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From: frimp@mcshh.hanse.de (Frank Heinzius)
Newsgroups: comp.lang.pascal
Subject: Re: File Locking In Turbo Pascal v6.0
Message-ID: <9110@mcshh.hanse.de>
Date: 29 Apr 91 08:07:42 GMT
References: <1991Apr26.032900.24012@cs.odu.edu> <27400@hydra.gatech.EDU>
Lines: 28

In <27400@hydra.gatech.EDU> gt0652b@prism.gatech.EDU (I don't know man, he was just here) writes:

]Forgive me for being stupid but I haven't ever had the need to write a 
]file locking program. Wouldn't this be all that was necessary?

]type
]   LockedRecType = record
]		     Locked: Boolean
]		     Data: DataType
]		     end;

]Var
]   File: LockRecType;

]begin
]   if File.Locked then 
]      writeln('It's locked stupid')
At this point another process could set the locked-flag, because
your if...then-construct is no *atomic* instruction. there are some
possibilities to avoid this. one is to embed the if...then-construct
in CLI/STI-commands or to implement semaphores with inline-assembler.
]    else
]      AccessFile(File)
]end       

-- 
Real : Frank Heinzius             Voice: +49 40 814 416 
Smart: frimp@mcshh.hanse.de       Blah : 2B or not 2B, that's FF