Viewing an HDF File with HDF Tools

The file SDS.hdf created by the program can be inspected with a number of HDF utilities. For example you can do 

gustav@blanc:../src 14:40:29 !552 $ hdfls SDS.hdf
SDS.hdf:
File library version: Major= 4, Minor=1, Release=2
String=NCSA HDF Version 4.1 Release 2, March 1998

Version Descriptor            : (tag 30)
        Ref nos: 1
Number type                   : (tag 106)
        Ref nos: 5
SciData dimension record      : (tag 701)
        Ref nos: 5
Numeric Data Group            : (tag 720)
        Ref nos: 2
Vdata                         : (tag 1962)
        Ref nos: 3
Vdata Storage                 : (tag 1963)
        Ref nos: 3
Vgroup                        : (tag 1965)
        Ref nos: 4 6 7
gustav@blanc:../src 14:40:36 !553 $

Program hdfls lists tags, reference numbers, and lengths of various data elements in SDS.hdf. You can see clearly that SDS.hdf is a structured file. It is, in fact, a miniature, specialised data base for storing scientific data. The HDF package provides utilities for a non-sequential access to various records within that data base. HDF files are completely portable across computer architectures and operating systems. Integer and floating point numbers are stored within HDF files as binary streams of data, which shrinks the size of the file very significantly and which speeds reading and writing of such files. But that data is stored in an architecture independent manner, while at the same time remaining fully compliant with IEEE specifications. Furthermore HDF provides its own facilities for data compression. So, not only do you get full portability, annotations and structuring of your data. The data can be automatically compressed if so desired. And this compression is also architecture and operating system independent.

You can zoom on a specific tag number and ask hdfls to list more information about that particular field:

gustav@blanc:../src 14:40:36 !553 $ hdfls -l -t 701 SDS.hdf
SDS.hdf:
File library version: Major= 4, Minor=1, Release=2
String=NCSA HDF Version 4.1 Release 2, March 1998

SciData dimension record      : (tag 701)
        Ref no      5         14 bytes

gustav@blanc:../src 14:43:13 !554 $

This time hdfls tells us that our program has written 14 bytes of data into the scientific data dimension record.

We can have an even more detailed view at what's inside the file with the command hdfed: 

gustav@blanc:../src 14:40:53 !608 $ hdfed SDS.hdf
hdfed> info -all
 (1)    Version Descriptor            : (Tag 30) Ref 1
 (2)    Vdata Storage                 : (Tag 1963) Ref 3
 (3)    Vdata                         : (Tag 1962) Ref 3
 (4)    Vgroup                        : (Tag 1965) Ref 4
 (5)    Number type                   : (Tag 106) Ref 5
 (6)    SciData dimension record      : (Tag 701) Ref 5
*(7)    Numeric Data Group            : (Tag 720) Ref 2
 (8)    Vgroup                        : (Tag 1965) Ref 6
 (9)    Vgroup                        : (Tag 1965) Ref 7
hdfed> info -long
 (7)    Numeric Data Group            : (Tag 720)
        Ref: 2, Offset: 2617, Length: 12 (bytes)
hdfed> prev tag=701
hdfed> info -long
 (6)    SciData dimension record      : (Tag 701)
        Ref: 5, Offset: 2603, Length: 14 (bytes)
hdfed> prev tag=30
hdfed> info -long
 (1)    Version Descriptor            : (Tag 30)
        Ref: 1, Offset: 2410, Length: 92 (bytes)
hdfed> dump -asci
       0:          NCSA HDF Version 4.1 Release 
      41: 2, March 1998                            
      82:           
hdfed> next tag=106
hdfed> info -long
 (5)    Number type                   : (Tag 106)
        Ref: 5, Offset: 2599, Length: 4 (bytes)
hdfed> dump -byte
       0:      1     24     32      1

The last command, dump -byte performed on the Number type record shows something readable:

24

is the value of flag DFNT_INT32

32

is the bit-length of a DFNT_INT32 variable

If you were to repeat the whole process, but creating a scientific data set for DFNT_INT16 instead (treat it as an exercise), you'd get:

hdfed> dump -byte
       0:      1     22     16      1 
hdfed>

where

22

is the value of flag DFNT_INT16 

16

is the bit-length of a DFNT_INT16 variable