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I've written a code in C++ for a bio-metric lock which I want to add to the log in screen beside the password.

Any idea from where should I start?

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Consider using libpam. You'll need to add account required [your-code-in-libpam-format].so to /etc/pam.d/lightdm Be warned that your biometric software must be compatible with PAM. – hexafraction Jul 18 '12 at 17:47
a livecd it's enough to acquire a complete root access in a tipical GNU/linux distro, if you are aiming to secure your computer it's probably not a good way or in better terms, this would not worth the effort. – user827992 Jul 18 '12 at 18:01

PAM is probably the way to do this. However, keep in mind that PAM works as a stack of modules, they get run and as soon as one says "yeah you're authenticated" you get signed in. You can mess with the ordering in some pam config file (that I can't remember off hand). Once you make this a PAM module, lightdm will automatically use it for authentication.

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