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Can HOFFSET return different answers on different machines?
When creating compound types, I rely on the HOFFSET macro:
if (H5Tinsert(typeid, LIQUOR, HOFFSET(struct s2, i2),
H5T_NATIVE_INT) < 0) ERR;
I see that HOFFSET, in turn, is just using the C macro offsetof.
From reading around, it seems that offsetof might return different numbers on different platforms, since, as I understand it, the internal padding of structs is platform dependent.
Is this true? Can HOFFSET give me one answer on one machine, and another on another machine, or with a different compiler?
Response:
Indeed, yes. However, it is appropriate because you want to get that
machine's native settings for the struct in memory. If you'd like to generate
a "packed" compound type for storing the data on disk (which may not always be
the best option, because it may require datatype conversions on more machines)
you can use H5Tpack(). To generate a "native" compound datatype for a
particular machine from a packed compound datatype, use H5Tget_native_type().
Question: Now I think I am starting to understand why you have H5Tpack. But to continue my line of questioning, in the example we've been discussing, it would be possible to run identical code on two machines, and get a different data file as a result?
Response:
Yes, although I believe that the different files would be
type-convertable.
Question:
I just re-read the H5Tpack documentation. Probably I need to pack all
my compound types to make sure that they come out the same way every
time.
Response:
You could do that, but it may be slower for users on one particular
machine.
Question:
But if I have a packed compound type, and I read it onto a array of
the struct that the compound type represents, then how does all the
data come out OK? Do you guys really read it member by member, and
adjust all the byte boundaries behind the scenes?
Answer:
Well, if we don't need to adjust anything, we'll read it directly
into the user's buffer. If we do need to adjust sizes & offsets, we read
it into an internal buffer and then copy it member by member into the
user's buffer (as you say).
- - Last modified: 13 February 2014