Re: hpd command for DisplayValues

[Andrew Cagney <cagney at redhat dot com>]

BTW, a gdb interaction looks like the below.  The background to know is 
that "display" and "examine" are closely related vis:

(gdb) x/i $pc
0x807b6c0 <main>:       lea    0x4(%esp),%ecx
(gdb) stepi
0x0807b6c4      28      {
(gdb) x/i $pc
0x807b6c4 <main+4>:     and    $0xfffffff0,%esp
(gdb) stepi
0x0807b6c7      28      {
(gdb) display/i $pc
1: x/i $pc  0x807b6c7 <main+7>: pushl  0xfffffffc(%ecx)
(gdb) stepi
0x0807b6ca in main (argc=0x1, argv=0xbff4d0d4) at ../../gdb/gdb.c:28
28      {
1: x/i $pc  0x807b6ca <main+10>:        push   %ebp
(gdb)
0x0807b6cb      28      {
1: x/i $pc  0x807b6cb <main+11>:        mov    %esp,%ebp

The output reflects how "display" is implemented using the lower-level 
"examine" command - it doesn't detect change.

Andrew


Adam Jocksch wrote:
> Andrew Cagney wrote:
> > Adam Jocksch wrote:
> > > Hrm, maybe. Although reading in the hpd spec: "By defining 
> > > actionpoints, the user may request in advance that target program 
> > > execution stop
> >
> > Nice quote, the display object doesn't fit that model since it is 
> > passive - it doesn't directly trigger a stop since it doesn't 
> > implement a mechanism for detecting changes in the data (that would 
> > be watch).  Instead it detects when the variable has changed after a 
> > stop.
> >
> > For the command line, consider this:
> >
> > > [] break main
> > > [] run ....
> > > main hit
> > > [] print argc
> > > ...
> > > [] step
> > > [] print argc
> > > [] step
> >
> > vs:
> >
> > > [] break main
> > > [] run ....
> > > main hit
> > > [] display argc
> > > ...
> > > [] step
> > > ... value of argc changed from 4 to -1
> > > [] step
> > > [] step
> > > Value of argc changed from -1 to 4
> >
> > For a graphical interface the object facilitates the efficient 
> > implementation of a variable display window - it will only trigger a 
> > redraw of the display elements of the X display when there's 
> > something new to draw.
> >
> > Andrew
> >
> > > under certain conditions". Do we want the display to be similar to a 
> > > watchpoint, except it applies to an expression rather than a single 
> > > variable? Or do we want it to be an expression that is displayed 
> > > when the program stops (I remember Andrew saying something about how 
> > > gdb does this, and it's bad, so we probably want to avoid that).
> > >
> > > While what the DisplayValue object is capable is well defined, I 
> > > guess I'm not 100% sure of how it will be used.
> That sounds similar to what I had envisioned.