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    <p>In this case you don't need to mock <code>getaddrinfo</code>, rather, you need to test without relying on its functionality. Both Patrick and Noah have good points but you have at least two other options:</p> <h2>Option 1: Subclass to Test</h2> <p>Since you already have your object in a class, you can subclass to test. For example, assume the following is your actual class:</p> <pre><code>class DnsClass { int lookup(...); }; int DnsClass::lookup(...) { return getaddrinfo(...); } </code></pre> <p>Then, for testing, you would subclass like this:</p> <pre><code>class FailingDnsClass { int lookup(...) { return 42; } }; </code></pre> <p>You can now use the <code>FailingDnsClass</code> subclass to generate errors but still verify that everything behaves correctly when an error condition occurs. Dependency Injection is often your friend in this case.</p> <p>NOTE: This is quite similar to Patrick's answer but doesn't (hopefully) involve changing the production code if you aren't already setup for dependency injection.</p> <h2>Option 2: Use a link seam</h2> <p>In C++, you also have link-time seams which Michael Feathers describes in <a href="http://rads.stackoverflow.com/amzn/click/0131177052" rel="noreferrer"><em>Working Effectively with Legacy Code</em></a>.</p> <p>The basic idea is to leverage the linker and your build system. When compiling the unit tests, link in your own version of <code>getaddrinfo</code> which will take precedence over the system version. For example:</p> <p>test.cpp:</p> <pre><code>#include &lt;sys/types.h&gt; #include &lt;sys/socket.h&gt; #include &lt;netdb.h&gt; #include &lt;iostream&gt; int main(void) { int retval = getaddrinfo(NULL, NULL, NULL, NULL); std::cout &lt;&lt; "RV:" &lt;&lt; retval &lt;&lt; std::endl; return retval; } </code></pre> <p>lib.cpp:</p> <pre><code>#include &lt;sys/types.h&gt; #include &lt;sys/socket.h&gt; #include &lt;netdb.h&gt; int getaddrinfo(const char *node, const char *service, const struct addrinfo *hints, struct addrinfo **res ) { return 42; } </code></pre> <p>And then for testing:</p> <pre><code>$ g++ test.cpp lib.cpp -o test $ ./test RV:42 </code></pre>
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    1. COI have used option 2 to test production code. It works as long as you never need to call the real version in the unit tests.
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    2. CO+1 Option 2 is very cool, with of course the drawback mentioned by Caspin.
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      1. USRobert S. Barnes
 

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