Is this the right way to determine available heap?

[OUTWATER]

All-

The GDB command 

p *((Region_Control *)_Region_Information->local_table[1])->Memory->first

was shown in the  RTEMS Frequently Asked Questions Manual as a way to determine the 
amount of heap remaining.

I'm debugging an application and I start with this much heap:
$1 = {back_flag = 0x8d, front_flag = 0x40c, next = 0x3c6514, previous = 0x17bdf4} 
using the command in the FAQ.

But doing this: 
p *((Region_Control *)_Region_Information->local_table[1])->Memory->last

returns 
$2 = {back_flag = 0x1, front_flag = 0x23bdf8, next = 0x17bdf8, previous = 0x3cd798}
which seems more like what I expect.

When I run my application (tclsh), it crashes with this backtrace snippet:
#0  0x400 in _uhoh ()
#1  0x20100017 in ?? ()
#2  0x96f80 in _API_extensions_Run_postswitch ()
    at ../../../../../../../rtems-4.5.0-beta3/c/src/exec/score/src/apiext.c:100
#3  0x8e7ee in _Thread_Dispatch () at 
../../../../../../../rtems-4.5.0-beta3/c/src/exec/score/src/threaddispatch.c:126
#4  0x84776 in _Thread_Enable_dispatch () at 
../../../../../../gen68360/lib/include/rtems/score/thread.inl:199
#5  0x84696 in rtems_region_get_segment (id=604045313, size=8, option_set=1, 
timeout=0, segment=0x177548)
    at 
../../../../../../../rtems-4.5.0-beta3/c/src/exec/rtems/src/regiongetsegment.c:84
#6  0x7ef88 in malloc (size=8) at 
../../../../../../rtems-4.5.0-beta3/c/src/lib/libc/malloc.c:160
#7  0x6fcfa in TclpAlloc (nbytes=8) at ./../generic/tclAlloc.c:671
#8  0x1793a in Tcl_Alloc (size=8) at ./../generic/tclCkalloc.c:813


It seems that malloc is involved, so I try to get the heap size with:
 p *((Region_Control *)_Region_Information->local_table[1])->Memory->last
 
and get:
  $4 = {back_flag = 1, front_flag = 2313828, next = 0x17bdf8, previous = 0x3b8f7c}
  
Looks like there is still plenty of heap.

Can anyone point out any flaws or errors in my thinking?

Thanks!

Keith