_Heap_Walk hates me

Joel Sherrill joel.sherrill at OARcorp.com
Tue Sep 18 11:02:58 UTC 2001

Phil Torre wrote:
> I'm having this problem with RTEMS-4.5.0, running on an MPC860, with
> RTEMS built as VARIANT=DEBUG and RTEMS_DEBUG defined.
> I've got this application code which predictably aborts out while
> doing _Heap_Walk() on the malloc heap, sometimes after a malloc() and
> sometimes after a free().
> _Heap_Walk() is aborting at heapwalk.c:90 because the front and back
> flags of a block don't match, sometimes after dumping some stuff like
> this:
> PASS: 2  Front and back flags don't match
>          Current Block (0xffe22c10):  Back - 25,  Front - 32
>          Next Block (0xffe22c30):     Back - 56,  Front - 45
> Please correct me if I'm wrong, but this doesn't look like the result of
> leaking application code, but rather something corrupting the chain of
> heap blocks used by the malloc region.  (If we were freeing something
> twice or letting the heap leak away, I'd expect an error return from
> malloc() or free(), but this error seems to mean that the internal state
> of the heap region itself has become inconsistent.)

The front and back flags are immediately before and immediately after
the memory provided to the user.  They indicate the size and status
(used/free) of the memory block.  I won't try to draw an ASCII figure
this morning but suffice it to say that the application is writing
outside its allocated memory.  Maybe you are allocating too little,
maybe it is a stray pointer.

Being old the first thing that strikes me is that all of the flags
mentioned above appear to be ASCII characters.   

FYI Front flag should == back flag for each block and is the size
of the block in bytes with the LSB being a used flag.

> It seems unlikely that the RTEMS heap code is buggy, so it must be us.
> Is there something we could be doing (other than scribbling on the heap
> data structures through a wild pointer or the like) to cause this lossage?
> Thanks in advance...
> --
> =====================================================================
> Phil Torre                               phone: 425-820-6363 x234
> Design Engineer                          email: ptorre at zetron.com
> Switching Systems Group                    fax: 425-820-7031
> Zetron, Inc.                               web: http://www.zetron.com

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