MOO-cows Mailing List Archive


Re: Mutual Exclusion

On Tue, 7 Nov 1995, CJ Holmes wrote:

> Is there a way to gurantee that only one task can access a variable at a
> given time?
> I have a financial-flow simulation where hundreds of robotic "consumers"
> are engaging in various transactions.  I suspect I'm getting into trouble
> when I check an account to see if it has the required sum of currency
> withing, and then subtract from it (in the case of a debit).  What if
> another task modifies the account between those two statements?  Is there a
> way I can "lock" the variable in question?

That is already guaranteed with the design of LambdaMOO.  It is a
non-preemptive multitasking operating system, much like Windows 3.11 and

What that really means is that the only way you can have a problem is if
you design your code wrong.

ie.  Wrong design:

Variable        Current Balance (CB)    1000

Robot A         Reads CB (1000)

you suspend()

Robot B         Reads CB (1000)
Robot B         Deducts 400
Robot B         Writes CB (600)
task ends

Robot A         Deducts 100
Robot A         Writes CB (900)
task ends

Basically, all you need to ensure is that you do not suspend between
reading the variable, and writing to the variable, and all is well :^)



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