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Blog post about printing associative arrays from GDB
Oct 23
Dennis
Oct 23
kinke
Oct 24
Mihails
Oct 24
Mihails
Oct 25
Mihails
October 23
I've written a blog post about printing custom data structures from GDB using dlopen that I hope some of you find interesting and I'd also be happy to discuss it:

https://medium.com/dunnhumby-data-science-engineering/how-to-simplify-debugging-unit-tests-in-d-a2b52c5c1fa

Please let me know if there are any better ways to print data structures like associative arrays from GDB, maybe by using functions from the runtime library. Another exciting idea would be to automate the pretty printing, maybe also by looking how the pretty-printers for C++ in GDB 7+ are defined. I sadly haven't read much D code from outside our company; is there a Phobos function to automatically print expected and actual values for failed assertions? In our company we use ocean's test!("==").
October 23
Thanks for spitting through the GDB manual so I don't have to!
I would've guessed something like this was possible with GDB, but never bothered until now.

On Wednesday, 23 October 2019 at 20:52:55 UTC, Johannes Riecken wrote:
> is there a Phobos function to automatically print expected and actual values for failed assertions? In our company we use ocean's test!("==").

Not a phobos function, a compiler switch. In recent LDC and DMD, you can use -checkaction=context and an attempt is made to print the actual values involved in the assertion. It is still young so you might still encounter bugs or limitations, but it does the job really well for most asserts.
October 23
Hi,

> Now wouldn’t it be even more useful if we knew why the unit test failed? Apparently D doesn’t print that information automatically on assertion failure, supposedly for performance reasons.

Off-topic, but recent frontends feature a `--checkaction=context` switch, which for your example yields something like:

onlineapp.d(5): [unittest] [3: 30, 2: 20, 1: 10] != [1: 10]
1/1 unittests FAILED

https://run.dlang.io/is/XZy5PN
October 24
On Wednesday, 23 October 2019 at 21:07:24 UTC, Dennis wrote:
> Thanks for spitting through the GDB manual so I don't have to!
> I would've guessed something like this was possible with GDB, but never bothered until now.
>
> On Wednesday, 23 October 2019 at 20:52:55 UTC, Johannes Riecken wrote:
>> is there a Phobos function to automatically print expected and actual values for failed assertions? In our company we use ocean's test!("==").
>
> Not a phobos function, a compiler switch. In recent LDC and DMD, you can use -checkaction=context and an attempt is made to print the actual values involved in the assertion. It is still young so you might still encounter bugs or limitations, but it does the job really well for most asserts.

That looks promising. In the version I have installed (dmd 2.087.0) compilation crashes when using the flag and also stdio.writeln, but playing around with the newest version in run.dlang.io, it seems to work well with the cases I've tried, cool!
October 24
GDB has some dedicated functionality to improve pretty-printing and expression evaluation of things that too complex to figure out automatically:

- https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html
- https://sourceware.org/gdb/onlinedocs/gdb/Writing-an-Xmethod.html

GCC, for example, provides such scripts for improved debugging of C++ standard library as part of its distribution. Writing something similar for DMD/GDC/LDC shouldn't be too hard.
October 24
On Thursday, 24 October 2019 at 11:20:07 UTC, Mihails wrote:
> GDB has some dedicated functionality to improve pretty-printing and expression evaluation of things that too complex to figure out automatically:
>
> - https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing.html
> - https://sourceware.org/gdb/onlinedocs/gdb/Writing-an-Xmethod.html
>
> GCC, for example, provides such scripts for improved debugging of C++ standard library as part of its distribution. Writing something similar for DMD/GDC/LDC shouldn't be too hard.

GDB uses internal functions of C++'s runtime to do the pretty-printing for C++. Implementing that for D's runtime library in its current form would seem like a considerable effort to me, since D's runtime library seems to be a bit hairier with respect to the internal fields, no?
October 24
On Thursday, 24 October 2019 at 13:16:39 UTC, Johannes Riecken wrote:
> GDB uses internal functions of C++'s runtime to do the pretty-printing for C++. Implementing that for D's runtime library in its current form would seem like a considerable effort to me, since D's runtime library seems to be a bit hairier with respect to the internal fields, no?

That wouldn't be a considerable effort on its own (I think all necessary machinery is already part of druntime.rt package) but right now you would need to have druntime built with debugging symbols to get anything done.

For C++ standard containers it is not a problem because those are defined in header files. If AA implementation was switched to templates, it would "just work" for druntime too.
October 24
On Thursday, 24 October 2019 at 14:20:42 UTC, Mihails wrote:
> On Thursday, 24 October 2019 at 13:16:39 UTC, Johannes Riecken wrote:
>> GDB uses internal functions of C++'s runtime to do the pretty-printing for C++. Implementing that for D's runtime library in its current form would seem like a considerable effort to me, since D's runtime library seems to be a bit hairier with respect to the internal fields, no?
>
> That wouldn't be a considerable effort on its own (I think all necessary machinery is already part of druntime.rt package) but right now you would need to have druntime built with debugging symbols to get anything done.
>
> For C++ standard containers it is not a problem because those are defined in header files. If AA implementation was switched to templates, it would "just work" for druntime too.

Cool, if you (or someone else) can give me the fields or functions in the druntime.rt package that can be used to print associative arrays from GDB, then I can try my hand at building a pretty-printer using GDB's Python API next week.
October 25
> Cool, if you (or someone else) can give me the fields or functions in the druntime.rt package that can be used to print associative arrays from GDB, then I can try my hand at building a pretty-printer using GDB's Python API next week.

AA implementation is provided by https://github.com/dlang/druntime/blob/master/src/rt/aaA.d, and you can either try reimplementing key logic in Python (more work but cleaner impl) or just call relevant functions via gdb python api and parse results. It should actually be possible to call functions like __aaGetX even with release build of druntime, you just to do a manual cast to the correct function type from a raw pointer.
November 12
I created an initial version and would be glad to get some feedback. I couldn't figure out how to call the _AAGetX-style functions, so I just went with the approach in my blog post to get something working that can be refactored later.

Here is the code. It currently works with gdc, but not with dmd, because dmd calls the type like _AArray_int_int, not int[int]. Also note that it creates (or overwrites) two files in the directory, libhelpers.so and helpers.d:

https://github.com/johannes-riecken-dunnhumby/pretty

I kept the generated files in there to ease experimentation (which I would take out later) and still it's very little code. To get it working I had to add the directory containing the code to my .gdbinit with `add-auto-load-safe-path ~/workspace/pretty`
gdb can auto-load python files related to an executable if they are named as <executable_name>-gdb.py, so I did that for my example executable foo (which i create with `gdc-9 -g -o foo foo.d`. To check if my code works I used the command

    PYTHONPATH=. gdb ./foo -ex 'b 3' -ex run -ex 'p dict'

For troubleshooting, check the output of `info pretty-printer` in gdb.
Support for custom structs should be easy enough to add, but I need to work now. Cheers!
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