title: Get the memory address pointed to by a ctypes pointer
body: <p>Short version:
  How can I get the address that a ctypes pointer points to?</p>
  
  <p>Long version:
  I have registered a python function as a callback with a C library. The C library expects function signature of <code>void (*p_func)(char stat, char * buf, short buf_len)</code> so I register an appropriate python function. When I get into the python function, I want to know the memory address pointed to by <code>buf</code>. How can I do this?</p>