Re: gdb problem

[Stephen Berman <stephen.berman@gmx.net>]

On Sat, 14 Jun 2008 13:33:30 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Date: Sat, 14 Jun 2008 00:30:34 +0200
>> 
>> (gdb) br /Users/steve/lib/emacs-cvs-pre-unicode/src/window.c:3840
>> Internal: readin window.c pst for `/Users/steve/lib/emacs-cvs-pre-unicode/src/window.c' found when no symtab found.
>> (gdb) br window.c:3840
>> Breakpoint 3 at 0x8099956: file window.c, line 3840.
>> (gdb) c
>> Continuing.
>> 
>> Breakpoint 3, Fdisplay_buffer (buffer=137869892, not_this_window=137636041, frame=137636041)
>>     at window.c:3840
>> warning: Source file is more recent than executable.
>> 3840      p->next = o->next;
>> 
>> 
>> First, I do not understand the gdb response when I tried to set a
>> breakpoint using an absolute path, but it evidently failed to set the
>> breakpoint.  Then I just used the file name and that did set the
>> breakpoint, and when it hit the breakpoint, gdb said it was in
>> Fdisplay_buffer, which is what I wanted, so this seemed to be ok.  But
>> the next two lines, the last two listed above, are strange: the warning
>> that the source file is more recent than the executable, and the content
>> of the line at the breakpoint.  In fact, this line is from window.c in
>> the directory /Users/steve/cvsroot/emacs/src, which is my source
>> directory of the Emacs CVS trunk (which does not contain Fdisplay_buffer
>> any more).
>> 
>> What happened here?
>
> There's something I'd like you to explain first.  If, as you say, the
> Emacs binary you were debugging was produced from the sources in the
> same /Users/steve/lib/emacs-cvs-pre-unicode/ tree, then why did you
> use an absolute file name to set a breakpoint in the first place?
> That's a highly unusual thing to do on systems where the source file
> names and the directories they live in are recorded in the executable's
> debug info.

Actually, in my first attempt to debug Fdisplay_buffer I did just type
"br window.c:3840" and got the same result as above.  Since that was
obviously not what I expected, in my ignorance of gdb I thought it
wouldn't hurt to try using an absolute file name.  In the post I
included both for the sake of completeness.

> More to the point, my crystal ball says that you compiled that Emacs
> binary in the /Users/steve/cvsroot/emacs/ tree, and then moved it,
> together with the sources, into /Users/steve/lib/emacs-cvs-pre-unicode/.
> As I say above, the original directories are recorded in the debug
> info which GDB reads and uses, so it tries to access the source files
> which were since updated.

Your crystal ball is crystal clear; would you like to ask it who's going
to win the European football championship or the US presidential
election?

> To verify that my guess is correct, type "info sources" in GDB, and
> see where it thinks your source files live.

It indeed shows source in both of the directories.

> If my guess is correct, then the following command should tell GDB to
> try the sources in /Users/steve/lib/emacs-cvs-pre-unicode/src first:
>
>   (gdb) dir /Users/steve/lib/emacs-cvs-pre-unicode/src
>
> (you should probably do the same for the lwlib directory).
>
> GDB has a more powerful feature for this kind of situations: the "set
> substitute-path" command.  If the above doesn't help, read about that
> command in the GDB manual, and set up your substitution rules to point
> GDB at the /Users/steve/lib/emacs-cvs-pre-unicode/ tree.

Thanks for the pointer; before posting I did briefly look at the table
of contents of the gdb manual but didn't see anything recognizably
relevant.

> Alternatively, just rebuilding Emacs in the emacs-cvs-pre-unicode tree
> should resolve all these problems (again, if my guess about the root
> cause is correct).

Thanks for the help and the useful information.

Steve Berman