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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Compound Datatypes
</TITLE>
</HEAD>
<body bgcolor="#ffffff">
<!-- BEGIN MAIN BODY -->
[ <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>
<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/compound.f90">Fortran 90 Example</A> ]
- <code>compound.f90</code>
<BR>
[ <A HREF="examples/java/Compound.java">Java Example</A> ] - <code>Compound.java</code>
</UL>
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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<BR> <H6>Last Modified: June 22, 2001</H6><BR>
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