Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!ucbvax!dewey.soe.berkeley.edu!oster
From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Question on Handle locking example
Message-ID: <27801@ucbvax.BERKELEY.EDU>
Date: 29 Jan 89 01:33:09 GMT
References: <379@lclark.UUCP> <2580011@hpausla.HP.COM> <1219@sw1e.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
Lines: 47

In article <1219@sw1e.UUCP> uucibg@sw1e.UUCP ([5-3929] Brian Gilstrap) writes:
>I'm not clear on why function 'a' below would die.  This was posted recently
>as an example of how not to code.  I'm sure I'm missing something extremely
>obvious (it's late...), but please explain....
>
>>a()
>>{	
>>	HLock(h);
>>	b(h);
>>	*h->x = ...	/* this would not survive a heap scramble */
>>	HUnlock(h);
>>}

B was defined as:
b(h)Handle h;{
	HLock(h);
	...
	HUnlock(h);
}
so the bad line in a gets executed with h already unlocked. One fix is to
write:
b(h)Handle h;{
	char state;
	state = HGetState(h);
	HLock(h);
	...
	HSetState(h);
}

Note: before you start coding with this style, disassemble the glue your
system uses for HGetState(). If it directly calls the system trap, you are
going to have to use:

unsigned char MyHGetState(h)Handle h;{
	if(world.machineType > envMac)
		return HGetState(h);
	else
		return *((unsigned char *) h);
}
and similarly for HSetState, as the code says, classic macs don't have
this trap.  (You'll have to do a SysEnvirons(1, &world); in your Init()
routine.)
}

--- David Phillip Oster            --"When we replace the mouse with a pen,
Arpa: oster@dewey.soe.berkeley.edu --3 button mouse fans will need saxophone
Uucp: {uwvax,decvax}!ucbvax!oster%dewey.soe.berkeley.edu --lessons." - Gasee