memory allocation

[Dr. T. Andrews]

In article <33422 at cca.CCA.COM>, g-rh at cca.CCA.COM (Richard Harter) writes:
) One point which I feel strongly about is that most allocators use the
) blocks (free and allocated) to hold control information.  I think that
) this is a mistake.  I put all control information in structures in a 
) separate node space.

Unless you hard-code a limit of <n> allocated blocks, you have to
allocate the separate node space from the same heap, leaving the
control information somewhat vulnerable (OK, so now it's arry[-3]
instead of [-1] that trashes you).

I stuck the control information in the blocks, and can still check
my free() calls.  Two things assist in this.

On free() calls, I trace the list until I find the block to be freed.
If it isn't a valid block, it isn't free()d.  As the block is being
freed, I coalesce the adjacent free block(s).  At most 3 blocks may
be collected (the freed block and the ones immediately above and
below).  It seems much better to do this work wen the block is freed
(if ever) rather than to perform trash-mashing at malloc() time.

Secondly: the third control word in the block (after a pointer to the
next block and the size of the current block) is the "XOR" of the two
other control words.  This gives a good indication if arry[-1] has
been tampered.
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