Re: Is this a bug in gdb

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?