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<html>
<head>
<title>
HDF5/H5Z API Specification
</title>
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<h1>H5Z: Filter and Compression Interface</h1>
</center>
<h2>Filter and Compression API Functions</h2>
These functions enable the user to configure new filters
for the local environment.
<table border=0 width=80%>
<tr><td valign=top>
<ul>
<li><a href="#Compression-Register">H5Zregister</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Compression-Unregister">H5Zunregister</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Compression-FilterAvail">H5Zfilter_avail</a>
</ul>
</td></tr>
</table>
<p>
HDF5 supports a filter pipeline that provides the capability for standard
and customized raw data processing during I/O operations.
HDF5 is distributed with a small set of standard filters such as
compression (gzip and a shuffling algorithm) and
error checking (Fletcher32 checksum).
For further flexibility, the library includes tools enabling a
user application to extend the pipeline through the
creation and registration of customized filters.
<p>
As mentioned above, one set of filters distributed with HDF5 provides
built-in methods for raw data compression.
The flexibility of the filter pipeline implementation enables the
definition of additional compression methods by a user application.
A compression method<br>
—
is associated with a dataset when the dataset is created,<br>
—
can be used only with chunked data (ie., datasets stored in the
<code>H5D_CHUNKED</code> storage layout), and<br>
—
is applied independently to each chunk of the dataset.
<p>
The HDF5 library does not support compression for contiguous datasets
because of the difficulty of implementing random access for partial I/O.
Compact dataset compression is not supported because it would not produce
significant results.
<p>
See <a href="Datasets.html"><cite>The Dataset Interface (H5D)</cite></a>
in the <cite>HDF5 User's Guide</cite> for further information regarding
data compression.
<p>
<strong>The FORTRAN90 Interfaces:</strong>
<br>
<font size=-1>
<i>In general, each FORTRAN90 subroutine performs exactly the same task
as the corresponding C function. The links below go to the C function
descriptions, which serve as general descriptions for both. A button,
under <strong>Non-C API(s)</strong> at the end of the C function description,
opens an external browser window displaying the FORTRAN90-specific
information. You will probably want to adjust the size and location of
this external window so that both browser windows are visible and to
facilitate moving easily between them.</i>
</font>
<table border=0 width=80%>
<tr><td valign=top>
<ul>
<!--<li><a href="#Compression-Register">h5zregister_f</a> -->
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Compression-Unregister">h5zunregister_f</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Compression-FilterAvail">h5zfilter_avail_f</a>
</ul>
</td></tr>
</table>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Compression-Register">H5Zregister</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Zregister</code>(<em>const H5Z_class_t</em> <code>filter_class</code>)
)
<dt><strong>Purpose:</strong>
<dd>Registers new filter.
<dt><strong>Description:</strong>
<dd><code>H5Zregister</code> registers a new filter with the
HDF5 library.
<p>
Making a new filter available to an application is a two-step
process. The first step is to <span class="termEmphasis">define</span>
the three filter callback filter functions described below:
<code>can_applyr_func</code>, <code>set_local_func</code>, and
<code>filter_func</code>.
This step can be skipped only when the filter is predefined, as is
the case with the Fletcher32 checksum and shuffle filters that
are distributed with the HDF5 Library.
This call to <code>H5Zregister</code>,
<span class="termEmphasis">registering</span> the filter with the
library, is the second step.
<p>
<code>H5Zregister</code> accepts a single parameter,
the <code>filter_class</code> data structure,
which is defined as follows:
<pre>
typedef struct H5Z_class_t {
H5Z_filter_t filter_id;
const char *comment;
H5Z_can_apply_func_t can_apply_func;
H5Z_set_local_func_t set_local_func;
H5Z_func_t filter_func;
} H5Z_class_t;
</pre>
<p>
<code>filter_id</code> is the identifier for the new filter.
<p>
<code>comment</code> is used for debugging and may be
the null pointer.
<p>
<code>can_apply_func</code>, described in detail below,
is a user-defined callback function which determines whether
the combination of the dataset creation property list setting,
the datatype, and the dataspace represent a valid combination
to apply this filter to.
<p>
<code>set_local_func</code>, described in detail below,
is a user-defined callback function which sets any parameters that
are specific to this dataset, based on the combination of the
dataset creation property list values, the datatype, and the
dataspace.
<p>
<code>filter_func</code>, described in detail below,
is a user-defined callback function which performs the action
of the filter.
<p>
The statistics associated with a filter are not reset
by this function; they accumulate over the life of the library.
<p>
<strong>The callback functions</strong>
<br>
Before <code>H5Zregister</code> can link a filter into an
application, three callback functions must be defined
as described in the HDF5 Library header file <code>H5Zpublic.h</code>.
<p>
<u>The <i>can apply</i> callback function</u> is defined as follows:<br>
<dir>
<em>typedef herr_t</em> (*<code>H5Z_can_apply_func_t</code>)
(<em>hid_t</em> <code>dcpl_id</code>,
<em>hid_t</em> <code>type_id</code>,
<em>hid_t</em> <code>space_id</code>)
</dir>
<p>
Before a dataset is created, the <i>can apply</i> callbacks for
any filters used in the dataset creation property list are called
with the dataset's dataset creation property list, <code>dcpl_id</code>,
the dataset's datatype, <code>type_id</code>, and
a dataspace describing a chunk, <code>space_id</code>,
(for chunked dataset storage).
<p>
This callback must determine whether the combination of the
dataset creation property list settings, the datatype, and the
dataspace represent a valid combination to which to apply this filter.
For example, an invalid combination may involve
the filter not operating correctly on certain datatypes,
on certain datatype sizes, or on certain sizes of the chunk dataspace.
<p>
This callback can be the <code>NULL</code> pointer, in which case
the library will assume that the filter can be applied to a dataset with
any combination of dataset creation property list values, datatypes,
and dataspaces.
<p>
The <i>can apply</i> callback function must return
a positive value for a valid combination,
zero for an invalid combination, and
a negative value for an error.
<p>
<u>The <i>set local</i> callback function</u> is defined as follows:<br>
<dir>
<em>typedef herr_t</em> (*<code>H5Z_set_local_func_t</code>)
(<em>hid_t</em> <code>dcpl_id</code>,
<em>hid_t</em> <code>type_id</code>,
<em>hid_t</em> <code>space_id</code>)
</dir>
<p>
After the <i>can apply</i> callbacks are checked for a new dataset,
the <i>set local</i> callback functions for any filters used in the
dataset creation property list are called.
These callbacks receive
<code>dcpl_id</code>, the dataset's private copy of the dataset
creation property list passed in to <code>H5Dcreate</code>
(i.e. not the actual property list passed in to <code>H5Dcreate</code>);
<code>type_id</code>, the datatype identifier passed in to
<code>H5Dcreate</code>,
which is not copied and should not be modified; and
<code>space_id</code>, a dataspace describing the chunk
(for chunked dataset storage), which should also not be modified.
<p>
The <i>set local</i> callback must set any parameters that are
specific to this dataset, based on the combination of the
dataset creation property list values, the datatype, and the dataspace.
For example, some filters perform different actions based on
different datatypes, datatype sizes, numbers of dimensions,
or dataspace sizes.
<p>
The <i>set local</i> callback may be the <code>NULL</code> pointer,
in which case, the library will assume that there are
no dataset-specific settings for this filter.
<p>
The <i>set local</i> callback function must return
a non-negative value on success and
a negative value for an error.
<p>
<u>The <i>filter operation</i> callback function</u>,
defining the filter's operation on the data, is defined as follows:
<dir>
<em>typedef size_t</em> (*<code>H5Z_func_t</code>)
(<em>unsigned int</em> <code>flags</code>,
<em>size_t</em> <code>cd_nelmts</code>,
<em>const unsigned int</em> <code>cd_values[]</code>,
<em>size_t</em> <code>nbytes</code>,
<em>size_t *</em><code>buf_size</code>,
<em>void **</em><code>buf</code>)
</dir>
<p>
The parameters <code>flags</code>, <code>cd_nelmts</code>,
and <code>cd_values</code> are the same as for the function
<a href="RM_H5P.html#Property-SetFilter"><code>H5Pset_filter</code></a>.
The one exception is that an additional flag,
<code>H5Z_FLAG_REVERSE</code>, is set when
the filter is called as part of the input pipeline.
<p>
The parameter <code>*buf</code> points to the input buffer
which has a size of <code>*buf_size</code> bytes,
<code>nbytes</code> of which are valid data.
<p>
The filter should perform the transformation in place if
possible. If the transformation cannot be done in place,
then the filter should allocate a new buffer with
<code>malloc()</code> and assign it to <code>*buf</code>,
assigning the allocated size of that buffer to
<code>*buf_size</code>.
The old buffer should be freed by calling <code>free()</code>.
<p>
If successful, the <i>filter operation</i> callback function
returns the number of valid bytes of data contained in <code>*buf</code>.
In the case of failure, the return value is <code>0</code> (zero)
and all pointer arguments are left unchanged.
<dt><strong>Note:</strong>
<dd>The <code>H5Zregister</code> interface is substantially revised
from the HDF5 Release 1.4.x series.
The <code>H5Z_class_t</code> struct and
the <i>set local</i> and <i>can apply</i> callback functions
first appeared in HDF5 Release 1.6.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>const H5Z_class_t</em> <code>filter_class</code>
<dd>IN: Struct containing filter-definition information.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
<!--
<dt><strong>Non-C API(s):</strong>
<dd><a href="fortran/h5z_FORTRAN.html#h5zregister_f"
target="FortranWin"><img src="Graphics/FORTRAN.gif" border=0></a>
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</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Compression-Unregister">H5Zunregister</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Zunregister</code>(<em>H5Z_filter_t</em> <code>filter</code>)
<dt><strong>Purpose:</strong>
<dd>Unregisters a filter.
<dt><strong>Description:</strong>
<dd><code>H5Zunregister</code> unregisters the filter
specified in <code>filter</code>.
<p>
After a call to <code>H5Zunregister</code>, the filter
specified in <code>filter</code> will no longer be
available to the application.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>H5Z_filter_t</em> <code>filter</code>
<dd>IN: Identifier of the filter to be unregistered.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
<dt><strong>Non-C API(s):</strong>
<dd><a href="fortran/h5z_FORTRAN.html#h5zunregister_f"
target="FortranWin"><img src="Graphics/FORTRAN.gif" border=0></a>
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</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Compression-FilterAvail">H5Zfilter_avail</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Zfilter_avail</code>(<em>H5Z_filter_t</em> <code>filter</code>)
<dt><strong>Purpose:</strong>
<dd>Determines whether a filter is available.
<dt><strong>Description:</strong>
<dd><code>H5Zfilter_avail</code> determines whether the filter
specified in <code>filter</code> is available to the application.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>H5Z_filter_t</em> <code>filter</code>
<dd>IN: Filter identifier.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
<dt><strong>Non-C API(s):</strong>
<dd><a href="fortran/h5z_FORTRAN.html#h5zfilter_avail_f"
target="FortranWin"><img src="Graphics/FORTRAN.gif" border=0></a>
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