Message: 1 Date: Fri, 24 Jul 2009 10:21:04 +0200 From: Gustaf Neumann neumann@wu.ac.at Subject: Re: [Xotcl] xotcl crash To: Tibichte zak@isa-software.com Cc: xotcl@alice.wu-wien.ac.at Message-ID: 4A696EF0.7010906@wu.ac.at Content-Type: text/plain; charset=ISO-8859-1; format=flowed

...

2- This forwarding method causes xotcl to crash:

obj forward f2 list {%@end 13} %self %1

? { obj f2 1 2 3 }

There is a bug in the current xotcl versions when both %@end and %1 is used, which will be fixed in the next release. The %1 means that the first argument from the provided argument list (here "1") will be placed at the given position. The argument is consumed and the remaining arguments (here "2" and "3") are appended to the arguments from the forward definition. S

The fact that %1 consumes = mutates the caller arg list and the %@POS substitution operates on the mutated state of the resulting (=final) list (kind of) made me think. And ... I guess I found the culprit of this crashy interaction (see the patch attached):

once %1 consumes the first element of the caller arg list, the currently arg handling mechanism for forwards counts %1 as an extra arg in the overall arg list erroneously. however, it is rather a placeholder for an already counted arg (=the first in the caller arg list). This +1 overestimation in the internal assembly of the final arg list led to a memory access violation for the %@POS substitution under end offsets (end, end-1, end-2; in the Tcl sense of the word) because end offsets require a correct total args count internally (which used to report +1).

as for the patch: it is a minor and fairly non-invasive change. all regression tests pass, i added some extra cases to test for the %1 + %@POS-under-end-offsets interaction. the patch can be applied to the a xotcl-1.6.3 source snapshot.

cheers //stefan