Re: hpd command for DisplayValues

[Andrew Cagney <cagney@redhat.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.