can't 'continue' after 'next' in breakpoint's commands/end block

can't 'continue' after 'next' in breakpoint's commands/end block

[Andrey Butirsky]

Hello, I'm trying to debug KWin window manager,
to not halt the session it can't be paused so I created such script to
trace function calls:

$ cat ~/kde/gdb-x
set pagination off

break KGlobalAccelImpl::x11KeyPress(xcb_key_press_event_t*) 
break TabBox::keyPress(int )
break /home/bam/kde/src/kde/workspace/kwin/src/tabbox/tabbox.cpp:555

commands 1-20
# FIXME: for some reason KWin halts on any stepping command here, ideas?
#next
continue
end

continue &

I'm attaching to a KWin process such way:

gdb attach $(pidof kwin_wayland) -x ~/kde/gdb-x

The problem is I can't insert next/step and such commands in the
breakpoint's 'commands' block - the process halts just after the 'next'
there despite the presence of 'continue' command afterward.

So I need to switch to another VT and kill gdb process to bring my
session back.

I'm not quite understand why it happens and what I can do with it. Any
ideas?

RE: can't 'continue' after 'next' in breakpoint's commands/end block

[Aktemur, Tankut Baris]

On Thursday, June 10, 2021 6:45 PM, Andrey Butirsky wrote:
> Hello, I'm trying to debug KWin window manager,
> to not halt the session it can't be paused so I created such script to
> trace function calls:
> 
> $ cat ~/kde/gdb-x
> set pagination off
> 
> break KGlobalAccelImpl::x11KeyPress(xcb_key_press_event_t*)
> break TabBox::keyPress(int )
> break /home/bam/kde/src/kde/workspace/kwin/src/tabbox/tabbox.cpp:555
> 
> commands 1-20
> # FIXME: for some reason KWin halts on any stepping command here, ideas?
> #next
> continue
> end
> 
> continue &
> 
> I'm attaching to a KWin process such way:
> 
> gdb attach $(pidof kwin_wayland) -x ~/kde/gdb-x
> 
> The problem is I can't insert next/step and such commands in the
> breakpoint's 'commands' block - the process halts just after the 'next'
> there despite the presence of 'continue' command afterward.
> 
> So I need to switch to another VT and kill gdb process to bring my
> session back.
> 
> I'm not quite understand why it happens and what I can do with it. Any
> ideas?

Hi, 

GDB stops executing the breakpoint commands if a command modifies the stopped
thread's PC.  The "next" command hence causes the execution of the commands
to be truncated.

https://sourceware.org/gdb/current/onlinedocs/gdb/Break-Commands.html#Break-Commands
"Any other commands in the command list, after a command that resumes execution,
are ignored. This is because any time you resume execution (even with a simple
next or step), you may encounter another breakpoint—which could have its own
command list, leading to ambiguities about which list to execute."

Instead of "next", could you define another breakpoint at the next line and
define breakpoint commands for that new BP as well?

-Baris


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