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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.