Re: [patch] Fix to processing end of function stab in dbxread.c

[Daniel Jacobowitz <drow@mvista.com>]

On Thu, Jul 11, 2002 at 11:10:28AM -0700, Jim Ingham wrote:
> Daniel,
> 
> We are talking about two different things here - I am talking about  
> converting the value in the blank FUN stab that marks the end of a  
> function to a read address, NOT correcting an SLINE stab.  That may  
> seem confusing given that I was talking about linetables, so I will  
> give my previous explanation in a little more detail.

Thanks for the excellent explanation.

<snip explanation of the original SLINE problem, which I understand>

> Fred's fix was to emit a fake linetable entry - with linenumber 0, when  
> he sees the end of function stab.  That way, in the linetable there is  
> just a region with no associated linenumbers from the end of the  
> function to 00031fd4.  The problem was getting "the end of the  
> function" from the data in the null FUN stab.  The value of the stab is  
> the offset from the beginning of the function.  He was adding that to  
> function_start_offset which according to the comment in the file is NOT  
> the address of the last function on any system but Solaris.  It is the  
> offset to the text section on other platforms, (which is clever,  
> because then it is the correct thing to use to relocate the addresses  
> in the SLINE entries on all platforms).  But it is not an appropriate  
> thing to add to the offset from the end function stab.
> 
> Note also that in the one other place where we get the real address  
> from the end fun stab (dbxread.c around 1726), we do:
> 
>           /* See if this is an end of function stab.  */
>           if (pst && nlist.n_type == N_FUN && *namestring == '\000')
>           {
>             CORE_ADDR valu;
> 
>             /* It's value is the size (in bytes) of the function for
>                function relative stabs, or the address of the function's
>                end for old style stabs.  */
>             valu = nlist.n_value + last_function_start;
>             if (TEXTHIGH (pst) == 0 || valu > TEXTHIGH (pst))
>               TEXTHIGH (pst) = valu;
>             break;
>           }
> 
> again using last_function_start, not function_start_offset.

You've switched functions.  That code is in read_dbx_symtab.  There was
no variable in process_one_symbol by that name until quite recently. 
They do have the same meaning however.  That's what I meant about your
archeology being wrong.  The comment that function_start_offset is
only correct for Solaris is also wrong; I can verify that it is correct
on GNU/Linux.  That's not your fault, though, the comments in dbxread.c
range from mediocre to misleading.  What comments referencing Solaris 2
(rather than referencing something about Sun's lame tools) often
mean is "on SVR4-ish systems".

I judge from your example that MacOSX has resolved addresses attached
to N_SLINE stabs, but not in ending N_FUN stabs?  GDB assumes that
function_start_offset applies to both of them equally (and it will be
zero if we expect both to be resolved).  On GNU/Linux both N_SLINE and
final N_FUN have offsets within the function.  I suspect that on some
Solaris variant N_SLINE and final N_FUN will both have resolved values.
In that case using last_function_start + valu will put us well outside
of the actual function, causing mayhem.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer