From:	Andrew Crabtree <andrewc AT typhoon DOT rose DOT hp DOT com>
Message-Id:	<199706241550.AA012077402@typhoon.rose.hp.com>
Subject:	Re: I need help!!!
To:	darkstar AT user1 DOT channel1 DOT com
Date:	Tue, 24 Jun 1997 8:50:02 PDT
Cc:	djgpp AT delorie DOT com
In-Reply-To:	<33AC6429.3B57@user1.channel1.com>; from "darkstar@user1.channel1.com" at Jun 21, 97 4:30 pm

> 
> Ok,  I would like to know how you "lock" a variable.
Do you mean mutex semaphores?  You have to use assembly.  

>  Could
> someone give me an example? 
The following assembly is the shortest, though very non-ideal 
semaphore routine

getsem :    lock bts Reg1,Reg2; try to get the semaphore
            jc getsem         ; if we didn't get it try again

This does a busy wait.  If you want it to be C Callable you will
have to inline it or get a param off of the stack.  You will also need
a free routine which should be simple to write.  To use it you just
declare a global variable to be you sempahore and then call getsem
when you want it.  Your call will block until the sem becomes available
so you better have either a pre-emptive system or an ISR that changes it.

Note that typically your OS will provide functions like this if it
supports them.  It will usually put your task on a non-ready queue until
the sem becomes available and let other processes run.  

I probably should have checked DJ libc to see if he has support for 
semaphores already (duh!) before I wrote this, but I didn't.  
Maybe browse through the
posix stuff...


> Also, does Djgpp use function variables?
Do you mean function pointers? Sure.  They are standard in C.

instead of doing this

int *ptr

do this 

int (*fnctptr) ... ;

HTH

Andrew

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