Code review comment for lp:~bregma/geis/lp-754135

There's a duplicate instance problem?

This fix is for the case where a pointer to a variable with automatic storage duration is passed to geis_subscribe() and that variable goes out of scope, leaving a dangling pointer. That was the cause of the problem reported in lp:754135.

Dereferencing dangling pointers results in undefined behaviour: there is no reliable way to reproduce the problem within the C language itself. The test case is using inside knowledge of the GCC runtime implementation on at least three of the most common architectures (i386, amd64, arm) to reliably force a segfault under the reported circumstances by setting the area previously used by the automatic variable to zeros. The test case does in fact segfault on tested hardware without the rest of the patch but runs cleanly with it.

An additional standalone program to demonstrate the problem was supplied in the bug by the reporter.