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

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

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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.