Question

Does Gmail, Contacts and Calendar uses C2DM to do push?
The above is not open source app, but some one may know the answer.

Answer 1

The answer is no. Gmail doesn’t use C2DM. To identify if an app uses C2DM it should have the permission like below inside AndroidManifest.xml.

<!-- Cloud to Device Messaging permissions -->
<permission
    android:name="com.example.jumpnote.android.permission.C2D_MESSAGE"
    android:protectionLevel="signature" />
<uses-permission android:name="com.example.jumpnote.android.permission.C2D_MESSAGE" />
<uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />

Note: the above permission when you look in the market it should be equivalent below:

NETWORK COMMUNICATION
RECEIVE DATA FROM INTERNET
Allows the applications to accept cloud to device messages sent by the application's service. Using this service will incur data usage. Malicious applications may cause excess data usage.

Then see the Gmail app in the Market, it does not use such permission.