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From: "trippels at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/63645] Incorrect code generation
Date: Mon, 27 Oct 2014 07:52:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63645

--- Comment #12 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
1) Your malloc is too small. It has to be sizeof (biggest member).
So you're invoking undefined behavior.

2) In the if statement, where you probe the different members, you
also invoke undefined behavior.

So two bugs in one testcase.