My nim program crashes in release but not in debug mode. Some system code is
overwriting my json structures causing the crash. The problem is hard to narrow
down, it moves around as I add debugging code. I haven't been able to simplify
the error case to something that is easy to share.
The release and optimizer options must be on to reproduce the problem.
Compiling with debug and --opt:size or with --opt:speed works without error and
-d:release with --opt:none works.
I been using the lldb debugger and isolated the problem. I found the address of
the json structure being overwritten and set a hardware watchpoint there. At
this point, my code adds to a sequence filling it to capacity then the system
expands the sequence and moves memory around. This process overwrites the
unrelated json object.
I am working on a Mac with the latest release version of nim and am debugging a
python shared library.
Anyone have ideas where the bug is and how to fix it? Or have ideas I can try?
Here is part of my debugger session:
# dump a pointer to a json jArray object then set a watch point on it.
(lldb) x 4302866232
0x100788738: 18 ac 7d 00 01 00 00 00 58 aa 7d 00 01 00 00 00
.?}.X?}.
0x100788748: f0 a0 7d 00 01 00 00 00 58 a3 7d 00 01 00 00 00
?}.X?}.
(lldb) watchpoint set expression 4302866232
Watchpoint created: Watchpoint 1: addr = 0x100788738 size = 8 state =
enabled type = w
new value: 4303203352
# run the program
(lldb) c
Process 69738 resuming
# the watchpoint is hit by memmove.
Watchpoint 1 hit:
old value: 4303203352
new value: 4337886456
Process 69738 stopped
* thread #2, queue = 'com.apple.main-thread', stop reason = watchpoint 1
frame #0: 0x7fff7b79052b
libsystem_platform.dylib`_platform_memmove$VARIANT$Nehalem + 299
libsystem_platform.dylib`_platform_memmove$VARIANT$Nehalem:
-> 0x7fff7b79052b <+299>: movaps xmmword ptr [rdi + 0x20], xmm3
0x7fff7b79052f <+303>: movaps xmmword ptr [rdi + 0x30], xmm4
0x7fff7b790533 <+307>: movups xmmword ptr [rdi + rdx + 0x40], xmm5
0x7fff7b790538 <+312>: movups xmmword ptr [rdi + rdx + 0x50], xmm6
Target 0: (Python) stopped.
(lldb) x 4302866232
0x100788738: f8 e4 8e 02 01 00 00 00 58 aa 7d 00 01 00 00 00
??..X?}.
0x100788748: f0 a0 7d 00 01 00 00 00 58 a3 7d 00 01 00 00 00
?}.X?}.
# memove is called by copyMem.
(lldb) fr select 1
frame #1: 0x000102808726 metar.so`growObj_FZeyQYjWPcE9c06y1gNqZxw
[inlined] copyMem_E1xtACub5WcDa3vbrIXbwgsystem(dest=,
source=0x0001028d9038, size=) at system.nim:3242 [opt]
3239 when declared(memTrackerOp):
3240 memTrackerOp("zeroMem", p, size)
3241 proc copyMem(dest, source: pointer, size: Natural) =
-> 3242 c_memcpy(dest, source, size)
3243 when declared(memTrackerOp):
3244 memTrackerOp("copyMem", dest, size)
3245 proc moveMem(dest, source: pointer, size: Natural) =
# here is the stack
(lldb) bt
* thread #2, queue = 'com.apple.main-thread', stop reason = watchpoint 1
frame #0: 0x7fff7b79052b
libsystem_platform.dylib`_platform_memmove$VARIANT$Nehalem + 299
* frame #1: 0x000102808726 metar.so`growObj_FZeyQYjWPcE9c06y1gNqZxw
[inlined] copyMem_E1xtACub5WcDa3vbrIXbwgsystem(dest=,
source=0x0001028d9038, size=) at system.nim:3242 [opt]
frame #2: 0x000102808717
metar.so`growObj_FZeyQYjWPcE9c06y1gNqZxw(old=,
newsize=, gch=0x000101809000) at gc.nim:537 [opt]
frame #3: 0x000102808999 metar.so`incrSeqV2 [inlined]
growObj_AVYny8c0GTk28gxjmat1MA(old=, newsize=) at
gc.nim:586 [opt]
frame #4: 0x000102808985 metar.so`incrSeqV2(seq=,
elemSize=) at sysstr.nim:260 [opt]
frame #5: 0x000102844316
metar.so`getLeafString_Zo50qhbAAbCqf7Q3nRkzYA(node=0x0001007f0240,
maxLen=72) at readable.nim:109 [opt]
frame #6: 0x000102844fe7
metar.so`readable_v9cZ2WN4Rpw3hZ8FsvwdS4A(metadata=,
readerName=) at readable.nim:183 [opt]
frame #7: 0x000102801998
metar.so`readMetadata_2nQm9bwutYGCHUjTrSrQ5Aw_2(filename="testfiles/image.jpg")
at metar.nim:44 [opt]
frame #8: 0x000102801a18
metar.so`readMetadataPy_wrapper_ANfPuNIhXMYZOCUppM6Wug_4(self=,
args=) at metar.nim:38 [opt]
frame #9: 0x7fff5f89 Python`PyEval_EvalFrameEx + 2917
frame #10: 0x7fff5232 Python`PyEval_EvalCodeEx + 1551
frame #11: 0x7fff5dddcc1d Python`PyEval_EvalCode + 32
frame #12: 0x7fff5ddfbad1 Python`___lldb_unnamed_symbol1600$$Python
+ 49
frame #13: 0x7fff5ddfbb78 Python`PyRun_FileExFlags + 130
frame #14: 0x7fff5ddfb6fa Python`PyRun_SimpleFileExFlags + 706
frame #15: 0x7fff5de0c96c Python`Py_Main + 3064
frame #16:
