[asterisk-dev] How to find crashing problems (cause maybe double free) in my own module?

Fri May 22 03:27:08 CDT 2009

Dear Holger,

Three points here. More so related to the language C instead of asterisk:-

   1. free() does *NOT* set the ptr to NULL. It just marks the allocated
   memory as unused. The ptr will still keep pointing to the same address. Its
   the programmer's responsibility to ensure that ptr is not used again in this
   state.
   2. From the manual page of free (defined in stdlib.h),:-

   ""free()  frees the memory space pointed to by ptr, which must have been
   returned by a
          previous call to malloc(), calloc() or realloc().  Otherwise, or
   if  free(ptr)  has
          already  been  called before, undefined behaviour occurs.  If ptr
   is NULL, no opera-
          tion is performed. ""

   So, if ptr is NULL, you do not need to perform a check anyways, because
   free() will handle it cleanly.

   3. Thirdly, by putting some simple debug logs just before free() [Just
   print the address being free'd], you can determine if or not, you are
   calling free() on the same ptr twice.

Btw, what (and why) design or architectural differences are you making from
app_conference ?

Prince Singh
Drishti-Soft Solutions Pvt. Ltd., India
http://www.drishti-soft.com
http://blog.drishti-soft.com

On Fri, May 22, 2009 at 1:43 PM, Holger Wirtz <wirtz at dfn.de> wrote:

> Hi all,
>
> I am writing on an own conference bridge which should be used for a real
> radio simulation for the open source flight simulation FlightGear
> (current Asterisk module code can be found at
> https://sourceforge.net/projects/appfgcom/). The basic idea behind is
> taken from app_conference: Every caller has its own member thread where
> frames are put onto a member-in-queue. A conference thread than mixes
> the frames from every member-in-queue together in every meber-out-queues
> (currently not based on their virtual distance). From
> themember-out-queue the meber-threads picks the frames and sends them to
> the channel and frees them.
>
> Because this is my first development for Asterisk it seems that have
> some problems of understanding some basic technology. At the current
> state the conference bridge works - I can place several calls and mix
> them in slinear together.
>
> But I get crashes like the following when running two or three calls for
> some time:
>
> --- cut here ---
> ...
>    -- Hungup 'IAX2/193.174.1.6:4569-11496'
> *** glibc detected *** asterisk: double free or corruption (out):
> 0xb5b75778 ***
> ======= Backtrace: =========
> /lib/libc.so.6[0xb7d9b215]
> /lib/libc.so.6(cfree+0x9c)[0xb7d9caec]
> asterisk(ast_frame_free+0x3d)[0x809c39d]
> /usr/lib/asterisk/modules/app_fgcom.so(member_exec+0x3e4)[0xb66dac54]
> /usr/lib/asterisk/modules/app_fgcom.so[0xb66da049]
> asterisk(pbx_exec+0x6b)[0x80b8c7b]
> asterisk[0x80b9992]
> /usr/lib/asterisk/modules/app_macro.so[0xb649dd50]
> asterisk(pbx_exec+0x6b)[0x80b8c7b]
> asterisk[0x80b9992]
> asterisk[0x80bab15]
> asterisk[0x80bb5ae]
> asterisk[0x80e3ac7]
> /lib/libpthread.so.0[0xb7ee5f50]
> /lib/libc.so.6(clone+0x5e)[0xb7dfd69e]
> ...
> --- cut here ---
>
> Sometimes I geta simple "Segmentation fault"...
>
> I know what this means - but I don't know why this happens... Before
> every call to free() (or ast_frfree()) I check the pointer against NULL
> but it seems that frames are freed and the pointer isn't set to NULL.
>
> What I found out is that I had a dynamic sleep time at the start of
> every conference-mixing-loop. This was about 20000 usec (with usleep).
> It seems that decreasing this time to 1000 usec helps a little bit - the
> crash is not after 2 minutes or so but after 10 or 15 minutes...
>
> Has anyone an idea what I can do to find this problem?
> Are there helpful tools for this kind of problem?
>
> TIA, Holger
> --
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-- 
Regards,
Prince Singh

Drishti-Soft Solutions Pvt Ltd
62-A, First Floor,
Maruti Industrial Area,
Sector - 18, Gurgaon - 122002
Haryana, India.

P: 91 124 4771000
F: 91 124 4039120
W: http://www.drishti-soft.com
B: http://blog.drishti-soft.com
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