Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!ucbvax!CORY.BERKELEY.EDU!dillon
From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Who's got the lock?
Message-ID: <8805160340.AA23871@cory.Berkeley.EDU>
Date: 16 May 88 03:40:36 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 40

:>If so, how far can I trust the definition of the lock in the AmigaDOS Technical
:>Reference Manual? I know some of the other structures are off-base, but I have
:>not been able to find struct Lock in an include file.
:
:The routines would work for the AmigaDOS FS handler. (I haven't looked at the 
:FileLock struct recently so the member names may be off). The problem is that
:a "Lock" is an opaque structure that only the handler understands. These are

	Actually, Locks are quite rigid.  The workbench guarantees that...
that is, a bug in the workbench guarantees it.  You see, the workbench does
a very bad thing to locks ... it copies them manually.  So the size of a 
lock MUST be EXACTLY sizeof(struct FileLock) or you will crash the workbench.

	The fields within a struct FileLock are:

	fl_Link		Usually used to link locks together in a lock list
			(BPTR)
	fl_Key		disk block # (read: unique key identifying an object in
			this device.. see below)
	fl_Access	exclusive or shared access
	fl_Task		pointer to the handler's message port
	fl_Volume	BPTR to the Device or Volume node

	So you see, the only two fields that can possibly take on 
unspecified meanings are fl_Link and fl_Key... fl_Link would not be used by
every day user programs, which leaves fl_Key.

	The device driver has this one remaining field to discern between 
objects within the device...  Intuitively it will contain the 'same' value
for duplicate locks on the same object.  Realistically the easiest programming
approach IS for it to contain the 'same' value, but of course the device
driver writer can always make life harder for himself and not follow 
this reasoning.

	For the floppy drivers, disk block numbers are placed in fl_Key.
For my RAM: disk sample device driver, I stick a pointer to my directory/
fileheader structure.  I.E. you can't assume you know what the value in the
fl_Key field means, but can probably assume its uniqueness.

					-Matt