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From: tomc@mntgfx.mentor.com (Tom Carstensen)
Newsgroups: comp.sys.mac.programmer
Subject: Pointer arithmetic in C w/ or w/o locked handles
Keywords: pointer C arithmetic handles
Message-ID: <1988Apr15.125337.100@mntgfx.mentor.com>
Date: 15 Apr 88 19:53:34 GMT
Organization: Mentor Graphics Corporation, Beaverton Oregon
Lines: 39

Is the problem I'm noticing with the following code
fragment the compiler, or on error:

    Handle hdl;
    char *ptr;
    long length;
    
    HLock(hdl);
    ptr = *hdl;
    
    while ( <condition> ) {
        /* ... stuff here ... */
        ptr++;
    }
    HUnlock(hdl);
    length = (long) (ptr - *hdl);

Running under The Debugger, I notice that the value
for ptr = 80023678, and the value for *hdl = 00023670,
and therefore length becomes 80000008, which is actually
a negative number.

The '8' (or 1000 binary) occurs because it is Locked - 
(only on 68000 w/ 24 bit addressing).  but when I do
pointer arithmetic, is the compiler messing up or 
should I only add Locked handles or Unlocked handles
together.

If I move the length calculation before I unlock the 
handle, then length becomes 8, and I'm ok.

:------------------------------------------------------------:
: Tom Carstensen         Usenet: tomc@mntgfx.MENTOR.COM      :
: Mentor Graphics                Delphi: CARSTENSEN          :
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