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Noko
Joined: 06 Sep 2008
Posts: 23
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 Posted: Wed Sep 10, 2008 7:09 pm Post subject: very weird SIGBUS error |
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I have some very strange error in SDL. Here's the output of gdb:
| Code: | GNU gdb 6.4
Copyright 2005 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
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Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "--host=i686-pc-cygwin --target=psp"...
(gdb) target remote :10001
Remote debugging using :10001
[New thread 0]
_start (args=0, argp=0x0) at crt0.c:105
105 if ((&module_info != NULL) && (module_info.modattribute & 0x1000)) {
(gdb) c
Continuing.
[New thread 13506353]
Program received signal SIGBUS, Bus error.
[Switching to thread 13506353]
SDL_LowerBlit (src=0x8ad0780, srcrect=0x88f7cb0, dst=0x8ab4450, dstrect=0x88f7ca8) at SDL_surface.c:424
424 if ( (src->map->dst != dst) ||
(gdb) disas
Dump of assembler code for function SDL_LowerBlit:
0x089f47a8 <SDL_LowerBlit+0>: addiu sp,sp,-40
0x089f47ac <SDL_LowerBlit+4>: sw s3,28(sp)
0x089f47b0 <SDL_LowerBlit+8>: sw s2,24(sp)
0x089f47b4 <SDL_LowerBlit+12>: sw s1,20(sp)
0x089f47b8 <SDL_LowerBlit+16>: sw s0,16(sp)
0x089f47bc <SDL_LowerBlit+20>: sw ra,32(sp)
0x089f47c0 <SDL_LowerBlit+24>: lw t0,48(a0)
0x089f47c4 <SDL_LowerBlit+28>: move s3,a0
0x089f47c8 <SDL_LowerBlit+32>: move s2,a2
0x089f47cc <SDL_LowerBlit+36>: lw v0,0(t0)
0x089f47d0 <SDL_LowerBlit+40>: move s0,a1
0x089f47d4 <SDL_LowerBlit+44>: bne v0,a2,0x89f47ec <SDL_LowerBlit+68>
0x089f47d8 <SDL_LowerBlit+48>: move s1,a3
<omitted>
(gdb) p/x $pc
$1 = 0x89f47cc
(gdb) p/x $t0
$2 = 0x0
(gdb) p/x $a0
$3 = 0x8ad0780
(gdb) p/x *(int *)($a0+48)
$4 = 0x8b7e530
(gdb) set $t0=*(int *)($a0+48)
(gdb) p/x $t0
$5 = 0x8b7e530
(gdb) c
Continuing.
Can't send signals to this remote system. SIGBUS not sent. |
The faulty operation is lw t0,48(a0) because t0 is 0. But t0 is supposed to be set at 0x089f47c0, with lw t0,48(a0). And when I set $t0 to *(int *)($a0+48) manually, program continues to work like lnothing happened.
This error happens randomly, I call SDL_LowerBlit many times, and only in some rare cases this hasppens.
EDIT:
I am a fucking idiot and another thread deleted the surface, that's why I had SIGBUS, and then this thread allocated surface again and that's why everything worked in debugger.
Thank you for reading.
Last edited by Noko on Fri Sep 12, 2008 2:41 am; edited 1 time in total |
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hlide
Joined: 10 Sep 2006
Posts: 750
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| Posted: Wed Sep 10, 2008 7:57 pm Post subject: |
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Well, honestly, reading carefully your output of gdb, the only sensible explanation i might see is a cache issue : how is it possible for lw $t0, 48($a0) randomly to read different values in run-time (as you say it happens randomly) ?
I would first have thought something might randomly set src->map to 0 but your gdb output seems to prove that it wasn't the case since you had a zero value at run-time and then you had the right value in debug-time by manually setting t0 from the same memory reference just after the exception with t0 == 0.
I'm sure there is a logical reason why it messes up, but we haven't enough information to tell you why. |
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