On Sun, Feb 26, 2017 at 07:53:15PM -0500, Tom Lane wrote: > Noah Misch <n...@leadboat.com> writes: > > Dear 7b4ac19 authors, > > Field ps_snapshot_data usually receives four-byte alignment within > > ParallelIndexScanDescData, but it contains the eight-byte whenTaken field. > > The select_parallel test dies with SIGBUS on "Oracle Solaris 10 1/13 > > s10s_u11wos_24a SPARC", building with gcc 4.9.2. > > It's a little distressing that the buildfarm didn't find this already. > Is there some reason why it's specific to that particular compiler, > rather than generic to alignment-picky 64-bit machines?
I wondered the same thing; if nothing else, why don't protosciurus and castoroides fail the same way? They do use older compilers, "Sun C 5.10 SunOS_sparc 2009/06/03" and gcc 3.4.3. I have "Sun C 5.12 SunOS_sparc 2011/11/16" and gcc 4.9.2, both of which are alignment-sensitive in several configurations, according to the attached test program. However, in a 32-bit build with this Sun C, I don't get alignment-related bus errors. (Those animals build 64-bit, so this isn't the full story.) > In general, though, I agree that using a char[] member to represent > anything that has any alignment requirement at all is seriously bad > coding style that is almost certain to fail eventually. > > A solution you didn't mention is to change the ParallelIndexScanDescData > field to be a pointer, perhaps "struct SerializedSnapshotData *", while > leaving that struct opaque so far as relscan.h is concerned. This could > avoid the need to use the unsafe blind casts that I'm sure must be > involved in accesses to that field at present. That, too, would be reasonable.
/* run with: for n in 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15; do ./a.out $n; done */ #include <stdio.h> #include <stdlib.h> int main(int argc, char **argv) { char foo[] = "abcdefghijklmnopqrstuvwxyz"; if (argc != 2) { puts("bad usage"); return 1; } printf("%llx\n", *(long long *)(foo + atoi(argv[1]))); return 0; }
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