how to fetch ID from other site?

Just l开发者_JS百科et me know how one site fetch Logged in ID of other site? e.g. If i already logged-in in Gmail,then how any other site fetch gmail email ID into their user name textbox?

Thanks

This would only be possible if Gmail had an API for it. You can't read an other domain's cookies, for security reasons.

Such behavior is obtained via a common loggin system. On Gmail a lot other (like StackOverflow), they use OpenID, some other uses OAuth like Twitter.

But, I don't know if you're allowed to access user's data, like your their Gmail address. I've never used OpenID before.