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<div id="projectname">HDF5<span id="projectnumber"> Last Updated on 2026-09-02</span>
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<div id="projectbrief">The HDF5 Field Guide</div>
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<div class="headertitle"><div class="title">Extendible Datasets </div></div>
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<div class="textblock"><p>Navigate back: <a class="el" href="index.html" title="notitle">Main</a> / <a class="el" href="_getting_started.html" title="Getting Started with HDF5">Getting Started with HDF5</a> / <a class="el" href="_learn_basics.html" title="Learning the Basics">Learning the Basics</a> </p><hr />
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<h1 class="doxsection"><a class="anchor" id="secLBExtDsetCreate"></a>
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Creating an Extendible Dataset</h1>
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<p>An extendible dataset is one whose dimensions can grow. HDF5 allows you to define a dataset to have certain initial dimensions, then to later increase the size of any of the initial dimensions.</p>
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<p>HDF5 requires you to use chunking to define extendible datasets. This makes it possible to extend datasets efficiently without having to excessively reorganize storage. (To use chunking efficiently, be sure to see the advanced topic, <a class="el" href="hdf5_chunking.html" title="Chunking in HDF5">Chunking in HDF5</a>.)</p>
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<p>The following operations are required in order to extend a dataset: </p><ul>
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<li>Declare the dataspace of the dataset to have unlimited dimensions for all dimensions that might eventually be extended. </li>
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<li>Set dataset creation properties to enable chunking. </li>
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<li>Create the dataset. </li>
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<li>Extend the size of the dataset.</li>
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</ul>
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<h1 class="doxsection"><a class="anchor" id="secLBExtDsetProg"></a>
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Programming Example</h1>
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<h2 class="doxsection"><a class="anchor" id="subsecLBExtDsetProgDesc"></a>
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Description</h2>
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<p>See <a class="el" href="_l_b_examples.html" title="Examples from Learning the Basics">Examples from Learning the Basics</a> for the examples used in the <a class="el" href="_learn_basics.html" title="Learning the Basics">Learning the Basics</a> tutorial.</p>
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<p>The example shows how to create a 3 x 3 extendible dataset, write to that dataset, extend the dataset to 10x3, and write to the dataset again.</p>
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<p>For details on compiling an HDF5 application: [ <a class="el" href="_l_b_compiling.html" title="Compiling HDF5 Applications">Compiling HDF5 Applications</a> ]</p>
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<h2 class="doxsection"><a class="anchor" id="subsecLBExtDsetProgRem"></a>
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Remarks</h2>
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<ul>
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<li>An unlimited dimension dataspace is specified with the <a class="el" href="group___h5_s.html#ga8e35eea5738b4805856eac7d595254ae" title="Creates a new simple dataspace and opens it for access.">H5Screate_simple</a> call, by passing in <a class="el" href="_h5_spublic_8h.html#a5af9ab788797b2ea9a4843857674ac18">H5S_UNLIMITED</a> as an element of the maxdims array. </li>
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<li>The <a class="el" href="group___p_l_c_r.html#gaf1b11da01d4d45d788c45f8bc5f0cbfa" title="Creates a new property list as an instance of a property list class.">H5Pcreate</a> call creates a new property as an instance of a property list class. For creating an extendible array dataset, pass in <a class="el" href="_h5_ppublic_8h.html#afcd7f8186c404f3a1d768632eacba102">H5P_DATASET_CREATE</a> for the property list class. </li>
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<li>The <a class="el" href="group___d_c_p_l.html#ga3584d592e377da3604b7604e266dcf5b" title="Sets the size of the chunks used to store a chunked layout dataset.">H5Pset_chunk</a> call modifies a Dataset Creation Property List instance to store a chunked layout dataset and sets the size of the chunks used. </li>
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<li>To extend an unlimited dimension dataset use the <a class="el" href="group___h5_d.html#gad31e1e0129f4520c531ce524de2a056f" title="Changes the sizes of a dataset's dimensions.">H5Dset_extent</a> call. Please be aware that after this call, the dataset's dataspace must be refreshed with <a class="el" href="group___h5_d.html#gad42a46be153d895d8c28a11ebf5a0d0a" title="Returns an identifier for a copy of the dataspace for a dataset.">H5Dget_space</a> before more data can be accessed. </li>
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<li>The <a class="el" href="group___d_c_p_l.html#ga4ef814034f601f48ab1ed6db79b4354c" title="Retrieves the size of chunks for the raw data of a chunked layout dataset.">H5Pget_chunk</a> call retrieves the size of chunks for the raw data of a chunked layout dataset. </li>
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<li>Once there is no longer a need for a Property List instance, it should be closed with the <a class="el" href="group___p_l_c_r.html#ga5dce61149211d3ef319452aa598887fb" title="Terminates access to a property list.">H5Pclose</a> call.</li>
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</ul>
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<hr />
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<p> Previous Chapter <a class="el" href="_l_b_dset_layout.html" title="Dataset Storage Layout">Dataset Storage Layout</a> - Next Chapter <a class="el" href="_l_b_com_dset.html" title="Compressed Datasets">Compressed Datasets</a></p>
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<p>Navigate back: <a class="el" href="index.html" title="notitle">Main</a> / <a class="el" href="_getting_started.html" title="Getting Started with HDF5">Getting Started with HDF5</a> / <a class="el" href="_learn_basics.html" title="Learning the Basics">Learning the Basics</a> </p>
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