55 lines
2.4 KiB
ReStructuredText
55 lines
2.4 KiB
ReStructuredText
Ok, so your nice Nim program crashes under various hard-to-reproduce conditions. What can you do about that?
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Generally speaking the most important question when you have a corruption is **what piece of code wrote to this memory location**. This question is answered by *watchpoints*, breakpoints are mostly useless for this. In general I found breakpoints only useful to set *watchpoints* in them.
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Classical debugging
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===================
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Use ``--debugger:native`` and use GDB or LLDB or Visual Studio for debugging. Note that currently local variables are mangled and a ``0`` is attached to the name.
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Test with different GCs
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=======================
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* ``--gc:boehm``
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* ``--gc:refc``
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* ``--gc:markAndSweep``
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Note that if your program does not crash with a different GC, it doesn't imply you found a GC bug! It's just a weak indicator.
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Test under different OSes
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=========================
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* Linux
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* MacOS X
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* Windows
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Test under different CPUs
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=========================
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* i386 (32bit)
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* amd64 (64bit)
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* arm
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Test different compiler options
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===============================
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``-d:release`` vs debug mode is the obvious choice, but the Nim GC, allocator and standard library have many more checks you can should enable:
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``-d:useSysAssert``
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enables assertions in the system.nim, especially in Nim's allocator.
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``-d:useGcAssert``
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enables assertions in Nim's GC.
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``-d:nimBurnFree``
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overwrite deallocated memory with 0xff bytes so that "access after free" triggers a segfault.
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``--tlsEmulation:on|off```
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turn "thread local storage emulation on/off". This is helpful when you do have an issue in a threaded program. As I recently learned, the fact that TLS is cleaned at thread exit is often an overlooked cause for crashes. TLS is *unsafer* than global storage in this respect.
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Edit lib/system/mmdisp.nim and gc.nim
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=====================================
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Even more debugging options can be enabled by editing ``lib/system/mmdisp.nim``. Most of these have no ``--define`` equivalent, unfortunately.
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One problem with corruptions is their non-deterministic nature, in particular heap and stack addresses change from run to run. Define ``-d:corruption`` to enable "cell IDs", so that every "cell" (that is every ``ref``/``string``/``seq``) gets a unique ID. It's often interesting to see if the corrupted cell has the same ID from run to run or if it differs. If it differs the bug is non-deterministic. Within the GC ``writeCell`` can be used to output offending cells.
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