All,
Apparently the header of a heap item is a data structure we’re not supposed to
know what it is like, but some seem (or mean) to know. Apparently, this is also
different for QDOS and SMSQ/E:
The QDOS Technical guide describes the heap item header a bit differently:
"Each heap item is an area of memory (which is a multiple of 8 bytes long),
together with a pair of longwords: the first is the length of the heap item,
while the second is a pointer (relative to itself) to the next heap item in the
list. The use of relative pointers ensures that heaps may be moved. „
That would look like
ds.l hp_len ; length of heap item
ds.l hp_nxfree ; next free item in free list (relative pointer)
hpi: ds.b heap_data
So, in your case, -4(a) would point to the next free heap item in the free
list. As the free list entry is not used for /allocated/ heap chunks (only on
those that are worked into a free list), Mr. Goodwin is free to use it for some
other purpose - On QDOS. Apparently, on SMSQ/E -4(ax) points to something
different, and thus looks weird to be changed by applications.
Thus, this code is a good example of why you shouldn’t use undocumented data
structures: They might change over time :)
Regards,
Tobias
> Am 23.01.2016 um 07:41 schrieb Wolf <[email protected]>:
>
> Hi,
>
> I don't quite agree with Per's explanation, at least not as far as SMSQ/E is
> concerned.
>
> Once the memory is allocated, A0 points past the header, as Per said.
>
> So, at -4(a0) you will find chp_flag, not chp_len.
>
> Moreover, chp_flag is used by the system :
>
> Consider how SMSQ/E releases memory reserved on the common heap
> (in smsq_mem_rchp_asm)
>
> movem.l reglist,-(sp) save volatiles
> moveq #chp.free,d0
> move.l d0,chp_ownr(a0) set owner free
> move.l chp_flag(a0),d0 flag address
> beq.s mrc_rehp is empty, do nothing
> move.l d0,a1 set flag...
> st (a1) ...now
> *
> So what happens here is that, if chp_flag isn't empty, the address
> it points to will have its MSB set. If you put "grab" in there, then address
> $67726162 will have its MSB set to 1.
>
> OK, that most likely won't be a problem because in most cases that address
> doesn't exist. But what if you put something else than "grab" at chp_flag and
> that points to somewhere in real memory? Oops...
>
> So your initial way of doing things definitely is the right way.
>
>
> HTH
>
> Wolfgang
>
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