title: malloc_usable_size() returns the wrong size
body: <p>I want to know the size allocated by malloc.<br />
  I have written the source code below.</p>
  <h2>test.c</h2>
  <pre class="lang-c prettyprint-override"><code>#include &lt;stdio.h&gt;
  #include &lt;stdint.h&gt;
  #include &lt;malloc.h&gt;
  void main(void)
  {
      uint8_t *test;
  
      test = (uint8_t *)malloc(sizeof(uint8_t)*4);
      printf(&quot;sizeof(test) = %d\n&quot;,malloc_usable_size(test));
  
      free(test); 
  }  
  </code></pre>
  <p>I expected size to be 4.<br />
  But the result is 12.</p>
  <pre><code>sizeof(test) = 12
  </code></pre>
  <p>Can you tell me what's wrong?<br />
  I hope that size 4 correctly comes out.</p>