Re: [Xenomai-help] Correct use of xeno_sigshadow_install()

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Peter Soetens wrote:
> On Monday 10 January 2011 17:50:05 Gilles Chanteperdrix wrote:
>> Peter Soetens wrote:
>>> On Monday 10 January 2011 17:24:28 Gilles Chanteperdrix wrote:
>>>> Peter Soetens wrote:
>>>>> Hi,
>>>>>
>>>>> A while ago, I was advised to call xeno_sigshadow_install() after a
>>>>> library installed a sigwinch signal handler (I believe this was Xenomai
>>>>> 2.5.1). Could it be that this is no longer supported in recent versions
>>>>> ? What is the correct way to avoid a segfault if an application
>>>>> installed a sigwinch handler after Xenomai did ?
>>>> I think we already answered that at the time, but if you install your
>>>> own sigwinch handler, you should:
>>>> - register it with the SA_SIGINFO flag
>>>> - call xeno_sigwinch_handler and consider that the signal is for the
>>>> application only if this function returns 0.
>>> This is indeed in the API docs, but it's not an option since it's a third
>>> party library (readline) doing this, it doesn't depend on or know
>>> Xenomai.
>>>
>>>> If you can not do that, then redefine SIGSHADOW in
>>>> include/asm-generic/syscall.h to a different value, and recompile both
>>>> Xenomai user-space and kernel-space support. Note however that when
>>>> starting applications in gdb you will then have to type:
>>>>
>>>> handle <the_new_signal> pass nostop print
>>>>
>>>> If you want to be able to debug Xenomai applications.
>>> Hmm....
>>>
>>> So defining the xeno_sigshadow_install() function ourselves (since it's
>>> not in any Xenomai header) and calling it after the readline signal
>>> handlers were installed is not gonna work ?
>> The thing is that xeno_sigshadow_installi is an internal function,
>> particularily, it is not thread safe. The exported function is
>> xeno_sigshadow_install_once, but it will not do anything the second time
>> it gets called.
>>
>> If you take care of this thread-safety issue, calling
>> xeno_sigshadow_install should be OK, since it just installs
>> xeno_sigshadow_handler as a handler for SIGSHADOW, calling
>> xeno_sigwinch_handler, then the handler your application registered
>> before. All this code is relatively simple, and you will find it in
>> src/skins/common/sigshadow.c
>>
>> Now, I suspect the issue you have is something else, so, could you post
>> the faulting code, reduced to a simple self-contained test case?
> 
> main.cpp:
> #include <readline/readline.h>
> #include <readline/history.h>
> #include <xenomai/native/task.h>
> #include <malloc.h>
> #include <sys/mman.h>
> #include <stdio.h>
> 
> char* line_read = 0;
> 
> char *rl_gets ()
> {
>   /* If the buffer has already been allocated,                                                                                                                                                                 
>      return the memory to the free pool. */
>   if (line_read)
>     {
>       free (line_read);
>       line_read = 0;
>     }
> 
>   if (rl_set_signals() != 0)
>     printf("Error setting signals !\n");
> 
>   line_read = readline ( ">" );
> 
>   /* If the line has any text in it,                                                                                                                                                                           
>      save it on the history. */
>   if (line_read && *line_read) {
>     add_history (line_read);
>   }
>   return (line_read);
> }
> 
> int main(void)
> {
>   mlockall(MCL_CURRENT | MCL_FUTURE);
> 
>   RT_TASK xenotask;
>   rt_task_shadow( &(xenotask),0, 10, 0);
>   rl_completion_append_character = '\0'; // avoid adding spaces                                                                                                                                                
> 
>   if ( read_history(".history") != 0 ) {
>     read_history("~/.history");
>   }
> 
>   // Let readline intercept relevant signals                                                                                                                                                                   
>   if(rl_catch_signals == 0)
>     printf("Error catching signals !\n");
> 
>   if (rl_set_signals() != 0)
>     printf("Error setting signals !\n");
> 
>   while (1)
>     {
>       rl_gets();
>     }
>   return 0;
> }
> 
> make main CXXFLAGS=-I/usr/include/xenomai LDFLAGS="-lnative -lpthread -
> lncurses -lreadline -lxenomai"
> 
> ./main
> <now resize your xterm window>
> Segmentation fault

You can not do that, Xenomai requires that xeno_sigwinch_handler be
called when SIGWINCH is received. To do that, you have either:
- to arrange for Xenomai xeno_sigshadow_install_once to be called after
the time when readline installs its handlers, that is what calling
xeno_sigshadow_install achieved, but this was fragile
- use another signal than SIGWINCH as internal signal used by Xenomai,
at the expense of some additional line launching gdb.
- define your own signal and handle correctly both Xenomai and
libreadline signals in that handler.


-- 
                                                                Gilles.