#13961: Compute the root graph of a graph (inverse of Graph.line_graph)
---------------------------------+------------------------------------------
Reporter: ncohen | Owner: jason, ncohen, rlm
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.10
Component: graph theory | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Nathann Cohen | Merged in:
Dependencies: #13787 | Stopgaps:
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Comment (by dcoudert):
The segfault happens in the isomorphism test at the end. I tried to track
it back.
Inside the isomorphic function (file
sage/groups/perm_gps/partn_ref/refinement_graphs.pyx), it happens during
the call to
{{{
cdef bint isomorphic = double_coset(<void *>GS1, <void *>GS2, part,
ordering, n, &all_children_are_equivalent, &refine_by_degree,
&compare_graphs, NULL, NULL, output)
}}}
Inside sage/groups/perm_gps/partn_ref/double_coset.pyx:
{{{
work_space = allocate_dc_work_space(n)
}}}
and inside the {{{SC_new}}} function in
sage/groups/perm_gps/partn_ref/data_structures_pyx.pxi, we have:
* an assignment before a NULL test: {{{SC.gen_used.bits[limbs-1] = 0}}}
and {{{SC.gen_is_id.bits[limbs-1] = 0}}}
* a call to the {{{SC_dealloc}}} in which we test if {{{SC.generators}}}
is not NULL to dealloc both {{{SC.generators}}} AND {{{SC.gen_inverses}}}.
Quite unsafe, no?
At this point I'm no longer able to understand what's going on, but there
is a pointer allocation problem somewhere.
The call to {{{sage_free(SC.generators[i])}}} with i=0 fails but that
pointer is apparently not NULL. O_o
-------
In your code, you should change this:
* "The this is that this implementation" -> "the problem is that this
implementation"
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13961#comment:11>
Sage <http://www.sagemath.org>
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