Taking the address of a convenience variable value

Andrew Cagney cagney@gnu.org
Wed May 26 17:24:00 GMT 2004

> On Tue, May 25, 2004 at 12:55:46PM -0400, Paul Dubuc wrote:
>>> In the June 2004 issue of the C/C++ User's Journal (p. 24) there is an 
>>> article on how to write user-defined commands for gdb to examine the 
>>> contents of STL vectors, sets and maps.  It looks extremely useful, so I 
>>> decided to try it modifying the commands for use with the GCC STL, but I 
>>> can't get some of the commands for sets and maps to work.  It relies on a 
>>> tecnique that involves being able to take the address of a convenience 
>>> variable value, for example:
>>>   set $maptype = &$arg0._M_t._M_header->_M_value_field
>>>   set $maptypep = &$maptype
>>> When I try this the 2nd statement gives me the error message
>>>   Attempt to take address of value not located in memory.

As you note, its trying to take the address of a convenience variable - 
since convenience variables do not live in the inferior they don't have 
an address.


  set $maptype = &$arg0._M_t._M_header->_M_value_field
  set $maptypep = &&$arg0._M_t._M_header->_M_value_field


  set $maptype = $arg0._M_t._M_header->_M_value_field
  set $maptypep = &$arg0._M_t._M_header->_M_value_field

make sense?

The other, sigh, possability is that this was a ``feature'' and there's 
been a regression :-/


>>> It doesn't work with gdb 5.3 or 6.1 on Solaris.  The author claims that it 
>>> works on HP-UX, but I don't know why it would be any different.
>>> Is there a way around this?  Or is there another source of user-defined 
>>> commands that can be used to print the contents of STL containers in gdb?  
>>> Any help would be very much appreciated.
> I read that article and was wondering if it was necessary to compile the
> STL with -g and not with -O2. I don't think the author mentioned it, but
> how else could all of the symbols in the STL work properly with GDB?

What does:

   (gdb) paddr &$arg0._M_t._M_header->_M_value_field


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