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The issue that we are talking about is access synchronisation. In a multi-tasking or networked environment, it happens that two or more programs simultaneously access the message base. Therefore, the Squish Message API (this is the piece of software that all Fidonet programs internally use to access the message base files) needs a way to ensure that no other Fidonet program can gain access to the message base file while itself is doing vital modifications on it. Otherwise, data could be screwed up or even lost if two programs would modify the same file simultaneously.
As long as your message base resides on a single computer, you do not have a big problem with access synchronisation. On OS/2, Windows NT and DOS, access synchronisation is done via so called record locks, which works reliably. On Linux and other Unix system, access control is performed using so called voluntary locks by all programs that use the SMAPI edition of the Squish Message API. This is not as safe as the "record locks" feature. Voluntary locks can only synchronize programs that use them, i.E. programs that use the SMAPI, like MsgEd TE or hpt. If you wish to use other Fidonet programs on Unix, you should ask the author if he employs voluntary locks for access synchronisation. If he doesn’t, do not use his software.
Things get more complicated in a networked environment, because there, not only different applications have to synchronize among themselves, but different computers also have to be synchronized. Or in other words, the network file system that you use must correctly propagate record and/or voluntary locks that have been set on one computer to all other computers.