Re: Is this a bug in gdb

["Volker Weißmann" <volker.weissmann@gmx.de>]

On 2/17/20 1:37 PM, Luis Machado wrote:

> On 2/16/20 7:40 PM, Volker Weißmann wrote:
>> Hello,
>>
>> The help text of the watch command claims that the -l option watches the
>> memory of the variable. When I tried this, I was surprised by the
>> outcome (reproducible):
>>
>>
>> (gdb) watch this->v_
>> Hardware watchpoint 2: this->v_
>
> This command tells GDB to watch the value of this->v_, whatever
> address &(this->v_) points to. That, of course, can change across the
> execution of the program.
>
>> (gdb) watch -l this->v_
>
> This command tells GDB to watch for changes in a particular location.
> Since this->v_ is a value rather than a location, the error is thrown.
>
> The correct invocation would be ...
>> A syntax error in expression, near `restrict *) 0x00007fffffffd398'.
>> (gdb)
>>
>> Note: Using print &(this->v_) and  watch (char[8])
>> *outputoftheprintcommand worked.
>>
>
> ... the above. It points to an address that will be watched.
>
>>
>> I am asking you whether this is a bug in gdb or not, because if it is a
>> bug in gdb, I will try to make a minimal example an file a bug report.
>
> I don't think it is a bug. But maybe the documentation isn't doing a
> good enough job of making it clear how to invoke these commands?

The reason why I thought that this might be a bug, is that I wrote the
following toy program to test it:

class MyClass {
public:
     int var = 0;
     void member() {
         var = 1;
     }
};
int main() {
     MyClass obj;
     obj.member();
     obj.var = 2;
     return obj.var;
}

Lets say I want to know why my program returns 2 and not 1. I can do
this like this:

(gdb) break main.cpp:5
Breakpoint 1 at 0x118c: file main.cpp, line 5.
(gdb) run
Starting program: /home/volker/Sync/DatenVolker/bugreports/gdb/a.out

Breakpoint 1, MyClass::member (this=0x7fffffffe344) at main.cpp:5
5               var = 1;
(gdb) watch -l this->var
Hardware watchpoint 2: -location this->var
(gdb) continue
Continuing.

Hardware watchpoint 2: -location this->var

Old value = 0
New value = 1
MyClass::member (this=0x7fffffffe344) at main.cpp:6
6           }
(gdb) continue
Continuing.

Hardware watchpoint 2: -location this->var

Old value = 1
New value = 2
main () at main.cpp:12
12          return obj.var;
(gdb)

In this toy program, watch -l this->var works, but in my large program
watch -l this->v_ did not. And I do not understand the difference.