Re: [RFC] Set unwindonsignal to on by default

[Phil Muldoon <pmuldoon at redhat dot com>]

Jan Kratochvil wrote:
> Hi Phil,
>
> just my opinion of a C user:
>
> On Wed, 17 Sep 2008 19:55:40 +0200, Phil Muldoon wrote:
>   
> > I propose via this patch that we set the flag to "on" as default. It can  
> > always be turned off if the user desires it.  Defaulting to "on" will  
> > improve the C++ user experience
> >     
>
> and worsen the C user experience.  I find it common the called function
> segfaults and I would like to know why.
>   

Fair point. But in your case if a segfault did happen as in your example 
below, you could just call the function again with unwindonsignal off. 

So:

set unwindsignal on
p func()

<sigsegv> need to debug that.

Because the first call was unwound and state returned to before the 
inferior function call was made, you can call it again in the same debug 
session without restarting the inferior with the flag off:

set unwindsignal off
p func()

<sigsegv> can now debug that

In the C++ case once your inferior terminates there is no going back and 
switching flags. It's dead, and you have to start all over again. It's a 
little more trouble for a C user, but a much better usability gain for a 
C++ user. You can also just explicitly set unwindonsignal in  .gdbinit 
if you absolutely have no interest in C++.

I do take your point well, however. It is not optimal to trade one 
user's pain for another if at all possible.  Do you think a breakpoint 
on std::terminate and the new flag to unwind only on termination signals 
a better solution?

Regards

Phil

> Regards,
> Jan
>
> ------------------------------------------------------------------------------
>
> (gdb) set unwindonsignal on
> (gdb) start
> Temporary breakpoint 1 at 0x40048f: file /tmp/callfunc.c, line 12.
> Starting program: /tmp/callfunc 
>
> Temporary breakpoint 1, main () at /tmp/callfunc.c:12
> 12	  return 0;
> (gdb) p func()
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x0000000000000000 in ?? ()
> The program being debugged was signaled while in a function called from GDB.
> GDB has restored the context to what it was before the call.
> To change this behavior use "set unwindonsignal off"
> Evaluation of the expression containing the function (func) will be abandoned.
> (gdb) bt
> #0  main () at /tmp/callfunc.c:12
> (gdb) 
>
> ------------------------------------------------------------------------------
>
> but before:
>
> (gdb) start
> Temporary breakpoint 1 at 0x40048f: file /tmp/callfunc.c, line 12.
> Starting program: /tmp/callfunc 
>
> Temporary breakpoint 1, main () at /tmp/callfunc.c:12
> 12	  return 0;
> (gdb) p func()
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x0000000000000000 in ?? ()
> The program being debugged was signaled while in a function called from GDB.
> GDB remains in the frame where the signal was received.
> To change this behavior use "set unwindonsignal on"
> Evaluation of the expression containing the function (func) will be abandoned.
> (gdb) bt
> #0  0x0000000000000000 in ?? ()
> #1  0x0000000000400489 in func () at /tmp/callfunc.c:6
> #2  <function called from gdb>
> #3  main () at /tmp/callfunc.c:12
> (gdb) up
> #1  0x0000000000400489 in func () at /tmp/callfunc.c:6
> 6	  return (*hook) ();
> (gdb) 
>