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<title>
HDF5/H5Z API Specification
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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.
<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>H5Z_filter_t</em> <code>filter</code>,
<em>const char *</em><code>comment</code>,
<em>H5Z_func_t</em> <code>function</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 new filter <code>function</code> as described below.
This step can be skipped only when the filter is predifined, 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>
The new filter is specified by the filter identifier
<code>filter</code>.
<p>
<code>comment</code> is used for debugging and may be
the null pointer.
<p>
<code>function</code> is a user-defined 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>New filters.</strong> Before a filter can be
linked into an application with <code>H5Zregister</code>,
the filter must be defined as described in the HDF5 Library
header file <code>H5Zpublic.h</code>:
<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
<code>H5Pset_filter</code>.
The additional flag <code>H5Z_FLAG_REVERSE</code> is set when
the filter is called as part of the input pipeline.
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 and return the number of valid bytes or zero for
failure. 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>.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>H5Z_filter_t</em> <code>method</code>
<dd>IN: Filter identifier.
<dt><em>const char *</em><code>comment</code>
<dd>IN: String associated with the filter;
used for debugging purposes only.
May be the null pointer.
<dt><em>H5Z_func_t</em> <code>function</code>
<dd>IN: Filter function.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</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.
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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.
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Describes HDF5 Release 1.5, Unreleased Development Branch
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Last modified: 2 April 2003
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