Greetings!
I have a conceptual question involving “datasets (H5Ds)”, “Dataspaces (H5Ss)”,“hyperslab selections”, and datatypes (H5Ts).
Background:
The motivation for asking is that I am working with the CFD General Notation System (CGNS) API in C, which is implemented via HDF5. Some of the H5Ds in that format are routinely permitted to be 32-bit or 64-bit as per CGNS specifications. Because of the need-to-know aspect of the element size, my C program will have something like:
int32_t* Buff32;
int64_t* Buff64;
// assume apriori knowledge of array size
if(eSize == 4){//32-bit
Buff32 = (int32_t*)malloc(nElem*eSize);
H5Dread(..., Buff32);
}else if(eSize == 8){//64-bit
Buff64 = (int64_t*)malloc(nElem*eSize);
H5Dread(...., Buff64);
}
I would prefer something like:
void* Buff;
Buff = malloc(nElem*eSize);
*H5Dread(..., Buff)*
Why? Because the next step in my program is to effectuate data conversions via typecasting.
MyDatatype_t outBuff*;
outBuff = (MyDatatype_t*)malloc(nElem*sizeof(MyDatatype_t));
for(int idx = 0; idx < nElem; idx++){
outBuff[idx] = (MyDatatype_t)Buff32[idx];
}
I expect the arrays to read in to be substantially large and would like to avoid the overhead of typecasting in C.
The Question
- Data under file’s H5D is 64-bit, however, the actual numbers are small enough to fit in 32-bit.
- I want to read data to buffer as 32-bit in spite of the file having a 64-bit H5D.
- Say there are
nElemsuch 64-bit numbers under the H5D on file. Can I then:
- Define a 32-bit H5T.
- Define an H5S for the file with
2*nELempoints (because 64-bit = 32-bit*2). - Define an H5S for the buffer as H5S_ALL or some other custom H5S.
- Make a hyperslab selection on the file H5S with a stride of
2(i.e., 64-bit worth of memory), but block size of1(i.e., 32-bit worth of memory). - OPTIONAL: Set an offset of
1(32-bits worth of memory) in the hyperslab for the FIle H5S if data is Little-Endian. - Call
H5Dread(loc_id,H5T,fileH5S,buffH5S,dxpl_id,Buff)and read directly to a 32-bit buffer?
Picture this:
Data in the H5D: [64-bit ][64-bit ][64-bit ]....[64-bit ]
How I see the H5D:[32-bit][32-bit][32-bit][32-bit][32-bit][32-bit]....[32-bit][32-bit]
Hyperslab (BE): [ ^ ] [ ^ ] [ ^ ] ....[ ^ ]
Hyperslab (LE): [ ^ ] [ ^ ] [ ^ ].... [ ^ ]