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Comment by chancho | Hacker News Reader

I've not debugged Lua but I am familiar with it's integration into C. I imagine debugging Lua+C is no harder than debugging Matlab+C. You load up the REPL in GDB and set your C breakpoints in GDB and your Lua breakpoints in Lua. After control leaves your C function and goes back into the REPL's guts just hit 'continue' to return to the REPL command line. It's more laborious but still straightforward. The only thing you really can't do is step from the high level language to the low one but you can set the proper breakpoint in GDB right before the call so it's more or less the same.

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Edit: sorry not sure if you were talking about Lua or a C-based Scheme, though I imagine it's the same.

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I'm not concerned about which language right now, anything with a REPL should be able to work in this way. But the context I'm working in is perhaps a bit trickier. The call stack would normally cross between languages multiple times so just inspecting the stack becomes really clumsy. Since both languages have access to the same data structures, a watchpoint set on the C side can break at a fairly arbitrary point in the interpreter, from which you need to gain control on the other side.

And it's important to be able to attach to a running process and move through the inter-language call stack (e.g. when debugging a deadlock under MPI (distributed-memory)). I haven't found a high-level language that offers this sort of interoperability, but I'd love to hear suggestions.

Lua has a simple API for debugging built in. Because of this, there are several free and a few commercial Lua debuggers available.

I'm not concerned about single-language debugging. Can these debuggers move through a mixed C-Lua stack? My stack might start and end in C, but cross through Lua two or more times. Can I set a watchpoint and get the program to stop wherever the memory is modified, regardless of which language is modifying it (i.e. I'd like to see C source when C touches the memory and Lua source (not interpreter inernals) when Lua code touches it).

Which of the Lua debuggers is your favorite?

There is a commercial Visual Studio plug-in that will do what you want, but I don't recall it's name.

I wrote a Lua remote debugger myself years ago. A nice trick was that you can associate a string with a chunk of code when you load the code. Then you can retrieve that string from the debug API based on the current instruction. I used that to make a table of "chunk name" to "source" as a global variable in LUA. That made it easy for the debugger to retrieve the source regardless of where it came from (file vs interactive console vs who knows what).

I'm more of an assert / test suite / REPL and printf debugging person. I've used gdb when debugging wrappers for C libraries, but my use of the Lua debugging API has mostly been for profiling, code coverage, etc.

GDB works across the language boundary, though. As far as it's concerned, Lua is just a C library.

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