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    <p>Your linked list looks something like this:</p> <pre class="lang-none prettyprint-override"><code>+------+----+----+ | Head | hd | tl | ----------&gt;-------- +------+----+----+ \ | ----&gt;------ | NULL | / \ | | +------+-----+------+------+ +------+-----+------+------+ | Node | x=0 | next | prev | | Node | x=1 | next | prev | +------+-----+------+------+ +------+-----+------+------+ | | | \ NULL / -----------------------&lt;---------------------- </code></pre> <p>(I've simplified it to two nodes).</p> <p>Now, we can just write out what this code does:</p> <pre><code>linkNode = hdd-&gt;hd; while(linkNode!=0) { free(linkNode-&gt;prev); linkNode = linkNode-&gt;next; } </code></pre> <ol> <li><code>linkNode = hdd-&gt;hd</code> leaves <code>linkNode</code> pointed at the first node</li> <li><code>(linkNode!=0)</code> is true (the first node is not NULL), so we enter the while loop</li> <li><code>free(linkNode-&gt;prev)</code> calls <code>free(NULL)</code> since <code>hdd-&gt;hd-&gt;prev == NULL</code> (you set the first node up like this explicitly). This is fine, but does nothing.</li> <li><code>linkNode = linkNode-&gt;next</code> leaves <code>linkNode</code> pointed at the last node</li> <li><code>linkNode!=0</code> is still true (the last node is also not NULL), so we go round the loop again</li> <li><code>free(linkNode-&gt;prev)</code> frees the previous node (which is the first one)</li> <li><code>linkNode = linkNode-&gt;next</code> leaves <code>linkNode == NULL</code></li> <li><code>linkNode!=0</code> is false now, so the loop terminates.</li> </ol> <p>So, we free'd all but the last node. No node's <code>prev</code> member points to that node, so calling <code>free(linkNode-&gt;prev)</code> can never free it. You could, however, free it via <code>hdd-&gt;tl</code>.</p>
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    1. POhow to free memory for doubly linked list program in c
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    1. COhow did u draw that?
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      1. USSibi Rajasekaran
    2. COtediously, but sometimes pictures are just clearer (even ASCII art)
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    3. COin a notepad or some tool?
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      1. USSibi Rajasekaran
 

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