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PODetect usage of macro? (errno)
 

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    <p>This is very specific, and a bit difficult to explain, and quite likely impossible, but here goes. </p> <p>I want to implement <strong>&lt;errno.h&gt;</strong>. (My hobby project is implementing a Standard <strong>C</strong> library.)</p> <p>The naïve way to go about it is:</p> <pre><code>// in &lt;errno.h&gt; extern int errno; // in some .c file int errno = 0; </code></pre> <p>This works. But it has one disadvantage: If a math library function is called, it <em>always</em> has to query the FPU status after execution to set <code>errno</code> as appropriate. This stalls the FPU in math-heavy applications.</p> <p>The <strong>C</strong> standard endorses lazy evaluation of <code>errno</code> by making it a macro instead:</p> <pre><code>int * __errno(); #define errno *__errno() </code></pre> <p>This way, <code>errno</code> is only "set" when its value is actually requested:</p> <pre><code>// "__errno()" returns the location of the current errno value, // the "errno" macro turns it into a value. x = errno; </code></pre> <p>Given some logic in the rest of the library, the FPU status needs only be queried if the last library function called was actually one <em>using</em> the FPU, <em>and the value of <code>errno</code> is actually requested</em>.</p> <p>So far, everything is fine. But it's the other way around that is giving me headaches:</p> <pre><code>errno = 0; </code></pre> <p>The value of <code>errno</code> is not requested at all. <em>But <code>__errno()</code> does not know that, and will query the FPU status if the last library function called was using the FPU.</em></p> <p>Right now I don't see a way to avoid that (i.e., have the <code>errno</code> macro or the <code>__errno()</code> function somehow work differently depending on whether they are used to the left or the right of the assignment operator), and am almost content to accept this.</p> <p>But perhaps any of you SO'ers has a brilliant idea?</p>
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