Watchpoints stopping GDB on specific threads

Watchpoints stopping GDB on specific threads

[Luis Machado]

Folks,

GDB currently has support for conditional breakpoint stops on specific
threads, by issuing the command "break <location> thread <id>".

I'm looking forward to have the same functionality for watchpoints. So
whenever GDB detects a watchpoint trigger, it would verify the current
thread and would stop only if the thread matches the ID we provided in a
command like "watch <address> thread <id>". if no ID is provided, GDB
would stop at every thread as usual (assuming a threaded watchpoint
support).

This is useful to conduct debugging sessions on applications with a
great number of threads, being able to inspect just the thread we're
interested in. It would also take advantage of the upcomming threaded
watchpoint support, hopefully.

At a first glance, i'd need to work in a different way than the
breakpoint case, because the entire watchpoint expression is handled by
a expression parser. It automatically processes the "thread <id>"
parameter and of course gives an error because that's not recognized. Do
you have any idea on how this could be solved in a clever way? We could
start parsing the expression from the end, and try to locate the "thread
<id>" parameter, and then hand over the remaining parameters to the
expression parser. What do you think?

Best regards,

-- 
Luis Machado
Software Engineer 
IBM Linux Technology Center
LoP Toolchain/Debuggers' team
Phone: +55 19-2132-2218
T/L: 839-2218
e-mail: luisgpm@vnet.linux.ibm.com

Re: Watchpoints stopping GDB on specific threads

[Daniel Jacobowitz]

On Tue, Aug 14, 2007 at 01:20:58PM -0300, Luis Machado wrote:
> At a first glance, i'd need to work in a different way than the
> breakpoint case, because the entire watchpoint expression is handled by
> a expression parser. It automatically processes the "thread <id>"
> parameter and of course gives an error because that's not recognized. Do
> you have any idea on how this could be solved in a clever way? We could
> start parsing the expression from the end, and try to locate the "thread
> <id>" parameter, and then hand over the remaining parameters to the
> expression parser. What do you think?

You might want to take a look at this:

  http://sourceware.org/ml/gdb-patches/2005-04/msg00092.html

break *EXPRESSION thread N is the same as watch EXPRESSION thread N.
Eli suggested basically the same thing you did; it might work :-)

-- 
Daniel Jacobowitz
CodeSourcery

Re: Watchpoints stopping GDB on specific threads

[Eli Zaretskii]

> From: Luis Machado <luisgpm@linux.vnet.ibm.com>
> Date: Tue, 14 Aug 2007 13:20:58 -0300
> 
> I'm looking forward to have the same functionality for watchpoints. So
> whenever GDB detects a watchpoint trigger, it would verify the current
> thread and would stop only if the thread matches the ID we provided in a
> command like "watch <address> thread <id>". if no ID is provided, GDB
> would stop at every thread as usual (assuming a threaded watchpoint
> support).

Unless I misunderstand something fundamental, if the current thread ID
is in some variable, you could define a condition for a watchpoint,
no?

Re: Watchpoints stopping GDB on specific threads

[Daniel Jacobowitz]

On Tue, Aug 14, 2007 at 10:41:06PM +0300, Eli Zaretskii wrote:
> Unless I misunderstand something fundamental, if the current thread ID
> is in some variable,

I don't believe it is.

I see that Frederic Riss posted a "threadnum" command patch, in July
2006 - that's also related to this problem.

-- 
Daniel Jacobowitz
CodeSourcery

Re: Watchpoints stopping GDB on specific threads

[Luis Machado]

On Tue, 2007-08-14 at 22:41 +0300, Eli Zaretskii wrote:
> > From: Luis Machado <luisgpm@linux.vnet.ibm.com>
> > Date: Tue, 14 Aug 2007 13:20:58 -0300
> > 
> > I'm looking forward to have the same functionality for watchpoints. So
> > whenever GDB detects a watchpoint trigger, it would verify the current
> > thread and would stop only if the thread matches the ID we provided in a
> > command like "watch <address> thread <id>". if no ID is provided, GDB
> > would stop at every thread as usual (assuming a threaded watchpoint
> > support).
> 
> Unless I misunderstand something fundamental, if the current thread ID
> is in some variable, you could define a condition for a watchpoint,
> no?

Using the "if" statement? But in which kind of variable? A user defined
variable on GDB or a variable inside the inferior?

-- 
Luis Machado
Software Engineer 
IBM Linux Technology Center
LoP Toolchain/Debuggers' team
Phone: +55 19-2132-2218
T/L: 839-2218
e-mail: luisgpm@linux.vnet.ibm.com

Re: Watchpoints stopping GDB on specific threads

[Eli Zaretskii]

> From: Luis Machado <luisgpm@linux.vnet.ibm.com>
> Cc: gdb@sourceware.org
> Date: Tue, 14 Aug 2007 17:18:49 -0300
> 
> Using the "if" statement?

"if" or "conditions".

> But in which kind of variable? A user defined
> variable on GDB or a variable inside the inferior?

I meant the latter, but Daniel says there's no such variable.