[Lazarus] [?? Probable Spam] Re: function StabBackTraceStr(addr:Pointer):shortstring; - problem with subsequent back traces

Bogusław Brandys brandys at o2.pl
Thu Apr 16 17:00:57 CEST 2009

Sergei Gorelkin pisze:
> Bogusław Brandys пишет:
>> There is a problem with this function. To avoid recursion there is a 
>> hack there to  point BackTraceStrFunc to system implementation (which 
>> only shows addresses in hex).Long back trace which mostly ends with 
>> incorrect frame address as I suppose (OpenStabs is returning false for 
>> nil address) - thus function pointer is not returned back to lineinfo 
>> stabs processing method and next back trace contains only addresses .
> Right you are. However, many backtraces *start* with an invalid frame: 
> e.g. when you call a virtual method of an object that is already 
> destroyed, you'll likely end up at a random address.

I attached my proposition of patch here : 

and here (only windows) for external debug files (.dbg) :


Do you have testcases for testing such invalid frames like in example 
above and for testing recursion ?

Would be great if someone looked at proposed solution and checked under 
unix,win64 and other supported systems.

>> This problem is minor if program crashes with exception and stack trace 
>> but in other cases , like in bigger program (any Lazarus ones) , ONLY 
>> first back trace has useful information (source,line number) attached to it.
>> That eliminates any useful logging like multilog for example.
>> And nobody really is able to know correct stack frames length before 
>> calling any of such functions to obtain back trace (that would resolve 
>> problem).
>> How we could fix it ? Is this recursion still a subject here ?
> I fixed one of the causes recently (see exeinfo.pp, rev.12802), but that 
> wasn't enough. In general, code of lineinfo, lnfodwrf and exeinfo has to 
> be checked very carefully to make sure it would not crash at any 
> circumstances. Including possibly unknown/corrupted state of the RTL in 
> case when backtrace is called due to a crash.
>> I have one idea, probably not perfect.
>> How about  a function to initialize
>>   BackTraceStrFunc  : TBackTraceStrFunc;
>> The only way now is to set :
>> BackTraceStrFunc := @SysBackTraceStr; (displays only hex of addresses)
>> Something generic is needed, able to set to StabBackTraceStr if lineinfo 
>> unit (option -gl) is used and only to  SysBackTraceStr in other case.
>> It might be used also to set to user defined function.
>> That way in any such functions in lcl 
>> (DumpExceptionBackTrace,GetStackTrace) or external package (like 
>> multilog), we could initialize BackTraceStrFunc to point again to 
>> correct function before obtaining stack trace.
> For me, it looks simpler and better to restore BackTraceStrFunc 
> unconditionally at the end of StabsBackTraceFunc/DwarfBackTraceFunc.
> Or even better, have it set to stub only for calling GetLineInfo.
> Regards,
> Sergei

That's what I did among other changes.


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