Android User IDs and File Access

At install time, Android gives each package a distinct Linux user ID. The identity remains constant for the duration of the package's life on that device. On a different device, the same package may have a different UID; what matters is that each package has a distinct UID on a given device.

Because security enforcement happens at the process level, the code of any two packages can not normally run in the same process, since they need to run as different Linux users. You can use the sharedUserId attribute in theAndroidManifest.xml's manifest tag of each package to have them assigned the same user ID. By doing this, for purposes of security the two packages are then treated as being the same application, with the same user ID and file permissions. Note that in order to retain security, only two applications signed with the same signature (and requesting the same sharedUserId) will be given the same user ID.

 

Any data stored by an application will be assigned that application's user ID, and not normally accessible to other packages. When creating a new file with getSharedPreferences(String, int), openFileOutput(String, int), oropenOrCreateDatabase(String, int, SQLiteDatabase.CursorFactory), you can use the MODE_WORLD_READABLE and/orMODE_WORLD_WRITEABLE flags to allow any other package to read/write the file. When setting these flags, the file is still owned by your application, but its global read and/or write permissions have been set appropriately so any other application can see it.

 

R.attr

public static final int sharedUserId

Since: API Level 1

Specify the name of a user ID that will be shared between multiple packages. By default, each package gets its own unique user-id. By setting this value on two or more packages, each of these packages will be given a single shared user ID, so they can for example run in the same process. Note that for them to actually get the same user ID, they must also be signed with the same signature.

Must be a string value, using '\\;' to escape characters such as '\\n' or '\\uxxxx' for a unicode character.

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

Constant Value: 16842763 (0x0101000b)

 

public static final int sharedUserLabel

Since: API Level 3

Specify a label for the shared user UID of this package. This is only used if you have also used android:sharedUserId. This must be a reference to a string resource; it can not be an explicit string.

Must be a reference to another resource, in the form "@[+][package:]type:name" or to a theme attribute in the form "?[package:][type:]name".

Constant Value: 16843361 (0x01010261)

 

public static final int largeHeap

Since: API Level 11

Request that your application's processes be created with a large Dalvik heap. This applies to all processes created for the application. It only applies to the first application loaded into a process; if using a sharedUserId to allow multiple applications to use a process, they all must use this option consistently or will get unpredictable results.

Must be a boolean value, either "true" or "false".

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

Constant Value: 16843610 (0x0101035a)