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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html>
<head>
<title>Reference (H5R) and Identifier Interfaces (H5I)</title>

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<hr>
<!-- #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&nbsp;&nbsp;</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&nbsp;&nbsp;</code></td>
    <td>Invalid reference type</td></tr>
-->
<tr><td><code>H5R_OBJECT</code></td>
    <td align=right><code>0&nbsp;&nbsp;</code></td>
    <td>Object reference</td></tr>
<tr><td><code>H5R_DATASET_REGION</code></td>
    <td align=right><code>1&nbsp;&nbsp;</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; i++)
          {
            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];
    hssize_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&nbsp;&nbsp;</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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