On 28 Jan 2009 at 13:06, Howard Chu wrote:

Ulrich.Windl@rz.uni-regensburg.de wrote:

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Full_Name: Ulrich Windl Version: 2.4.11 OS: Linux URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (132.199.156.178)

Looking for a threaded AVL tree implementation that is supposed to work, I found libraries/liblutil/tavl.c in openLDAP. I'm no expert on threaded AVL trees, but there's thing that worries me: tavl_delete uses two stacks (pptr, pdir) with a size of 8 elements on the routine' stack. While locating the element to delete, it pushes the parent node onto the stack.

You're misreading the code. The stack is 8*sizeof(void *) which means 32 elements deep on a 32 bit machine, and 64 deep on a 64 bit machine. There is no possibility of overflow now or for several years to come.

OK! I was confused by the terrible programming style at that point: as the array elements are of the proper type (and not bytes), there is no need for a sizeof here: I see no reason why the depth of the tree to be allowed should depend on the length of a pointer, maning: If you want 32, why don't you write 32? So the worst case stack depth would be something like "log2(2^32/(sizeof(void *) * 5))" ("5" being 2 links + 2 threading marks + 1 data pointer), something near 32-5 == 27 (for 32-bit machines) and 64-6 == 58. Practically the latter could be safely reduced to a value around 42 IMHO.

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So from my knowledge this means that for a perfectly filled balanced tree, the stack will overflow when the tree consists of 2^8 (2^0 + 2^1 + 2^2 + ... + 2^7) or more nodes, and you are locating a leaf node. Maybe even with fewer nodes. As the algorithm also pushes an other element onto the stack, the problem could appear even before 128 nodes.

-- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/