uid gid pid appid等的区别

1.uid
android中uid用于标识一个应用程序。uid在应用安装时被分配，并且在应用存在于手机上期间，都不会改变。
一个应用程序只能有一个uid，多个用户可以使用sharedUserId 方式共享同一个uid，前提是这些应用的签名要相同。
Uid是在apk安装的时候确认的，具体代码可以参考：
frameworks\base\services\core\java\com\android\server\pm\Settings.java

    /**
     * Registers a user ID with the system. Potentially allocates a new user ID.
     * @throws PackageManagerException If a user ID could not be allocated.
     */
    void addUserToSettingLPw(PackageSetting p) throws PackageManagerException {
        if (p.appId == 0) {
            // Assign new user ID
            p.appId = newUserIdLPw(p);
        } else {
            // Add new setting to list of user IDs
            addUserIdLPw(p.appId, p, p.name);
        }
        if (p.appId < 0) {
            PackageManagerService.reportSettingsProblem(Log.WARN,
                    "Package " + p.name + " could not be assigned a valid UID");
            throw new PackageManagerException(INSTALL_FAILED_INSUFFICIENT_STORAGE,
                    "Package " + p.name + " could not be assigned a valid UID");
        }
    }

    // Returns -1 if we could not find an available UserId to assign
    private int newUserIdLPw(Object obj) {
        // Let's be stupidly inefficient for now...
        final int N = mUserIds.size();
        for (int i = mFirstAvailableUid; i < N; i++) {
            if (mUserIds.get(i) == null) {
                mUserIds.set(i, obj);
                return Process.FIRST_APPLICATION_UID + i;
            }
        }

        // None left?
        if (N > (Process.LAST_APPLICATION_UID-Process.FIRST_APPLICATION_UID)) {
            return -1;
        }

        mUserIds.add(obj);
        return Process.FIRST_APPLICATION_UID + N;
    }

另外，补充一点比较容易产生的误解：关于uid 10000的范围，uid大于10000，只能代表当前是应用的uid，不能区分这个应用是系统应用还是非系统应用。uid小于10000，基本都是系统进程的uid。

/**
     * Defines the root UID.
     * @hide
     */
    public static final int ROOT_UID = 0;

    /**
     * Defines the UID/GID under which system code runs.
     */
    public static final int SYSTEM_UID = 1000;

    /**
     * Defines the UID/GID under which the telephony code runs.
     */
    public static final int PHONE_UID = 1001;

    /**
     * Defines the UID/GID for the user shell.
     * @hide
     */
    public static final int SHELL_UID = 2000;

    /**
     * Defines the UID/GID for the log group.
     * @hide
     */
    public static final int LOG_UID = 1007;

    /**
     * Defines the UID/GID for the WIFI supplicant process.
     * @hide
     */
    public static final int WIFI_UID = 1010;

    /**
     * Defines the UID/GID for the mediaserver process.
     * @hide
     */
    public static final int MEDIA_UID = 1013;

    /**
     * Defines the UID/GID for the DRM process.
     * @hide
     */
    public static final int DRM_UID = 1019;

    /**
     * Defines the UID/GID for the group that controls VPN services.
     * @hide
     */
    public static final int VPN_UID = 1016;

    /**
     * Defines the UID/GID for keystore.
     * @hide
     */
    public static final int KEYSTORE_UID = 1017;

    /**
     * Defines the UID/GID for the NFC service process.
     * @hide
     */
    public static final int NFC_UID = 1027;

    /**
     * Defines the UID/GID for the Bluetooth service process.
     * @hide
     */
    public static final int BLUETOOTH_UID = 1002;

    /**
     * Defines the GID for the group that allows write access to the internal media storage.
     * @hide
     */
    public static final int MEDIA_RW_GID = 1023;

    /**
     * Access to installed package details
     * @hide
     */
    public static final int PACKAGE_INFO_GID = 1032;

    /**
     * Defines the UID/GID for the shared RELRO file updater process.
     * @hide
     */
    public static final int SHARED_RELRO_UID = 1037;

    /**
     * Defines the UID/GID for the audioserver process.
     * @hide
     */
    public static final int AUDIOSERVER_UID = 1041;

    /**
     * Defines the UID/GID for the cameraserver process
     * @hide
     */
    public static final int CAMERASERVER_UID = 1047;

    /**
     * Defines the UID/GID for the WebView zygote process.
     * @hide
     */
    public static final int WEBVIEW_ZYGOTE_UID = 1053;

    /**
     * Defines the UID used for resource tracking for OTA updates.
     * @hide
     */
    public static final int OTA_UPDATE_UID = 1061;

    /**
     * Defines the UID used for incidentd.
     * @hide
     */
    public static final int INCIDENTD_UID = 1067;

    /**
     * Defines the UID/GID for the Secure Element service process.
     * @hide
     */
    public static final int SE_UID = 1068;

    /** {@hide} */
    public static final int NOBODY_UID = 9999;

    /**
     * Defines the start of a range of UIDs (and GIDs), going from this
     * number to {@link #LAST_APPLICATION_UID} that are reserved for assigning
     * to applications.
     */
    public static final int FIRST_APPLICATION_UID = 10000;

    /**
     * Last of application-specific UIDs starting at
     * {@link #FIRST_APPLICATION_UID}.
     */
    public static final int LAST_APPLICATION_UID = 19999;

2.appid

与uid相关，uid相同，则appid相同。

    public static @AppIdInt int getAppId(int uid) {
        return uid % PER_USER_RANGE;//PER_USER_RANGE = 100000
    }

2.pid
进程id，可变的。是各进程的身份标识，程序一运行系统就会自动分配给进程一个独一无二的PID。
进程中止后pid被系统回收，可能会被继续分配给新运行的程序，但是在android系统中一般不会把已经kill掉的进程ID重新分配给新的进程。
新产生进程的进程号，一般比产生之前所有的进程号都要大。

3.gid
对应于linux中用户组的概念，android 中 gid 等于uid。

4.UserID
在Android中，多用户情况下，userlist.xml中会定义一个user id,表示当前是哪个user，此时的user id是从0开始递增。
userlist.xml中的内容：

<?xml version='1.0' encoding='utf-8' standalone='yes' ?>
<users nextSerialNumber="10" version="7">
    <guestRestrictions>
        <restrictions no_sms="true" no_install_unknown_sources="true" no_config_wifi="true" no_outgoing_calls="true" />
    </guestRestrictions>
    <deviceOwnerUserId id="-10000" />
    <user id="0" />
</users>