Re: [Libunwind-devel] use_prev_instr breaking unwind - how to stop it?

[Doug Moore]

Documentation has to change anyway, since the unw_init_local docs are  
currently broken.

Decrementing the ip for my case (using unw_set_reg) gets me through a  
few unw_step calls successfully, then crashes the program.

But I can't win this fight, so I'll quit.

Maybe I can get a new cursor init function added to the API.  Would  
the powers that be accept a change that added unw_init_local_signal,  
which is just like unw_init_local except that it doesn't set  
use_prev_instr?  If not that name, the same thing by some other name?   
I can post that change pretty quickly, if only someone will consider it.

Doug


Quoting Dave Watson <address@hidden>:

> On 03/29/17 04:51 PM, Doug Moore wrote:
> > Can’t we fix this whole problem by having getcontext subtract from  
> > the pc to start with, so that nobody ever has to decrement the pc  
> > on the first call to unw_step?
>
> That would assume that the ucontext is usually from a signal frame
> (third argument from a sigaction handler), which is probably pretty
> common.  It would break anyone that grabbed it outside a handler using
> getcontext() instead of unw_getcontext() though.
>
> The current documentation says:
>
> ```
> On IA-64, unw_context_t has a layout that is compatible with that of
> ucontext_t and such structures can be initialized with getcontext()
> instead of unw_getcontext()
> ```
>
> so it might be a somewhat interface breaking change.
>
> Conversely, for your use case you might just be able to bump the ip by
> one in the ucontext before the first unw_step()?
>
>
> !DSPAM:10223,58dd225839344861828991!