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<title>Reference (H5R) and Identifier Interfaces (H5I)</title>
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<!-- #EndLibraryItem --><h1>The Reference Interface (H5R) and <br> the Identifier Interface (H5I)</h1>
<h2>1. Introduction</h2>
This document discusses the kinds of references implemented
(and planned) in HDF5 and the functions implemented (and planned)
to support them.
<h2>2. References</h2>
This section contains an overview of the kinds of references
implemented, or planned for implementation, in HDF5.
<p>
<dl>
<dt>Object reference
<dd>Reference to an entire object in the current HDF5 file.
<p>
An object reference points to an entire object in the
current HDF5 file by storing the relative file address
(OID) of the object header for the object pointed to.
The relative file address of an object header is
constant for the life of the object.
An object reference is of a fixed size in the file.
<p>
<dt>Dataset region reference
<dd>Reference to a specific dataset region.
<p>
A dataset region reference points to a region of a
dataset in the current HDF5 file by storing the OID
of the dataset and the global heap offset of the
region referenced. The region referenced is located
by retrieving the coordinates of the areas in the
region from the global heap. A dataset region
reference is of a variable size in the file.
</dl>
<b>Note:</b> All references are treated as soft links for the
purposes of reference counting. The library does not keep track of
reference links and they may dangle if the object they refer to
is deleted, moved, or not yet available.
<h2>3. Reference Types</h2>
Valid HDF5 reference types for use in the H5R functions
are as follows:
<center>
<table>
<tr><th align=left>Reference Type</th><th align=left>Value </th><th align=left>Description</th></tr>
<!-- NOT USER-LEVEL INFORMATION; DELETED FROM HDF5 USER'S GUIDE
<tr><td><code>H5R_BADTYPE</code></td>
<td align=right><code>-1 </code></td>
<td>Invalid reference type</td></tr>
-->
<tr><td><code>H5R_OBJECT</code></td>
<td align=right><code>0 </code></td>
<td>Object reference</td></tr>
<tr><td><code>H5R_DATASET_REGION</code></td>
<td align=right><code>1 </code></td>
<td>Dataset region reference</td></tr>
</table>
</center>
<h2>4. Functions</h2>
Five functions, four in the H5R interface and one in the
H5I interface, have been implemented to support references.
The H5I function is also useful outside the context of references.
<p>
<dl>
<dt><em>herr_t</em> <code>H5Rcreate(</code><em>void *</em><code>reference,</code>
<em>hid_t</em> <code>loc_id,</code>
<em>const char *</em><code>name,</code>
<em>H5R_type_t</em> <code>type,</code>
<em>hid_t</em> <code>space_id)</code>
<dd><code>H5Rcreate</code> creates an object which is a
particular type of reference (specified with the
<code>type</code> parameter) to some file object and/or
location specified with the <code>space_id</code> parameter.
For dataset region references, the selection specified
in the dataspace is the portion of the dataset which
will be referred to.
<p>
<dt><em>hid_t</em> <code>H5Rdereference(</code><em>hid_t</em> <code>dset,</code>
<em>H5R_type_t</em> <code>rtype,</code>
<em>void *</em><code>reference)</code>
<dd><code>H5Rdereference</code> opens the object referenced
and returns an identifier for that object.
The parameter <code>reference</code> specifies a reference of
type <code>rtype</code> that is stored in the dataset
<code>dset</code>.
<p>
<dt><em>int</em> <code>H5Rget_object_type(</code><em>hid_t</em> <code>obj_id,</code>
<em>void *</em><code>reference)</code>
<dd><code>H5Rget_object_type</code> retrieves the type of object
that an object reference points to.
The parameter <code>obj_id</code> specifies the dataset
containing the reference object or the location identifier
of the object that the dataset is located within.
The parameter <code>reference</code> specifies the
reference being queried.
<p>
<dt><em>H5S_t</em> <code>H5Rget_region(</code><em>H5D_t</em> <code>dataset,</code>
<em>H5R_type_t</em> <code>type,</code>
<em>void *</em><code>reference)</code>
<dd><code>H5Rget_region</code> creates a copy of dataspace of
the dataset that is pointed to and defines a selection in
the copy which is the location (or region) pointed to.
The parameter <code>ref</code> specifies a reference of
type <code>rtype</code> that is stored in the dataset
<code>dset</code>.
<p>
<dt><em>H5I_type_t</em> <code>H5Iget_type(</code><em>hid_t</em> <code>id)</code>
<dd>Returns the type of object referred to by the
identifier <code>id</code>. Valid return values appear
in the following list:
<center>
<table>
<tr valign=bottom><td><code>H5I_FILE</code></td>
<td>File objects</td></tr>
<tr valign=bottom><td><code>H5I_GROUP</code></td>
<td>Group objects</td></tr>
<tr valign=bottom><td><code>H5I_DATATYPE</code></td>
<td>Datatype objects</td></tr>
<tr valign=bottom><td><code>H5I_DATASPACE</code></td>
<td>Dataspace objects</td></tr>
<tr valign=bottom><td><code>H5I_DATASET</code></td>
<td>Dataset objects</td></tr>
<tr valign=bottom><td><code>H5I_ATTR</code></td>
<td>Attribute objects</td></tr>
</table>
</center>
<p>
This function was inspired by the need of users to figure
out which type of object closing function
(<code>H5Dclose</code>, <code>H5Gclose</code>, etc.)
to call after a call to <code>H5Rdereference</code>,
but it is also of general use.
<p>
</dl>
<h2>5. Examples</h2>
<b>Object Reference Writing Example</b>
<br>
Create a dataset which has links to other datasets as part
of its raw data and write the dataset to the file.
<p>
<pre>
{
hid_t file1;
hid_t dataset1;
hid_t datatype, dataspace;
char buf[128];
hobj_ref_t link;
hobj_ref_t data[10][10];
int rank;
size_t dimsf[2];
int i, j;
/* Open the file */
file1=H5Fopen("example.h5", H5F_ACC_RDWR, H5P_DEFAULT);
/* Describe the size of the array and create the data space */
rank=2;
dimsf[0] = 10;
dimsf[1] = 10;
dataspace = H5Screate_simple(rank, dimsf, NULL);
/* Define datatype */
datatype = H5Tcopy(H5T_STD_REF_OBJ);
/* Create a dataset */
dataset1=H5Dcreate(file1,"Dataset One",datatype,dataspace,H5P_DEFAULT);
/* Construct array of OIDs for other datasets in the file */
/* somewhat hokey and artificial, but demonstrates the point */
for(i=0; i<10; i++)
for(j=0; j<10; j++)
{
sprintf(buf,"/Group/Linked Set %d-%d",i,j);
if(H5Rcreate(&link,file1,buf,H5R_OBJECT,-1)>0)
data[i][j]=link;
} /* end for */
/* Write the data to the dataset using default transfer properties. */
H5Dwrite(dataset, H5T_STD_REF_OBJ, H5S_ALL, H5S_ALL, H5P_DEFAULT, data);
/* Close everything */
H5Sclose(dataspace);
H5Tclose(datatype);
H5Dclose(dataset1);
H5Fclose(file1);
}
</pre>
<b>Object Reference Reading Example</b>
<br>
Open a dataset which has links to other datasets as part of
its raw data and read in those links.
<p>
<pre>
{
hid_t file1;
hid_t dataset1, tmp_dset;
href_t data[10][10];
int i, j;
/* Open the file */
file1=H5Fopen("example.h5", H5F_ACC_RDWR, H5P_DEFAULT);
/* Open the dataset */
dataset1=H5Dopen(file1,"Dataset One",H5P_DEFAULT);
/*
* Read the data to the dataset using default transfer properties.
* (we are assuming the dataset is the same and not querying the
* dimensions, etc.)
*/
H5Dread(dataset, H5T_STD_REF_OBJ, H5S_ALL, H5S_ALL, H5P_DEFAULT, data);
/* Analyze array of OIDs of linked datasets in the file */
/* somewhat hokey and artificial, but demonstrates the point */
for(i=0; i<10; i++)
for(j=0; j<10; i++)
{
if((tmp_dset=H5Rdereference(dataset, H5T_STD_REF_OBJ, data[i][j]))>0)
{
<perform operations on linked dataset>
} /* end if */
H5Dclose(tmp_dset);
} /* end for */
/* Close everything */
H5Dclose(dataset1);
H5Fclose(file1);
}
</pre>
<b>Dataset Region Reference Writing Example</b>
<br>
Create a dataset which has links to other dataset regions
(single elements in this case) as part of its raw data and
write the dataset to the file.
<p>
<pre>
{
hid_t file1;
hid_t dataset1, dataset2;
hid_t datatype, dataspace1, dataspace2;
char buf[128];
href_t link;
href_t data[10][10]; /* HDF5 reference type */
int rank;
size_t dimsf[2];
hsize_t start[3],count[3];
int i, j;
/* Open the file */
file1=H5Fopen("example.h5", H5F_ACC_RDWR, H5P_DEFAULT);
/* Describe the size of the array and create the data space */
rank=2;
dimsf[0] = 10;
dimsf[1] = 10;
dataspace1 = H5Screate_simple(rank, dimsf, NULL);
/* Define Dataset Region Reference datatype */
datatype = H5Tcopy(H5T_STD_REF_DATAREG);
/* Create a dataset */
dataset1=H5Dcreate(file1,"Dataset One",datatype,dataspace1,H5P_DEFAULT);
/* Construct array of OIDs for other datasets in the file */
/* (somewhat artificial, but demonstrates the point) */
for(i=0; i<10; i++)
for(j=0; j<10; i++)
{
sprintf(buf,"/Group/Linked Set %d-%d",i,j);
/* Get the dataspace for the object to point to */
dataset2=H5Dopen(file1,buf,H5P_DEFAULT);
dataspace2=H5Dget_space(dataspace2);
/* Select the region to point to */
/* (could be different region for each pointer) */
start[0]=5; start[1]=4; start[2]=3;
count[0]=2; count[1]=4; count[2]=1;
H5Sselect_hyperslab(dataspace2,H5S_SELECT_SET,start,NULL,count,NULL);
if(H5Rcreate(&link,file1,buf,H5R_REF_DATAREG,dataspace2)>0)
/* Store the reference */
data[i][j]=link;
H5Sclose(dataspace2);
H5Dclose(dataspace2);
} /* end for */
/* Write the data to the dataset using default transfer properties. */
H5Dwrite(dataset, H5T_STD_REF_DATAREG, H5S_ALL, H5S_ALL, H5P_DEFAULT, data);
/* Close everything */
H5Sclose(dataspace);
H5Tclose(datatype);
H5Dclose(dataset1);
H5Fclose(file1);
}
</pre>
<b>Dataset Region Reference Reading Example</b>
<br>
Open a dataset which has links to other datasets regions
(single elements in this case) as part of its raw data and
read in those links.
<p>
<pre>
{
hid_t file1;
hid_t dataset1, tmp_dset;
hid_t dataspace;
href_t data[10][10]; /* HDF5 reference type */
int i, j;
/* Open the file */
file1=H5Fopen("example.h5", H5F_ACC_RDWR, H5P_DEFAULT);
/* Open the dataset */
dataset1=H5Dopen(file1,"Dataset One",H5P_DEFAULT);
/*
* Read the data to the dataset using default transfer properties.
* (we are assuming the dataset is the same and not querying the
* dimensions, etc.)
*/
H5Dread(dataset, H5T_STD_REF_DATAREG, H5S_ALL, H5S_ALL, H5P_DEFAULT, data);
/* Analyze array of OIDs of linked datasets in the file */
/* (somewhat artificial, but demonstrates the point) */
for(i=0; i<10; i++)
for(j=0; j<10; i++)
{
if((tmp_dset=H5Rdereference(dataset, H5D_STD_REF_DATAREG,data[i][j]))>0)
{
/* Get the dataspace with the pointed to region selected */
dataspace=H5Rget_space(data[i][j]);
<perform operation on linked dataset region>
H5Sclose(dataspace);
} /* end if */
H5Dclose(tmp_dset);
} /* end for */
/* Close everything */
H5Dclose(dataset1);
H5Fclose(file1);
}
</pre>
<!--
<h1>Material to Be Omitted!!!</h1>
Additional material above will also need to be deleted or
commented out.
<p>
<h2>"Kinds of Reference" Information</h2>
<dl>
<dt>Dataset Offset Reference
<dd>Reference to a specific byte sequence in a dataset (3)
<dt>Disk Offset Reference
<dd>Reference to a specific byte sequence in a file (3)
<dt>External Object Reference
<dd>Reference to an entire object in another HDF5 file (3)
<dt>External Dataset Region Reference
<dd>Reference an a specific dataset region in another HDF5 file (3)
<dt>External Dataset Offset Reference
<dd>Reference to a specific byte sequence in a dataset in
another HDF5 file (3)
<dt>External Disk Offset Reference
<dd>Reference to a specific byte sequence in another HDF5 file (3)
<dt>Generic Reference
<dd>A reference which may be any of the types defined above. (3)
</dl>
Notes:
<ul>
<li>1 - Planned for in final release.
<li>2 - Most useful reference types to tackle next
<li>3 - Optional reference types.
</ul>
<h2>Comments</h2>
<pre>
Reference types are atomic types and may be included as fields in compound
datatypes.
There are (at least) three levels of reference strength:
Weak - We allow the user to store any type of reference in an array
of references. (u.e., the array of references in the example above
could be a mix of Object, Dataset Region and Internal references)
Medium - We force the user to stick with a particular type of
reference within a dataset, but the datasets pointed to (with
Object and Dataset Region references) may be of any datatype
or dataspace.
Strong - We force the user to stick with a particular type of
reference and Object and Dataset Region references must point to
datasets with the same datatype.
Extra Strong - We force the user to stick with a particular type of
reference and Object and Dataset Region references must point to
datasets with the same datatype <i>and</i> dataspace.
The library is currently implemented with "medium" strength references.
</pre>
<h2>Reference Type</h2>
<center>
<table>
<tr><td><code>H5R_MAXTYPE</code></td>
<td align=right><code>3 </code></td>
<td>Highest type in group (invalid as true type)</td></tr>
</table>
</center>
<h2>Information Regarding Specific Kinds of References</h2>
<dl>
<dt>Dataset Offset Reference
<dd>Points to a sequence of bytes within a dataset in
the current HDF5 file by storing the OID of the dataset and the byte
length and offset of the sequence within the dataset. The offset is
the logical byte offset within the dataset, meaning that the data is
de-compressed before returning the sequence of bytes requested. No
interpretation of the data at that location is provided. However,
if the dataset is extendible and the size of the dimensions are changed,
the element(s) that the sequence is located within may vary. Fixed size
in file.
<dt>Disk Offset Reference
<dd>Points to a sequence of bytes in the current
HDF5 file by storing the byte length and offset of the sequence within
the file, relative to the super block (as are all the other high-level
addresses used in the file). The offset is the absolute byte offset
within the file, no interpretation of the data at that location is
provided. Fixed size in file.
<dt>External Object Reference
<dd>Points to an entire object in another HDF5 file by
storing a global heap offset which points to the URL of the external
file and the OID of the object pointed to. Variable size in file.
<dt>External Dataset Region Reference
<dd>Points to a region of a dataset in
another HDF5 file by storing a global heap offset which points to the
URL of the external file, OID of the dataset and the coordinates of the
region. Variable size in file.
<dt>External Dataset Offset Reference
<dd>Points to a sequence of bytes within a
dataset in another HDF5 file by storing a global heap offset which
points to the URL of the external file, the OID of the dataset and the
byte length and offset of the sequence within the dataset. The offset
is the logical byte offset within the dataset, meaning that the data is
de-compressed before returning the sequence of bytes requested.
However, if the dataset is not stored contiguously and the size of the
dimensions are changed, the element(s) that the sequence is located
within may vary. Variable size in file.
<dt>External Disk Offset Reference
<dd>Points to a sequence of bytes in another
HDF5 file by storing a global heap reference which points to the URL
of the external file and the byte length and offset of the sequence
within the file. The offset is the absolute byte offset within the
file, no interpretation of the data at that location is provided.
Variable size in file.
<dt>Generic Reference
<dd>A reference which may contain any of the other
references defined above. (Mostly useful for implementing "weak"
strength pointers within the medium strength model we are using)
Variable size in file.
</dl>
<h2>Implementation Details</h2>
<dl>
<dt>File Storage
<dd>In order to efficiently index an array, each element must be the
same size when stored in the dataset on disk. Fixed-sized references
will be stored directly in the dataset array on disk; variable-sized
references will have a fixed-size head offset stored in the array on
disk with a file heap used to store the actual variable-sized
information stored in the heap.
<dt>Memory Storage
<dd>Each <code>href_t</code> object in memory is a struct containing
a pointer type and union of information required for each
pointer type.
Information in this structure is not designed for users to view.
Non-C APIs may have to mangle this structure in some way, in order
to provide users with access to references in a language-appropriate
way.
</dl>
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THG Help Desk: <img src="../Graphics/help.png" align=top height=16>
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Describes HDF5 Release 1.4.5, February 2003
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Last modified: 2 August 2004
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