debugging SIGSEV on pclose

[Christopher Faylor]

On Thu, Sep 08, 2011 at 04:15:47PM +0200, Marco atzeri wrote:
>On 9/5/2011 2:20 PM, Marco atzeri wrote:
>> Hi,
>> I am trying to identify the octave segfault, last reported on
>> http://cygwin.com/ml/cygwin-announce/2011-08/msg00003.html
>>
>> To reproduce: run octave from xterm and at prompt
>> -------------------------
>> graphics_toolkit ("fltk")
>> x=1:10;
>> plot(x,x)
>> print("fltk.png","-dpng")
>> -------------------------
>> the plot is built and printed as file but octave crashes.
>>
>>
>> After builiding cygwin (cvs), octave and fltk with
>> gcc-4.5.3 I was able to identify the SIGSEV in cygwin pclose.
>>
>> Program received signal SIGSEGV, Segmentation fault.
>> [Switching to Thread 5980.0x164c]
>> pclose (fp=0x7ffdf000) at
>> /pub/cygwin/cvs/src_new/winsup/cygwin/syscalls.cc:4026
>> 4026 if (fh->get_device () != FH_PIPEW && fh->get_device () != FH_PIPER)
>> (gdb) l
>> 4021 int
>> 4022 pclose (FILE *fp)
>> 4023 {
>> 4024 fhandler_pipe *fh = (fhandler_pipe *) cygheap->fdtab[fileno(fp)];
>
>Answering myself while kicking the head on the wall
>
>the SIGSEV on 4026 is obvious as fh is NULL
>
>(gdb) p fh
>$7 = (fhandler_pipe *) 0x0
>(gdb) p fp
>$8 = (FILE *) 0x20204f94
>
>> 4025
>> 4026 if (fh->get_device () != FH_PIPEW && fh->get_device () != FH_PIPER)
>
>Question:
>is a mistake in pclose to assume that fh could be invalid

I'm not sure what you're asking here.  It's not a mistake to assume that
pclose is being passed a valid fp.  Linux also crashes if the fp is
closed twice.

>or something is just trashing cygheap->fdtab ?

Who knows?  strace should show if the fp is closed twice.

cgf

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