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Crashing in _Block_release

From: Jens Alfke
Subject: Crashing in _Block_release
Date: Wed, 29 Feb 2012 14:30:38 -0800

I’m hitting a nasty crash that I can reproduce by running a single one of my 
unit tests. It’s impossible to “bt” at the point of the crash, but I was able 
to narrow it down to a _Block_release call made while draining an autorelease 
pool. Now I’m stuck, though — after the crash has occurred, the PC is bogus 
(0x0019) and the stack is smashed, so I can’t tell what happened.

Any ideas, or suggestions how to debug further? The block does appear to be a 
valid object before the call (see below).

One possible clue: gdb says the function is at blocks_runtime.m:283, but my 
copy of blocks_runtime.m only has 261 lines and _Block_release is at line 234. 
I hope there’s not a version mismatch between Base and the libobjc2 runtime; 
what would be the best way to tell if there is?


Breakpoint 1, _Block_release (src=<optimized out>, src=<optimized out>) at 
283     {
(gdb) bt
#0  _Block_release (src=<optimized out>, src=<optimized out>) at 
#1  0x0024578c in -[GSBlock release] (self=0x81b7330, _cmd=0x721ea8) at 
#2  0x002c7d01 in -[NSAutoreleasePool emptyPool] (self=0x8106f64, 
_cmd=0x721e90) at NSAutoreleasePool.m:658
#3  0x002c7825 in -[NSAutoreleasePool dealloc] (self=0x8106f64, _cmd=0x721ee8) 
at NSAutoreleasePool.m:538
#4  0x002c76a0 in -[NSAutoreleasePool drain] (self=0x8106f64, _cmd=0x8ff678) at 
#5  0x008b2841 in RunTestCase (test=0x8fec40) at vendor/MYUtilities/Test.m:69
#6  0x008b21f5 in RunTestCaseNamed (name=0xbffff439 "MYAfterDelay") at 
#7  0x008b2396 in RunTestCases (argc=2, argv=0xbffff2a4) at 
#8  0x08048752 in main (argc=2, argv=0xbffff2a4) at 
(gdb) print [0x81b7330 retainCount]
$2 = 34
(gdb) finish
Run till exit from #0  _Block_release (src=<optimized out>, src=<optimized 
out>) at blocks_runtime.m:283

Program received signal SIGSEGV, Segmentation fault.
0x00000019 in ?? ()
(gdb) bt
#0  0x00000019 in ?? ()
Backtrace stopped: Not enough registers or memory available to unwind further

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