Re: (ITS#8819) LMDB seg fault with MDB_DUPSORT on -O3

[hyc@symas.com]

nic@nicwatson.org wrote:
> That's news to me. Then I googled it. You're right.
> 
>>From https://wiki.sei.cmu.edu/confluence/display/c/EXP36-C.+Do+not+cast+pointers+into+more+strictly+aligned+pointer+types
> 
> The C Standard, 6.3.2.3, paragraph 7 [ISO/IEC 9899:2011], states
> 
> A pointer to an object or incomplete type may be converted to a
> pointer to a different object or incomplete type. If the resulting
> pointer is not correctly aligned for the referenced type, the behavior
> is undefined.
> 
> Nic
> 
> On Tue, Mar 20, 2018 at 9:41 AM, Hallvard Breien Furuseth
> <h.b.furuseth@usit.uio.no> wrote:
>> Looks like another type aliasing problem to me.  The data is accessed
>> through an MDB_page* variable.  This tells the compiler that the data
>> is word-aligned, like struct MDB_page.  Fix: Use a void/char pointer,
>> don't lie to the compiler.

Good catch.

We once discussed padding odd-length keys to make sure the data was still 
word-aligned. Maybe should do that in LMDB 1.0. This particular crash is now 
fixed in mdb.master. I've left other derefs of *fp alone for the moment but 
may need to revisit that later; older ARM and SPARC would probably choke on them.

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