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Frontend Interfaces
===================

This file will try to document some of the interfaces that dpkg makes
available to frontends or that expects them to use, which are currently
not covered by any other type of documentation.


Database Locking
----------------

Any frontend needing to make sure no write operation is currently happening,
and no other frontend is running should first acquire the frontend lock at
«<admindir>/lock-frontend», and then acquire the dpkg database lock at
«<admindir>/lock». When the frontend invokes dpkg, it should set the
environment variable DPKG_FRONTEND_LOCKED (to prevent dpkg from acquiring
the frontend lock), and then release the dpkg database lock, which will be
acquired by dpkg itself. This way no other frontend following this protocol
can race to perform operations while another one has one in progress.

These locks must be file record locks (i.e. fcntl(2) advisory locking), and
the whole file should be locked, as that's the most portable way to perform
this operation; this can be achieved by using start=0, len=0 and
whence=SEEK_SET.

Name	Size	Last Modified	Owner	Permissions
AUTHORS	1.205 KB	January 30 2021 6:07:17	root	0644
README	3.462 KB	January 30 2021 6:07:17	root	0644
README.api	1.514 KB	January 30 2021 6:07:17	root	0644
README.bug-usertags	5.068 KB	January 30 2021 6:07:17	root	0644
README.feature-removal-schedule	5.622 KB	January 30 2021 6:07:17	root	0644
THANKS	6.441 KB	January 30 2021 6:07:17	root	0644
TODO	5.199 KB	April 13 2021 10:18:32	root	0644
changelog	676.074 KB	April 13 2021 10:43:39	root	0644
dpkg.cron.daily	1.268 KB	January 30 2021 6:09:12	root	0644
frontend.txt	1.068 KB	January 30 2021 6:07:17	root	0644
protected-field.txt	3.09 KB	January 30 2021 6:09:02	root	0644
rootless-builds.txt	7.125 KB	January 30 2021 6:09:12	root	0644
triggers.txt	35.758 KB	January 30 2021 6:07:17	root	0644

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