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<TITLE>Crystal Space: Memory Debugger Internals</TITLE>
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<H3> 8.4.3 How It Works </H3>
<!--docid::SEC636::-->
<P>
Here are some technical details about the functioning of the memory debugger.
This can help you understand better how it works, and I think this can help
you use the debugger in a more productive fashion.
</P><P>
In each `<SAMP>new</SAMP>' or `<SAMP>delete</SAMP>' call, we check whenever we have been
initialized. If not, we call the initialization function that parses the
`<TT>.map</TT>' file, sets the debugging options and so on. When program exits
the memory debugger checks for unfreed memory block, displays statistics and
so on. This is done by declaring a dummy static variable at the very end of
memory debugger module and the shutdown function being called from its
destructor.
</P><P>
The only trick used in memory debugger is used to get the address from where
`<SAMP>new</SAMP>' and `<SAMP>delete</SAMP>' have been called. This is a processor-dependent
issue, alas we can't do it in a platform-independent fashion. To support
doing this on different platforms we define a macro called
<CODE>GET_CALL_ADDRESS()</CODE>. We pass the first argument of the procedure we're
invoking <CODE>GET_CALL_ADDRESS()</CODE> from and we get the address from were we
were called in a variable called `<SAMP>addr</SAMP>'.
</P><P>
Here are details on how we're doing it on different platforms:
</P><P>
<DL COMPACT>
<DT><EM>Intel x86</EM>
<DD>These processors use a stack for passing arguments, and the same stack is used
to push the return address. Since all machines uses a top-to-bottom stack
(i.e. stack increases in the direction of lower addresses), and also since
ANSI C standard requires that the addresses of function arguments
increase (that is, if you're defining a function such as `<SAMP>void some(int
a, int b)</SAMP>' you can be absolutely sure that address of `<SAMP>b</SAMP>' is higher than
address of `<SAMP>a</SAMP>'. It is a good guess to suppose that on most computers as
of today upon the invocation of the `<SAMP>new</SAMP>' and `<SAMP>delete</SAMP>' operators the
stack will look like this:
<P>
<TABLE><tr><td> </td><td class=example><pre>+-----------------+ higher addresses
| arg2 | ^
+-----------------+ |
| arg1 | |
+-----------------+ |
| return address | |
+-----------------+ lower addresses
</pre></td></tr></table></P><P>
This means that if we take the address of first argument of the procedure and
go one machine word back, we'll get the address of the function's <EM>return
address</EM> word. Now we peek it from there and we have the return address.
This is embedded into a macro that looks like this:
</P><P>
<TABLE><tr><td> </td><td class=example><pre>#define GET_CALL_ADDRESS(firstarg) \
address addr = ((address*)&firstarg) [-1];
</pre></td></tr></table></P><P>
Where `<SAMP>address</SAMP>' is a shortcut for type `<SAMP>void*</SAMP>'.
</P><P>
<DT><EM>An Almost Cross-Platform Solution</EM>
<DD>Fortunately for us, GCC versions above 2.8.0 have a extremely useful
built-in function called <CODE>__builtin_return_address()</CODE>. If we invoke it
with the argument zero, it returns just what we need; the address from where
our procedure was called. Thus, an almost-cross-platform
<CODE>GET_CALL_ADDRESS()</CODE> is implemented this way:
<P>
<TABLE><tr><td> </td><td class=example><pre>#if (__GNUC__ >= 2) && (__GNUC_MINOR__ >= 8)
# define GET_CALL_ADDRESS(firstarg) \
address addr = (address)__builtin_return_address(0);
#endif
</pre></td></tr></table></P><P>
Thus, if you can't get `<TT>memdbg.sh</TT>' to work, you should find a newer
GCC; this should always help.
</DL>
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