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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Compound Datatypes
</TITLE>
</HEAD>
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[ <A HREF="title.html"><I>HDF5 Tutorial Top</I></A> ]
<H1>
<BIG><BIG><BIG><FONT COLOR="#c101cd">Compound Datatypes</FONT>
</BIG></BIG></BIG></H1>
<hr noshade size=1>
<BODY>
<H2>Contents:</H2>
<UL>
<LI><A HREF="#def">Creating Compound Datatypes</A>
<LI>Programming Example
<UL>
<LI> <A HREF="#desc">Description</A>
<LI> <A HREF="#rem">Remarks</A>
<LI> <A HREF="#fc">File Contents</A>
</UL>
</UL>
<em><b>Note:</b> The FORTRAN API does not yet support compound datatypes.</em>
<HR>
<A NAME="def">
<H2>Creating Compound Datatypes</H2>
A compound datatype is similar to a struct in C or a common block in
FORTRAN. It is a collection of one or more atomic types or small arrays of
such types. To create and use a compound datatype you need to be familiar
with various properties of the compound datatype:
<UL>
<LI>It is of class <strong>compound</strong>.
<LI>It has a fixed total size, in bytes.
<LI>It consists of zero or more members (defined in any order) with
unique names and occupying non-overlapping regions within the datum.
<LI>Each member has its own datatype.
<LI>Each member is referenced by an index number between zero and <em>N</em>-1,
where <em>N</em> is the number of members in the compound datatype.
<LI>Each member has a name which is unique among its siblings in a
compound datatype.
<LI>Each member has a fixed byte offset, which locates the first byte
(smallest byte address) of that member in the compound datatype.
<LI>Each member can be a small array of up to four dimensions.
</UL>
Properties of members of a compound datatype are defined when the
member is added to the compound datatype and cannot be subsequently modified.
<P>
Compound datatypes must be built out of other datatypes. First, one
creates an empty compound datatype and specifies its total size. Then
members are added to the compound datatype in any order.
<H2> Programming Example</H2>
<A NAME="desc">
<H3><U>Description</U></H3>
This example shows how to create a compound datatype, write an array
to the file which uses the compound datatype, and read back subsets of
the members.
<p>
<UL>
[ <A HREF="examples/h5_compound.c">C Example</A> ] - <code>h5_compound.c</code>
<BR>
[ <A HREF="examples/java/Compound.java">Java Example</A> ] - <code>Compound.java</code>
</UL>
<PRE>
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#include "hdf5.h"
#define FILE "SDScompound.h5"
#define DATASETNAME "ArrayOfStructures"
#define LENGTH 10
#define RANK 1
int
main(void)
{
/* First structure and dataset*/
typedef struct s1_t {
int a;
float b;
double c;
} s1_t;
s1_t s1[LENGTH];
hid_t s1_tid; /* File datatype identifier */
/* Second structure (subset of s1_t) and dataset*/
typedef struct s2_t {
double c;
int a;
} s2_t;
s2_t s2[LENGTH];
hid_t s2_tid; /* Memory datatype handle */
/* Third "structure" ( will be used to read float field of s1) */
hid_t s3_tid; /* Memory datatype handle */
float s3[LENGTH];
int i;
hid_t file, dataset, space; /* Handles */
herr_t status;
hsize_t dim[] = {LENGTH}; /* Dataspace dimensions */
/*
* Initialize the data
*/
for (i = 0; i < LENGTH; i++) {
s1[i].a = i;
s1[i].b = i*i;
s1[i].c = 1./(i+1);
}
/*
* Create the data space.
*/
space = H5Screate_simple(RANK, dim, NULL);
/*
* Create the file.
*/
file = H5Fcreate(FILE, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
/*
* Create the memory datatype.
*/
s1_tid = H5Tcreate (H5T_COMPOUND, sizeof(s1_t));
H5Tinsert(s1_tid, "a_name", HOFFSET(s1_t, a), H5T_NATIVE_INT);
H5Tinsert(s1_tid, "c_name", HOFFSET(s1_t, c), H5T_NATIVE_DOUBLE);
H5Tinsert(s1_tid, "b_name", HOFFSET(s1_t, b), H5T_NATIVE_FLOAT);
/*
* Create the dataset.
*/
dataset = H5Dcreate(file, DATASETNAME, s1_tid, space, H5P_DEFAULT);
/*
* Wtite data to the dataset;
*/
status = H5Dwrite(dataset, s1_tid, H5S_ALL, H5S_ALL, H5P_DEFAULT, s1);
/*
* Release resources
*/
H5Tclose(s1_tid);
H5Sclose(space);
H5Dclose(dataset);
H5Fclose(file);
/*
* Open the file and the dataset.
*/
file = H5Fopen(FILE, H5F_ACC_RDONLY, H5P_DEFAULT);
dataset = H5Dopen(file, DATASETNAME);
/*
* Create a datatype for s2
*/
s2_tid = H5Tcreate(H5T_COMPOUND, sizeof(s2_t));
H5Tinsert(s2_tid, "c_name", HOFFSET(s2_t, c), H5T_NATIVE_DOUBLE);
H5Tinsert(s2_tid, "a_name", HOFFSET(s2_t, a), H5T_NATIVE_INT);
/*
* Read two fields c and a from s1 dataset. Fields in the file
* are found by their names "c_name" and "a_name".
*/
status = H5Dread(dataset, s2_tid, H5S_ALL, H5S_ALL, H5P_DEFAULT, s2);
/*
* Display the fields
*/
printf("\n");
printf("Field c : \n");
for( i = 0; i < LENGTH; i++) printf("%.4f ", s2[i].c);
printf("\n");
printf("\n");
printf("Field a : \n");
for( i = 0; i < LENGTH; i++) printf("%d ", s2[i].a);
printf("\n");
/*
* Create a datatype for s3.
*/
s3_tid = H5Tcreate(H5T_COMPOUND, sizeof(float));
status = H5Tinsert(s3_tid, "b_name", 0, H5T_NATIVE_FLOAT);
/*
* Read field b from s1 dataset. Field in the file is found by its name.
*/
status = H5Dread(dataset, s3_tid, H5S_ALL, H5S_ALL, H5P_DEFAULT, s3);
/*
* Display the field
*/
printf("\n");
printf("Field b : \n");
for( i = 0; i < LENGTH; i++) printf("%.4f ", s3[i]);
printf("\n");
/*
* Release resources
*/
H5Tclose(s2_tid);
H5Tclose(s3_tid);
H5Dclose(dataset);
H5Fclose(file);
return 0;
}
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
</PRE>
The program outputs the following:
<PRE>
Field c :
1.0000 0.5000 0.3333 0.2500 0.2000 0.1667 0.1429 0.1250 0.1111 0.1000
Field a :
0 1 2 3 4 5 6 7 8 9
Field b :
0.0000 1.0000 4.0000 9.0000 16.0000 25.0000 36.0000 49.0000 64.0000 81.0000
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
</PRE>
<A NAME="rem">
<H3><U>Remarks</U></H3>
<UL>
<LI><CODE>H5Tcreate</CODE> creates a new datatype of the specified class with
the specified number of bytes.
<PRE>
hid_t H5Tcreate ( H5T_class_t class, size_t size )
</PRE>
<UL>
<LI>The <I>class</I> parameter specifies the datatype to create.
Currently only the <CODE>H5T_COMPOUND</CODE> datatype class is supported with this
function.
<LI>The <I>size</I> parameter specifies the number of bytes in the
datatype to create.
</UL>
<P>
<LI><CODE>H5Tinsert</CODE> adds a member to the compound datatype specified by
<I>type_id</I>.
<PRE>
herr_t H5Tinsert ( hid_t type_id, const char * name, off_t offset,
hid_t field_id )
</PRE>
<UL>
<LI>The <I>type_id</I> parameter is the identifier of the compound datatype
to modify.
<LI>The <I>name</I> parameter is the name of the field to insert. The new
member name must be unique within a compound datatype.
<LI>The <I>offset</I> parameter is the offset in the memory structure of
the field to insert.
The library defines the <CODE>HOFFSET</CODE> macro to compute the offset of a member within
a struct:
<PRE>
HOFFSET ( s, m )
</PRE>
This macro computes the offset of member <I>m</I> within a struct
variable <I>s</I>.
<LI>The <I>field_id</I> parameter is the datatype identifier of the
field to insert.
</UL>
<P>
<LI><CODE>H5Tclose</CODE> releases a datatype.
<PRE>
herr_t H5Tclose ( hid_t type_id )
</PRE>
The <I>type_id</I> parameter is the identifier of the datatype to release.
</UL>
<A NAME="fc">
<H3><U>File Contents</U></H3>
<PRE>
HDF5 "SDScompound.h5" {
GROUP "/" {
DATASET "ArrayOfStructures" {
DATATYPE {
H5T_STD_I32BE "a_name";
H5T_IEEE_F32BE "b_name";
H5T_IEEE_F64BE "c_name";
}
DATASPACE { SIMPLE ( 10 ) / ( 10 ) }
DATA {
{
[ 0 ],
[ 0 ],
[ 1 ]
},
{
[ 1 ],
[ 1 ],
[ 0.5 ]
},
{
[ 2 ],
[ 4 ],
[ 0.333333 ]
},
{
[ 3 ],
[ 9 ],
[ 0.25 ]
},
{
[ 4 ],
[ 16 ],
[ 0.2 ]
},
{
[ 5 ],
[ 25 ],
[ 0.166667 ]
},
{
[ 6 ],
[ 36 ],
[ 0.142857 ]
},
{
[ 7 ],
[ 49 ],
[ 0.125 ]
},
{
[ 8 ],
[ 64 ],
[ 0.111111 ]
},
{
[ 9 ],
[ 81 ],
[ 0.1 ]
}
}
}
}
}
</PRE>
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Describes HDF5 Release 1.2.2, June 2000
<BR> <H6>Last Modified: April 5, 2000</H6><BR>
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