title: memcpy between 2 unsigned char* in c
body: <p>How to copy use <code>memcpy</code> with 
  two pointers <code>a</code>, <code>b</code> both are <code>unsigned char *</code>:</p>
  
  <ol>
  <li><code>a</code> pointing to memory filled with data (unsigned char)</li>
  <li><code>b</code> is even not allocated</li>
  </ol>
  
  <p>How to copy <code>a</code> to <code>b</code> using <code>memcpy</code>?  The third parameter of <code>memcpy</code> needs the size of the memory pointed to by <code>a</code> in bytes; how to get it?</p>