<code><span class="hljs-meta">#<span class="hljs-meta-keyword">include</span> <span class="hljs-meta-string"><stdio.h></span></span>
<span class="hljs-function"><span class="hljs-keyword">int</span> <span class="hljs-title">main</span><span class="hljs-params">()</span>
</span>{
    <span class="hljs-keyword">int</span> n1, n2;

    <span class="hljs-built_in">printf</span>(<span class="hljs-string">"Enter two positive integers: "</span>);
    <span class="hljs-built_in">scanf</span>(<span class="hljs-string">"%d %d"</span>,&n1,&n2);

    <span class="hljs-keyword">while</span>(n1!=n2)
    {
        <span class="hljs-keyword">if</span>(n1 > n2)
            n1 -= n2;
        <span class="hljs-keyword">else</span>
            n2 -= n1;
    }
    <span class="hljs-built_in">printf</span>(<span class="hljs-string">"GCD = %d"</span>,n1);

    <span class="hljs-keyword">return</span> <span class="hljs-number">0</span>;
}</code>

Output

<samp>Enter two positive integers: 81
153
GCD = 9</samp>