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<section id="error-handling">
<span id="errorhandling"></span><h1>Error handling<a class="headerlink" href="#error-handling" title="Permalink to this headline">¶</a></h1>
<p>PROJ maintains an internal error state, which is local to a
<a class="reference internal" href="reference/datatypes.html#c.PJ_CONTEXT" title="PJ_CONTEXT"><code class="xref c c-type docutils literal notranslate"><span class="pre">PJ_CONTEXT</span></code></a> thread context.</p>
<p>See <a class="reference internal" href="quickstart.html"><span class="doc">Quick start</span></a> for more information about how to create and use a thread
context object.</p>
<p>If you receive an abnormal return from a PROJ API function (e.g. a NULL
pointer) you may wish to discover more information about the error.</p>
<p>In this case you can make a call to <a class="reference internal" href="reference/functions.html#c.proj_context_errno" title="proj_context_errno"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_context_errno()</span></code></a>, passing in
your thread context. This will return an integer error code.</p>
<p>If the error code is zero, the last PROJ operation was deemed successful and no
error has been detected.</p>
<p>If the error code is non-zero, an error has been detected. You can pass your
thread context together with this error code to
<a class="reference internal" href="reference/functions.html#c.proj_context_errno_string" title="proj_context_errno_string"><code class="xref c c-func docutils literal notranslate"><span class="pre">proj_context_errno_string()</span></code></a> to retrieve a string describing the error
condition.</p>
<p>A basic example showing how a C program might catch and report errors follows:</p>
<div class="literal-block-wrapper docutils container" id="id1">
<div class="code-block-caption"><span class="caption-text">errorhandling.c</span><a class="headerlink" href="#id1" title="Permalink to this code">¶</a></div>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="w"> </span><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span><span class="cp"></span>
<span class="linenos"> 2</span><span class="w"> </span><span class="cp">#include</span><span class="w"> </span><span class="cpf"><proj.h></span><span class="cp"></span>
<span class="linenos"> 3</span>
<span class="linenos"> 4</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 5</span><span class="w"> </span><span class="n">PJ_CONTEXT</span><span class="w"> </span><span class="o">*</span><span class="n">c</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 6</span><span class="w"> </span><span class="n">PJ</span><span class="w"> </span><span class="o">*</span><span class="n">p</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 7</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">errno</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 8</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="n">errstr</span><span class="p">;</span><span class="w"></span>
<span class="linenos"> 9</span>
<span class="linenos">10</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_context_create</span><span class="p">();</span><span class="w"></span>
<span class="linenos">11</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_create_crs_to_crs</span><span class="p">(</span><span class="n">c</span><span class="p">,</span><span class="w"> </span><span class="s">"EPSG:4326"</span><span class="p">,</span><span class="w"> </span><span class="s">"EPSG:3857"</span><span class="p">,</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span><span class="w"></span>
<span class="linenos">12</span>
<span class="linenos">13</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">p</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">14</span><span class="w"> </span><span class="cm">/* Something is wrong, let's try to get details ... */</span><span class="w"></span>
<span class="linenos">15</span><span class="w"> </span><span class="n">errno</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_context_errno</span><span class="p">(</span><span class="n">c</span><span class="p">);</span><span class="w"></span>
<span class="linenos">16</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">errno</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">17</span><span class="w"> </span><span class="cm">/* This should be impossible. */</span><span class="w"></span>
<span class="linenos">18</span><span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">"Failed to create transformation, reason unknown.</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"></span>
<span class="linenos">19</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">20</span><span class="w"> </span><span class="n">errstr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">proj_context_errno_string</span><span class="p">(</span><span class="n">c</span><span class="p">,</span><span class="w"> </span><span class="n">errno</span><span class="p">);</span><span class="w"></span>
<span class="linenos">21</span><span class="w"> </span><span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">"Failed to create transformation: %s.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">errstr</span><span class="p">);</span><span class="w"></span>
<span class="linenos">22</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">23</span><span class="w"> </span><span class="n">proj_context_destroy</span><span class="p">(</span><span class="n">c</span><span class="p">);</span><span class="w"></span>
<span class="linenos">24</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
<span class="linenos">25</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos">26</span>
<span class="linenos">27</span><span class="w"> </span><span class="cm">/* transformation object is valid, do work ... */</span><span class="w"></span>
<span class="linenos">28</span>
<span class="linenos">29</span><span class="w"> </span><span class="n">proj_destroy</span><span class="p">(</span><span class="n">p</span><span class="p">);</span><span class="w"></span>
<span class="linenos">30</span><span class="w"> </span><span class="n">proj_context_destroy</span><span class="p">(</span><span class="n">c</span><span class="p">);</span><span class="w"></span>
<span class="linenos">31</span>
<span class="linenos">32</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="linenos">33</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
</pre></div>
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