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<title>Gwyddion Module Library Reference Manual: File Modules</title>
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<h2><span class="refentrytitle">File Modules</span></h2>
<p>File Modules —
More about file modules
</p>
</td>
<td valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="id-1.3.6.3"></a><h2>Overview</h2>
<p>
File modules implement loading and saving of SPM files. The structure
and operation of file modules will be explained on a simple but complete
sample file module. This module imports a sample data format that was
construed for this tutorial and does not occur in practice. However,
the format resembles many (binary) data formats used in practice,
except it contains no device and manufacturer information as they
are not essential for the example.
</p>
<p>
The sample file format our module will load looks as follows, all
values are stored in little-endian:
</p>
<div class="informaltable"><table border="0">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>Position</th>
<th>Type</th>
<th>Name</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr>
<td>0x00</td>
<td>
<span class="type">char</span>[4]</td>
<td><code class="varname">magic</code></td>
<td>Magic header <code class="literal">"SMPL"</code>
</td>
</tr>
<tr>
<td>0x04</td>
<td><span class="type">uint16</span></td>
<td><code class="varname">xres</code></td>
<td>Pixels per line</td>
</tr>
<tr>
<td>0x06</td>
<td><span class="type">uint16</span></td>
<td><code class="varname">yres</code></td>
<td>Number of lines</td>
</tr>
<tr>
<td>0x08</td>
<td><span class="type">float</span></td>
<td><code class="varname">measure</code></td>
<td>Size of one pixel [nm]</td>
</tr>
<tr>
<td>0x0c</td>
<td><span class="type">float</span></td>
<td><code class="varname">z0</code></td>
<td>Value corresponding to 0 in raw data [nm]</td>
</tr>
<tr>
<td>0x10</td>
<td><span class="type">float</span></td>
<td><code class="varname">gain</code></td>
<td>
Conversion factor from raw data to physical values [nm]
</td>
</tr>
<tr>
<td>0x14</td>
<td>
<span class="type">uint16</span>[<code class="varname">xres</code>*<code class="varname">yres</code>]</td>
<td><code class="varname">data</code></td>
<td>Raw data values</td>
</tr>
</tbody>
</table></div>
<p>
Furthermore we will assume files of this type get the extension
<code class="filename">.ssd</code> (Simple SPM Data). The complete module source
code follows, it will be described in detail below.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
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<td class="listing_code"><pre class="programlisting"><span class="preproc">#include</span><span class="normal"> </span><span class="string"><string.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><stdio.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><glib/gstdio.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwyddion/gwymacros.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwyddion/gwymath.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libprocess/stats.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwymodule/gwymodule-file.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><app/gwymoduleutils-file.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string">"err.h"</span>
<span class="preproc">#define</span><span class="normal"> EXTENSION </span><span class="string">".ssd"</span>
<span class="preproc">#define</span><span class="normal"> MAGIC </span><span class="string">"SMPL"</span>
<span class="preproc">#define</span><span class="normal"> </span><span class="function">MAGIC_SIZE</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">MAGIC</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">-</span><span class="normal"> </span><span class="number">1</span><span class="symbol">)</span>
<span class="keyword">enum</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> HEADER_SIZE </span><span class="symbol">=</span><span class="normal"> MAGIC_SIZE </span><span class="symbol">+</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="number">2</span><span class="normal"> </span><span class="symbol">+</span><span class="normal"> </span><span class="number">3</span><span class="symbol">*</span><span class="number">4</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</span>
<span class="keyword">typedef</span><span class="normal"> </span><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> xres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> yres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> measure</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> z0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> gain</span><span class="symbol">;</span>
<span class="cbracket">}</span><span class="normal"> SimpleFile</span><span class="symbol">;</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> </span><span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">gint</span><span class="normal"> </span><span class="function">simple_detect</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> <a href="../libgwyddion/GwyContainer.html">GwyContainer</a></span><span class="symbol">*</span><span class="normal"> </span><span class="function">simple_load</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">filename</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GwyRunType</span><span class="normal"> mode</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">GwyModuleInfo</span><span class="normal"> module_info </span><span class="symbol">=</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> <a href="libgwymodule-gwymoduleloader.html#GWY-MODULE-ABI-VERSION:CAPS">GWY_MODULE_ABI_VERSION</a></span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">&</span><span class="normal">module_register</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Imports simple data files."</span><span class="symbol">),</span>
<span class="normal"> </span><span class="string">"J. Random Hacker <hacker.jr@example.org>"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"1.0"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"Bit Rot Inc."</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"2006"</span><span class="symbol">,</span>
<span class="cbracket">}</span><span class="symbol">;</span>
<span class="function"><a href="libgwymodule-gwymoduleloader.html#GWY-MODULE-QUERY:CAPS">GWY_MODULE_QUERY</a></span><span class="symbol">(</span><span class="normal">module_info</span><span class="symbol">)</span>
<span class="keyword">static</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Basic-Types.html#gboolean">gboolean</a></span>
<span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="function"><a href="libgwymodule-gwymodule-file.html#gwy-file-func-register">gwy_file_func_register</a></span><span class="symbol">(</span><span class="string">"simple"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Simple AFM files (.afm)"</span><span class="symbol">),</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileDetectFunc">GwyFileDetectFunc</a></span><span class="symbol">)&</span><span class="normal">simple_detect</span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileLoadFunc">GwyFileLoadFunc</a></span><span class="symbol">)&</span><span class="normal">simple_load</span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#TRUE:CAPS">TRUE</a></span><span class="symbol">;</span>
<span class="cbracket">}</span>
<span class="keyword">static</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Basic-Types.html#gint">gint</a></span>
<span class="function">simple_detect</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">only_name</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="function"><a href="http://developer.gnome.org/doc/API/2.0/glib/glib-String-Utility-Functions.html#g-str-has-suffix">g_str_has_suffix</a></span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">name_lowercase</span><span class="symbol">,</span><span class="normal"> EXTENSION</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">?</span><span class="normal"> </span><span class="number">20</span><span class="normal"> </span><span class="symbol">:</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">buffer_len </span><span class="symbol">></span><span class="normal"> MAGIC_SIZE</span>
<span class="normal"> </span><span class="symbol">&&</span><span class="normal"> </span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">head</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">==</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">100</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="cbracket">}</span>
<span class="keyword">static</span><span class="normal"> <a href="../libgwyddion/GwyContainer.html">GwyContainer</a></span><span class="symbol">*</span>
<span class="function">simple_load</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">filename</span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Miscellaneous-Macros.html#G-GNUC-UNUSED:CAPS">G_GNUC_UNUSED</a> </span><span class="usertype">GwyRunType</span><span class="normal"> mode</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">SimpleFile</span><span class="normal"> simple</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwyContainer</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">container </span><span class="symbol">=</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwySIUnit</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">unit</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwyDataField</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">dfield</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">buffer</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">guchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">p</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">err </span><span class="symbol">=</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gsize</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> expected_size</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">guint16</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">rawdata</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">data</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gint</span><span class="normal"> i</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(!</span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-get-contents">gwy_file_get_contents</a></span><span class="symbol">(</span><span class="normal">filename</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">size</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">err</span><span class="symbol">))</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_GET_FILE_CONTENTS</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">err</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Error-Reporting.html#g-clear-error">g_clear_error</a></span><span class="symbol">(&</span><span class="normal">err</span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol"><=</span><span class="normal"> HEADER_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_TOO_SHORT</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">!=</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_FILE_TYPE</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Simple"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> p </span><span class="symbol">=</span><span class="normal"> buffer </span><span class="symbol">+</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> expected_size </span><span class="symbol">=</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">+</span><span class="normal"> HEADER_SIZE</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol">!=</span><span class="normal"> expected_size</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_SIZE_MISMATCH</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> expected_size</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> dfield </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-new">gwy_data_field_new</a></span><span class="symbol">(</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS">FALSE</a></span><span class="symbol">);</span>
<span class="normal"> data </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-get-data">gwy_data_field_get_data</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="normal"> rawdata </span><span class="symbol">=</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#guint16">guint16</a></span><span class="symbol">*)</span><span class="normal">p</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">for</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">i </span><span class="symbol">=</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span><span class="normal"> i </span><span class="symbol"><</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">;</span><span class="normal"> i</span><span class="symbol">++)</span>
<span class="normal"> data</span><span class="symbol">[</span><span class="normal">i</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS">GUINT16_FROM_LE</a></span><span class="symbol">(</span><span class="normal">rawdata</span><span class="symbol">[</span><span class="normal">i</span><span class="symbol">])*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">+</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0</span><span class="symbol">;</span>
<span class="normal"> unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-set-si-unit-xy">gwy_data_field_set_si_unit_xy</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal"> unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-set-si-unit-z">gwy_data_field_set_si_unit_z</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal"> container </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data"</span><span class="symbol">,</span><span class="normal"> dfield</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data/title"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="string">"Topography"</span><span class="symbol">));</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> container</span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.4"></a><h2>Administrative and Conventions</h2>
<p>
Beside standard headers our module includes a special header that
provides some common inline functions
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="preproc">#include</span><span class="normal"> </span><span class="string">"err.h"</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
It is located in <code class="filename">modules/file</code> for the use by
in-tree modules. External modules can copy it (or only the
actually used functions) to their source code.
</p>
<p>
Header <code class="filename">err.h</code> contains common error reporting
functions (called <code class="function">err_ERROR_TYPE</code>).
Its primary purpose it to ensure consistency of error
messages between file modules and to ease translators' work.
</p>
<p>
Next we define a few convenience symbols: the file extension, the magic
header and its size, and header size
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="preproc">#define</span><span class="normal"> EXTENSION </span><span class="string">".ssd"</span>
<span class="preproc">#define</span><span class="normal"> MAGIC </span><span class="string">"SMPL"</span>
<span class="preproc">#define</span><span class="normal"> </span><span class="function">MAGIC_SIZE</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">MAGIC</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">-</span><span class="normal"> </span><span class="number">1</span><span class="symbol">)</span>
<span class="keyword">enum</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> HEADER_SIZE </span><span class="symbol">=</span><span class="normal"> MAGIC_SIZE </span><span class="symbol">+</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="number">2</span><span class="normal"> </span><span class="symbol">+</span><span class="normal"> </span><span class="number">3</span><span class="symbol">*</span><span class="number">4</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
We also define a structure representing the imported file. It is a bit
superfluous in our module as we do not pass this information around, but
it is useful in many modules
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">typedef</span><span class="normal"> </span><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> xres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> yres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> measure</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> z0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> gain</span><span class="symbol">;</span>
<span class="cbracket">}</span><span class="normal"> SimpleFile</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.5"></a><h2>Registration</h2>
<p>
The feature registration is similar to data processing functions,
there are just no menu paths and stock icons, on the other hand there are
more functions the module can provide: one for each file operation. Our
module implements only file detection and import therefore we pass
<a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS"><code class="literal">NULL</code></a> for the save and export functions.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gboolean">gboolean</a></span>
<span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="function"><a href="libgwymodule-gwymodule-file.html#gwy-file-func-register">gwy_file_func_register</a></span><span class="symbol">(</span><span class="string">"simple"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Simple AFM files (.afm)"</span><span class="symbol">),</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileDetectFunc">GwyFileDetectFunc</a></span><span class="symbol">)&</span><span class="normal">simple_detect</span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileLoadFunc">GwyFileLoadFunc</a></span><span class="symbol">)&</span><span class="normal">simple_load</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#TRUE:CAPS">TRUE</a></span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
It is necessary to provide at least one of load, save and export
operations, otherwise the file type would be rejected. It is highly
recommended to provide a detection function, although it is not
required. If no detection function exists for a file type the user
has to always explicitly request the type to load and/or save files
in this format.
</p>
</div>
<div class="refsect1">
<a name="id-1.3.6.6"></a><h2>Detection</h2>
<p>
Two types of detection exist: on load when we have the file
available, and on save when the file does not exist and we know only
the requested file name. They are differentiated by
<code class="varname">only_name</code> argument. In both cases the detection
function gets a <a class="link" href="libgwymodule-gwymodule-file.html#GwyFileDetectInfo" title="GwyFileDetectInfo"><span class="type">GwyFileDetectInfo</span></a> structure, but if
<code class="varname">only_name</code> is <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS"><code class="literal">FALSE</code></a>, some fields are unset.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12
13</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gint">gint</a></span>
<span class="function">simple_detect</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">only_name</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-str-has-suffix">g_str_has_suffix</a></span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">name_lowercase</span><span class="symbol">,</span><span class="normal"> EXTENSION</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">?</span><span class="normal"> </span><span class="number">20</span><span class="normal"> </span><span class="symbol">:</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">buffer_len </span><span class="symbol">></span><span class="normal"> MAGIC_SIZE</span>
<span class="normal"> </span><span class="symbol">&&</span><span class="normal"> </span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">head</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">==</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">100</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
The latter case is usually trivial, we just compare the extension with
the normal extension of our file type.
</p>
<p>
The former case can be more complex. File name does not tell us much
about file type: consider just the number of formats using extension
<code class="filename">.afm</code> (not talking about <code class="filename">.dat</code>
which can be essentially anything). Therefore we use file names only
as the last resort and base the detection on actual file contents.
</p>
<p>
Many formats have well defined magic headers, precisely for the puropose
of easy detection. If out format has one, we are lucky and the detection
code can look as simple as in the example. To detect a particular
version or type of a file format a more detailed analysis is necessary,
but not substantially different from the simple case.
</p>
<p>
The most problematic case arises when the format has no detectable magic
header at all. Often we can employ the following approach: read the
header data and check whether it makes sense if interpreted as our
file format. For example we read the number of lines and pixels per
line, calculate expected file size and compare it to the real file size.
Or we read the physical dimensions and check they are positive (and
approximately of expected magnitude). If our detection routine requires
significant processing or <em class="structfield"><code>head</code></em> and
<em class="structfield"><code>tail</code></em> are not sufficient and we have to open
the file ourselves, it is advisable to perform a quick check that can
eliminate most files that are <span class="emphasis"><em>not</em></span> of our type first,
and then perform the sophisticated test only on the likely candidates.
</p>
</div>
<div class="refsect1">
<a name="id-1.3.6.7"></a><h2>Loading</h2>
<p>
The task of the load function is to read the file and create
a <a href="../libgwyddion/GwyContainer.html"><span class="type">GwyContainer</span></a> containing all importable data. It must not assume
anything about the file properties and contents, not even that the file
passed the detection because the user can request a file type
explicitly, bypassing detection. It has also deal with broken files,
checking whether the file is long enough to contain the data it claims
to and performing further parameter validation if necessary.
</p>
<p>
The file contents can be often read with <a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-get-contents"><code class="function">gwy_file_get_contents()</code></a> which
uses <code class="function">mmap()</code> (if available). The returned memory
is read-only. Sometimes destructive header parsing is useful, in such
a case it is possible to use <a href="/usr/share/gtk-doc/html/glib/glib-Memory-Allocation.html#g-memdup"><code class="function">g_memdup()</code></a> to duplicate the header,
keeping the data part mapped read-only.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol"><=</span><span class="normal"> HEADER_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_TOO_SHORT</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
The header is parsed using the
<a class="link" href="gwymodule-tutorial-file.html#module-tutorial-file-portability" title="Portability">portable</a>
binary buffer reading functions.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">p </span><span class="symbol">=</span><span class="normal"> buffer </span><span class="symbol">+</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp/libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Then we can create the data field and read the data into it. Since
<span class="application">Gwyddion</span> always works with values in base
SI units, we have to convert the raw data values to physical values.
If the file uses non-base units (which is common in SPM), we have to
multiply the data by the corresponding factor: in your case with
10<sup>-9</sup> as the physcial quantities are stored
in nanometers in the file.
Data reading can be most easily performed with GLibs function like
<a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS"><code class="function">GUINT16_FROM_LE()</code></a>.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">dfield </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-new">gwy_data_field_new</a></span><span class="symbol">(</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS">FALSE</a></span><span class="symbol">);</span>
<span class="normal">data </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-get-data">gwy_data_field_get_data</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="normal">rawdata </span><span class="symbol">=</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#guint16">guint16</a></span><span class="symbol">*)</span><span class="normal">p</span><span class="symbol">;</span>
<span class="keyword">for</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">i </span><span class="symbol">=</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span><span class="normal"> i </span><span class="symbol"><</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">;</span><span class="normal"> i</span><span class="symbol">++)</span>
<span class="normal"> data</span><span class="symbol">[</span><span class="normal">i</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS">GUINT16_FROM_LE</a></span><span class="symbol">(</span><span class="normal">rawdata</span><span class="symbol">[</span><span class="normal">i</span><span class="symbol">])*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">+</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
We have to set also the lateral and value units of the created data
field (by default, the dimensions and values are unitless).
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-set-si-unit-xy">gwy_data_field_set_si_unit_xy</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyprocess/GwyDataField.html#gwy-data-field-set-si-unit-z">gwy_data_field_set_si_unit_z</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Our sample file format can only store topography channels, therefore
the units are always meters. For different types of channels the units
have to be set accordingly. Note that
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"nA"</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
is exactly the same as
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"A"</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
In other words, the prefixes are ignored because <a href="../libgwyddion/GwySIUnit.html"><span class="type">GwySIUnit</span></a> represents
the unit type and contains no magnitude information. However, if the
format stores the units as a string including the prefix we can use
<a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new-parse"><code class="function">gwy_si_unit_new_parse()</code></a> to obtain the corresponding power of 10 (and
subsequently multiply data field values with the correct multiplier).
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwySIUnit.html#gwy-si-unit-new-parse">gwy_si_unit_new_parse</a></span><span class="symbol">(</span><span class="string">"nA"</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">power10</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Finally we create a new <a href="../libgwyddion/GwyContainer.html"><span class="type">GwyContainer</span></a>, put the data field at
<code class="literal">"/0/data"</code> and set the channel title
<code class="literal">"/0/data/title"</code> (if the file contains explicit
channel names, they are good candidates for a channel title, otherwise
we settle for channel type as Topography or Current), and return the
container. The application then takes care of adding it to the data
browser and showing it in a data window.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">container </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data"</span><span class="symbol">,</span><span class="normal"> dfield</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data/title"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="string">"Topography"</span><span class="symbol">));</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="keyword">return</span><span class="normal"> container</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.8"></a><h2>Loading More</h2>
<p>
Real file formats often contain more information we would like to
import. First, they can contain several data fields (channels or even
unrelated images). We store the second data at
<code class="literal">"/1/data"</code> and its title at
<code class="literal">"/1/data/title"</code>, the third channel data at
<code class="literal">"/2/data"</code>, etc. The number sequence does not have
to be contignuous but it is customary to make it so (in any case
small integers should be used, do not start counting from 12758933).
</p>
<p>
Auxiliary information about the data as date and time of scanning, scan
speed, tunneling current, tip oscillation frequency or user-defined
comments can be stored to metadata. Metadata is a nested <a href="../libgwyddion/GwyContainer.html"><span class="type">GwyContainer</span></a>
at <code class="literal">"/0/meta"</code> (for the first channel, the number of
course corresponds to channel id). The keys are simply names of the
values, the values are always strings. Note both keys and values have
to be converted to UTF-8.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">meta </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Comment"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="normal">myfile</span><span class="symbol">-></span><span class="normal">user_comment</span><span class="symbol">));</span>
<span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Tip oscillation frequency"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup-printf">g_strdup_printf</a></span><span class="symbol">(</span><span class="string">"%g Hz"</span><span class="symbol">,</span><span class="normal"> myfile</span><span class="symbol">-></span><span class="normal">freq</span><span class="symbol">));</span>
<span class="function"><a href="../libgwyddion/GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/meta"</span><span class="symbol">,</span><span class="normal"> meta</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="module-tutorial-file-portability"></a><h2>Portability</h2>
<p>
Gwyddion runs on many platforms and can be compiled with various
compilers. File import is the area most affected by the differences
between them, therefore certain care must be taken to make file modules
work across all platforms.
</p>
<div class="refsect2">
<a name="id-1.3.6.9.3"></a><h3>Text Data</h3>
<p>
Text files can encode <span class="emphasis"><em>line ends</em></span>
differently from the platform Gwyddion is running on. Therefore
opening files as text may lead to unexpected results. One usually
opens all files as binary and uses <a href="../libgwyddion/libgwyddion-gwyutils.html#gwy-str-next-line"><code class="function">gwy_str_next_line()</code></a> to parse text
files (or text parts of files) to lines.
</p>
<p>
The text representation of <span class="emphasis"><em>real numbers</em></span> is
locale-dependent. User's locale is unpredictable and a module must not
set locale to arbitrary values because it would affect the rest of the
application. Therefore one has to use functions as <a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-ascii-strtod"><code class="function">g_ascii_strtod()</code></a>
to read real numbers portably.
</p>
<p>
Many text strings, like comments and remarks, are stored in an
arbitrary <span class="emphasis"><em>character encoding</em></span>.
Typically it is ISO 8895-1 (Latin1) or UTF-16, but sometimes more
exotic encodings are used. Since Gwyddion uses UTF-8 for all text data
(like Gtk+ does) they must be converted, e.g., with <a href="/usr/share/gtk-doc/html/glib/glib-Character-Set-Conversion.html#g-convert"><code class="function">g_convert()</code></a>.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.3.6.9.4"></a><h3>Binary Data</h3>
<p>
The canonical bad example is
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="type">char</span><span class="normal"> c</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">LONG</span><span class="normal"> i</span><span class="symbol">;</span>
<span class="normal"> </span><span class="type">wchar_t</span><span class="normal"> remark</span><span class="symbol">[</span><span class="number">20</span><span class="symbol">];</span>
<span class="cbracket">}</span><span class="normal"> header</span><span class="symbol">;</span>
<span class="function">fread</span><span class="symbol">(&</span><span class="normal">header</span><span class="symbol">,</span><span class="normal"> </span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">header</span><span class="symbol">),</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span><span class="normal"> filehandle</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
For start, <span class="type">LONG</span> is unlikely to exist on non-Microsoft
platforms. So should we fix it to <span class="type">long int</span>? No, the
<span class="emphasis"><em>type size</em></span> of classic C types (<span class="type">short</span>,
<span class="type">long</span>, etc.) is different on different architectures (more
precisely ABIs) and the standard prescribes only the lower limit. If we
mean `32bit integer' we also have to express it in our code:
<a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gint32"><span class="type">gint32</span></a>.
</p>
<p>
Likewise, <span class="type">wchar_t</span> size is not well-defined. If the
file comes from a Microsoft platform, where <span class="type">wchar_t</span> is
used a lot, it is probably 16bit, so we write <a href="/usr/share/gtk-doc/html/glib/glib-Unicode-Manipulation.html#gunichar2"><span class="type">gunichar2</span></a> instead
and use <a href="/usr/share/gtk-doc/html/glib/glib-Unicode-Manipulation.html#g-utf16-to-utf8"><code class="function">g_utf16_to_utf8()</code></a> to get UTF-8.
</p>
<p>
Then we have <span class="emphasis"><em>byte order</em></span>. If we are on a platform
with different native byte order than the file uses, we have to reverse
the bytes in each multi-byte value. GLib provides macros as
<a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS"><code class="function">GUINT16_FROM_LE()</code></a> to ease this task.
</p>
<p>
And last but not least, different ABIs prescribe different
<span class="emphasis"><em>structure padding</em></span>. Typically there can be
from 0 to 7 unused bytes between <em class="structfield"><code>c</code></em> and
<em class="structfield"><code>i</code></em> fields. While most compilers provide
some packing <code class="literal">#pragma</code>s or other means to
specify desired structure padding, the portability of these
constructs is questionable at best.
</p>
<p>
The answer to (almost) all the problems above is: Don't do that then.
Avoid direct reading of structures, read the data into a flat byte
buffer instead and use the
file module utils
functions to obtain individual values.
</p>
</div>
</div>
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