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<div id="projectname">HDF5<span id="projectnumber"> Last Updated on 2025-10-28</span>
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<div id="projectbrief">The HDF5 Field Guide</div>
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<div class="headertitle"><div class="title">HDF5 Library initialization and shutdown</div></div>
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<div class="textblock"><p>Navigate back: <a class="el" href="index.html">Main</a> / <a class="el" href="_t_n.html">Technical Notes</a> </p><hr />
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<h1><a class="anchor" id="sec_initshut_app"></a>
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Application perspective</h1>
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<h2><a class="anchor" id="subsec_initshut_app_implicit"></a>
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Implicit initialization and shutdown</h2>
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<p>When a developer exports a new symbol as part of the HDF5 library, they should make sure that an application cannot enter the library in an uninitialized state through a new API function, or read an uninitialized value from a non-function HDF5 symbol.</p>
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<p>The HDF5 library initializes itself when an application either enters the library through an API function call such as <a class="el" href="group___h5_f.html#gaa3f4f877b9bb591f3880423ed2bf44bc" title="Opens an existing HDF5 file.">H5Fopen</a>, or when an application evaluates an HDF5 symbol that represents either a property-list identifier such as <a class="el" href="_h5_fpublic_8h.html#a1c406ffa89f4acf5a332144a2683d394">H5F_ACC_RDONLY</a> or <a class="el" href="_h5_fpublic_8h.html#a402673dec5c537b27a49a9a8bd6140b4">H5F_ACC_RDWR</a>, a property-list class identifier such as <a class="el" href="_h5_ppublic_8h.html#a60ec2d4334addfc0eda89614598ee38e">H5P_FILE_ACCESS</a>, a VFD identifier such as <a class="el" href="_h5_f_dfamily_8h.html#adbf24f060712550a2a9649589a6060c6">H5FD_FAMILY</a> or <a class="el" href="_h5_f_dsec2_8h.html#a15ae1f958e1cf11cb239916d76b10606">H5FD_SEC2</a>, or a type identifier such as <a class="el" href="group___p_d_t_c9x.html#ga8fa995f599c6c8203a8d4e7a7dd01805">H5T_NATIVE_INT64</a>.</p>
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<p>The library sets a flag when initialization occurs and as long as the flag is set, skips initialization.</p>
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<p>The library provides a couple of macros that initialize the library as necessary. The library is initialized as a side-effect of the <code>FUNC_ENTER_API*</code> macros used at the top of most API functions. HDF5 library symbols other than functions are provided through <code>#define</code>s that use <a class="el" href="_h5public_8h.html#abb549ba9fa5c4f3107d8ff19602705da">H5OPEN</a> to introduce a library-initialization call (<a class="el" href="group___h5.html#ga27fa33dc262dda95c5aa8df533837480" title="Initializes the HDF5 library.">H5open</a>) at each site where a non-function symbol is used.</p>
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<p>Ordinarily the library registers an <code>atexit(3)</code> handler to shut itself down when the application exits.</p>
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<h2><a class="anchor" id="subsec_initshut_app_explicit"></a>
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Explicit initialization and shutdown</h2>
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<p>An application may use an API call, <a class="el" href="group___h5.html#ga27fa33dc262dda95c5aa8df533837480" title="Initializes the HDF5 library.">H5open</a>, to explicitly initialize the library. <a class="el" href="group___h5.html#ga8a9fe81dcf66972ed75ea481e7750574" title="Flushes all data to disk, closes all open objects, and releases memory.">H5close</a> explicitly shuts down the library.</p>
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<h1><a class="anchor" id="sec_initshut_int"></a>
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Library internals perspective</h1>
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<p>No matter how library initialization begins, eventually the internal function <code>H5_init_library</code> will be called. <code>H5_init_library</code> is responsible for calling the initializers for every internal HDF5 library module (aka "package") in the correct order so that no module is initialized before its prerequisite modules. A table in <code>H5_init_library</code> establishes the order of initialization. If a developer adds a module to the library that it is appropriate to initialize with the rest of the library, then they should insert its initializer into the right place in the table.</p>
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<p><code>H5_term_library</code> drives library shutdown. Library shutdown is table-driven, too. If a developer adds a module that needs to release resources during library shutdown, then they should add a call at the right place to the shutdown table. Note that some entries in the shutdown table are marked as "barriers," and if a new module should only be shutdown <b>strictly after</b> the preceding modules, then it should be marked as a barrier. See the comments in <code>H5_term_library</code> for more information.</p>
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