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bug#17688: 24.3.90; segmentation fault in deselect_palette

From: Ken Brown
Subject: bug#17688: 24.3.90; segmentation fault in deselect_palette
Date: Thu, 05 Jun 2014 12:08:36 -0400
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 6/4/2014 11:58 AM, Eli Zaretskii wrote:
(gdb) bt full
#0  0x0000000100631d84 in deselect_palette (f=0x0, hdc=0x0)
     at /usr/src/debug/emacs-24.3.90-1/src/w32xfns.c:123
No locals.
#1  0x0000000100631e53 in release_frame_dc (f=0x0, hdc=0x0)
     at /usr/src/debug/emacs-24.3.90-1/src/w32xfns.c:154
         ret = 0
#2  0x00000001006351f9 in uniscribe_encode_char (
     font=0x101071d30 <bss_sbrk_buffer+6928560>, c=76)
     at /usr/src/debug/emacs-24.3.90-1/src/w32uniscribe.c:585
         context = 0x0
         f = 0x0
         old_font = 0x0
         code = 15
         ch = L"LC"
         len = 1
         items = 0x436980
         nitems = 1
         uniscribe_font = 0x101071d30 <bss_sbrk_buffer+6928560>

This backtrace makes no sense: uniscribe_encode_char calls
release_frame_dc only if the variable 'context' has a non-NULL value
(and then 'f' should also be non-NULL).  But here we see that
release_frame_dc is called by uniscribe_encode_char when both
'context' and 'f' are NULL, which cannot happen.  I was about to say
that this could be due to compiler optimizations that screw up the
backtrace, but then I saw that your Emacs binary was built with -O0.
So now I'm stumped how could this happen at all.

That's been the problem for several months. People have reported several crashes of the Cygwin-w32 build, always on 64-bit Cygwin, with backtraces that "can't happen". Can you think of any way to try to track this down?


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