uid) {
        if (MU_ENABLED) {
            return uid / PER_USER_RANGE;
        } else {
            return 0;
        }
    }

    public static final int getCallingUserId() {
        return getUserId(Binder.getCallingUid());
    }

    /**
     * Returns the uid that is composed from the userId and the appId.
     * @hide
     */
    public static final int getUid(int userId, int appId) {
        if (MU_ENABLED) {
            return userId * PER_USER_RANGE + (appId % PER_USER_RANGE);
        } else {
            return appId;
        }
    }

    /**
     * Returns the app id (or base uid) for a given uid, stripping out the user id from it.
     * @hide
     */
    public static final int getAppId(int uid) {
        return uid % PER_USER_RANGE;
    }

    /**
     * Returns the user id of the current process
     * @return user id of the current process
     */
    public static final int myUserId() {
        return getUserId(Process.myUid());
    }
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