No password pop-up for CVS diff inside Emacs

[Xin Shi]

Hello Experts,

I'm viewing a CVS controlled file inside Emacs and I want to look at the earlier version by:

C-x v ~ , then the version number, but it failed:

ad-Orig-error: Running cvs -Q update -r1.4 -p Example.cc...FAILED (status 1)

I tested the command in the terminal: 

cvs -Q update -r1.4 -p Example.cc

It poped-up for password, and after I entered, it is OK. 

Now the question is why the Emacs in the first place didn't ask me the password and just failed? 

Thanks!

Xin