[HN Gopher] Show HN: Dlg - Zero-cost printf-style debugging for Go
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Show HN: Dlg - Zero-cost printf-style debugging for Go
Hey HN, I tend to use printf-style debugging as my primary
troubleshooting method and only resort to gdb as a last resort.
While I like its ease of use printf debugging isn't without its
annoyances, namely removing the print statements once you're done.
I used to use trace-level logging from proper logging libraries but
adding trace calls in every corner quickly gets out of control and
results in an overwhelming amount of output. To scratch my own
itch I created dlg - a minimal debugging library that disappears
completely from production builds. Its API exposes just a single
function, Printf [1]. dlg is optimized for performance in debug
builds and, most importantly, when compiled without the dlg build
tag, all calls are eliminated by the Go linker as if dlg was never
imported. For debug builds it adds optional stack trace generation
configurable via environment variables or linker flags. GitHub:
https://github.com/vvvvv/dlg Any feedback is much appreciated.
[1]: Actually two functions - there's also SetOutput.
Author : 0xFEE1DEAD
Score : 57 points
Date : 2025-07-27 18:02 UTC (3 days ago)
(HTM) web link (github.com)
(TXT) w3m dump (github.com)
| kitd wrote:
| Very cool!
|
| Just one thought: I don't think your stack traces need to include
| the lines inside Printf itself and it adds to the noise a bit.
| Maybe just go up to the line where the call to Printf is made?
| 0xFEE1DEAD wrote:
| Thank you!
|
| You're absolutely right, it annoys me as well.
|
| It's a bit finicky to do so, but I'm currently in an
| exploratory phase of providing an even more fine grained stack
| tracing feature and filtering out dlg itself is definitly on my
| todo list.
| smcameron wrote:
| > printf debugging isn't without its annoyances, namely removing
| the print statements once you're done.
|
| Use something like stacked git[1], and then it's just one "stg
| pop" and poof, they're gone.
|
| [1] https://stacked-git.github.io/
| BobbyTables2 wrote:
| I've been doing this (manually) for about 10+ years.
|
| I rarely use branches, my local work area is about 50-100
| commits on top of master.
|
| I pick/reorder/edit with "git rebase -i"
|
| I'll prefix the commit summary with a word that helps me keep
| straight debug changes from ones that will go toward a MR.
| shayief wrote:
| I usually leave them as uncommitted changes, then git checkout
| to undo them. And using something like `git add -p` to skip
| Printfs if I need to commit other changes.
| sethammons wrote:
| In extreme processing needs, I suppose the elimination of a check
| that would happen with a level logger is a solid trade off.
|
| My favored approach is a context aware level logger with
| structured logs going to an aggregator like Splunk.
|
| Because it is context aware, custom filtering logic is available
| in a middleware.
| jerf wrote:
| See the standard library "slog" package:
| https://pkg.go.dev/log/slog
|
| Which is a relatively recent addition to the Go standard
| library, based on the collective experience with logging over
| the years. Compare with the original log standard library:
| https://pkg.go.dev/log and you can see the changes in even just
| the time since Go was developed in logging best practices.
|
| Slog, once you understand the interfaces involved in stuff, has
| performed most every trick I could desire, up to and including
| things like subloggers with their own priorities for outputs
| that override the global settings.
|
| I'd like to see some slog integration in this package, though I
| say that without a design in mind. As it stands, this is
| exactly what it says on the tin; debugging-only output that you
| fully intend to just disappear out of the production binary.
|
| The trick it uses is a fairly straightforward use of Go's tag-
| based compilation features, which I do not mean as a criticism,
| but to point out that any Go programmer who likes the looks of
| this but wants to adapt it to their own task will find it easy
| to simply take the technique directly and adapt it in not very
| many lines of code.
| negrel wrote:
| Cool lib! I took the same approach for my zero cost debugging
| assertion Go library
|
| https://github.com/negrel/assert
| mdaniel wrote:
| How does the dead code elimination work when using _args_ to
| Printf? If static strings are fed into an empty function, I can
| imagine it does nothing. However, this I have less of a firm grip
| upon dlg.Printf("the thing.Something is %+v",
| thing.Something())
|
| since surely golang will still call Something, and still call
| Printf
|
| And don't misunderstand me: it's a grave pet peeve of mine to do
| _action_ inside the args to Printf but I can tell you it 's a
| very common [anti-]pattern so I'd presume your users will want a
| story for how that interacts with the "disappears completely"
| claim
| llimllib wrote:
| It's easy to show that you're right and that code isn't dead so
| it can't be eliminated: package main
| import ( "fmt"
| "github.com/vvvvv/dlg" ) func risky()
| error { _, err := fmt.Printf("unexpected error\n")
| return err } func main() {
| dlg.Printf("something failed: %s", risky()) risky()
| }
|
| prints "unexpected error" twice
| exasperaited wrote:
| This is a thing I had a little "are you sure?" itch in my brain
| about, but you've expressed so clearly in four lines of
| comment, and this is me showing gratitude for the comment and
| the untangling of my brain.
|
| ETA: Presumably if there was at least an option for passing a
| closure that would provide a way to address this? Since the
| closure will only uselessly compile, and not actually uselessly
| run?
| 0xFEE1DEAD wrote:
| In short: You're right - Go will still evaluate argument
| expressions. In hindsight, I should've made that more obvious,
| not calling this out initially was an expert blind spot on my
| part, not an intent to mislead.
|
| If we consider this example: func risky() error
| { return fmt.Errorf("unexpected error") }
| func risky2() error{ return fmt.Errorf("also an error")
| } func main() { fmt.Printf("something
| failed: %v", risky()) dlg.Printf("something failed:
| %v", risky2()) }
|
| And look at the disassembly: 0x10009f354
| d503201f NOOP ; Dead code of dlg.Printf
| 0x10009f358 b0000000 ADRP 4096(PC), R0 ; Load "also an
| error" string address 0x10009f35c 9124ec00 ADD $2363,
| R0, R0 ; Calculate exact string address 0x10009f360
| d28001a1 MOVD $13, R1 ; Set string length=13 (R1)
| 0x10009f364 aa1f03e2 MOVD ZR, R2 ; Clear R2
| 0x10009f368 aa1f03e3 MOVD ZR, R3 ; Clear R3
| 0x10009f36c aa0303e4 MOVD R3, R4 ; Clear R4
| 0x10009f370 97ffd890 CALL fmt.Errorf(SB) ; Call fmt.Errorf
|
| What disappears is the logging work (formatting,
| interface/slice plumbing, I/O) and the call itself. Go cannot
| eliminate calls to functions inside of Printf because they
| could produce side-effects. Eliminating functions calls like
| this would be very expensive to do and would clashes with Go's
| focus on fast compilation times.
|
| I'll add a section to the README that explains this. Thanks for
| pointing it out.
| sigmonsays wrote:
| it doesn't appear to truly be zero cost if you log variables that
| can't be eliminated. The only way (I believe) to implement zero
| cost is some type of macro system which go does not support.
| lanstin wrote:
| Yeah I want a log package where the log statements that don't
| log also don't have their arguments evaluated. Half of my gc
| pressure is little strings created to call the log package that
| don't get logged.
| 9rx wrote:
| So what you want is slog, found in the standard library? The
| doc.go file found in the package even goes into detail about
| what you (seem to) describe and how it avoids that problem.
| badc0ffee wrote:
| A link to doc.go, for the lazy:
|
| https://cs.opensource.google/go/x/exp/+/645b1fa8:slog/doc.g
| o...
|
| Basically it says to pass in objects, not prepared strings,
| so the formatted output is only computed if the message is
| actually logged. That object can be a struct implementing
| the LogValuer interface to do any required work/produce
| formatted output.
| lanstin wrote:
| Oh cool, I hadn't run into LogValuer, that's cool. And
| passing pointers to strings rather than strings, hmmm.
| 9rx wrote:
| _> And passing pointers to strings rather than strings,
| hmmm._
|
| Are you referring to the "URL" example? That isn't a case
| of passing pointers to strings, that is passing a URL
| (fmt.Stringer) where the String method has a pointer
| receiver.
|
| To demonstrate why the pointer would be needed in that
| case, consider: type URL struct{}
| func (*URL) String() string { return "https://..." }
| func print(v any) {
| fmt.Println(v.(fmt.Stringer).String()) } func
| main() { print(&URL{}) // ok
| print(URL{}) // panic }
| dspillett wrote:
| _> it doesn 't appear to truly be zero cost if you log
| variables that can't be eliminated._
|
| I'd say it is fair to call it zero cost, if the costs you are
| seeing are due to the way you are using it. If the values being
| logged are values you are already computing and storing,
| constants, or some mix of the two (concatenated via its printf
| function), by my understanding (caveat: I've never actually
| used Go myself) all the logging code should be stripped out as
| dead code in the link stage.
|
| Obviously if you are performing extra computations with
| intermediate values to produce the log messages, your code
| there might produce something that is not reliably detected as
| eliminable dead code so there would be cost there.
|
| _> The only way (I believe) to implement zero cost is some
| type of macro system which go does not support._
|
| That looks to be effectively what it is doing, just at the link
| stage instead of in a preprocessor. Where C & friends would
| drop the code inside #ifdef when the relevant value is not set
| (so it won't be compiled at all) this should throw it out later
| in the process if DLG_STACKTRACE isn't set at compile time.
|
| So there will always be a compile-time cost (unlike with a
| preprocessor macro, something will be produced then thrown away
| and the analysis of what to throw away will take longer with
| more code present), but not a run-time cost if the logging is
| not enabled, _assuming_ you don 't have any logic in the trace
| lines beside the calls to this module.
| norir wrote:
| The problem is calling anything zero cost is inherently
| wrong. Nothing in life has no cost. I know I am being
| pedantic but I think a more accurate description is "zero
| production runtime cost," which is also how I interpret
| rust's zero cost abstraction. In that case too, I find the
| terminology grating because there are very obviously costs to
| using rust and its abstractions.
|
| One obvious cost to code that is removed in production is
| that there is now a divergence between what your source code
| says it does and what it actually does. I now need to keep in
| mind the context the program is running in. There is a cost
| to that. It might be worth it, but it isn't zero.
| latchkey wrote:
| I'm curious why you prefer this over writing tests.
| 0xFEE1DEAD wrote:
| I do write tests but I'm not a fan of aiming for 100% coverage
| because it's incredibly tedious to achieve, and the gains in my
| experience are marginal.
|
| That being said even with 100% coverage your code may still
| contain bugs. If you've never had to debug code with extensive
| tests then hats off to you.
| 9rx wrote:
| When you write a test you see the cute little "debug test"
| button show up beside it in your editor. If you press it,
| your logpoints spill the logging information you seek without
| having to modify the code.
|
| I expect that is what the parent is really asking about: What
| does this meaningfully offer over the "built-in" debug
| logging?
| 0xFEE1DEAD wrote:
| Well, my vim is missing buttons like these and I wasn't
| aware such a feature existed.
|
| Maybe I'm just old, maybe it's just a different approach,
| but I built dlg for myself because it fits my workflow.
|
| > What does this meaningfully offer over the "built-in"
| debug logging?
|
| For you, it most likely doesn't offer anything meaningful.
|
| For devs who use fmt.Printf a lot, it maybe does.
| latchkey wrote:
| I'm 52 and coding since I was about 10 years old, so I'm
| not sure age is the right thing to go off.
| latchkey wrote:
| Crazy that I get downvoted for a perfectly valid question, go
| HN!
|
| I'm not saying that tests are perfect, but if you do find a
| bug, you write a new test and fix it. You don't add print
| statements that you expect to keep around any longer than the
| amount of effort there is to write the test and fix the code.
|
| Maybe this product is for people who don't write tests, but
| even in this codebase, there is a pretty well done set of
| tests and zero dogfood usage that I noticed.
| 0xFEE1DEAD wrote:
| > but if you do find a bug, you write a new test and fix
| it.
|
| Maybe there's a misunderstanding here but this library is
| not meant to replace tests. It's for initially finding the
| bug for which you then write a test case (or not).
| latchkey wrote:
| It isn't clear how this library helps one find bugs.
| danudey wrote:
| If you're not sure how logging debug information can help
| you find bugs then I'm not sure what to say. Especially
| if you've been programming for four decades, I would
| think it would be obvious how logging information can
| help you find logic or data errors in your
| code/inputs/outputs.
| latchkey wrote:
| I know how to write failing tests, and use a debugger.
| danudey wrote:
| I think you're getting downvoted because it's not a valid
| question - debug logging and unit testing are two separate
| things, and the OP didn't say anywhere that he prefers this
| to unit tests. Your question presupposes a dichotomy that
| doesn't exist.
| latchkey wrote:
| My statement certainly had some implied subtext that I
| can expand upon now...
|
| If you write tests, you don't need a specialized library
| for debug logging.
|
| Run the test, and set a breakpoint in your debugger.
| danudey wrote:
| This is only mildly offtopic, but I wonder if the
| following is true:
|
| Software developers who...
|
| 1. Experienced actual instruction on software
| development;
|
| 2. Learned to write software in
| C/C++/FORTRAN/Pascal/whatever; or
|
| 3. Spent a lot of time in more niche situations like
| system programming, embedded programming, enterprise
| systems, etc.
|
| ...are familiar with using debuggers as part of their
| workflow to inspect internal state and diagnose. In
| contrast, developers who...
|
| 1. Were entirely self-taught;
|
| 2. Grew up on interpreted languages (JS, Python, PHP,
| Perl, etc); and
|
| 3. Generally tend to write smaller, self-contained
| projects
|
| ...tend to just add debug logging and re-run the program.
|
| I definitely _have_ used debuggers in the past, but as
| someone who almost exclusively has written in interpreted
| languages (or languages where the write-compile-run loop
| is fast, e.g. Golang), my habit is to add debug logging
| to more places to validate internal state as the program
| runs, rather than attaching a debugger and inspecting
| specific things manually.
|
| Either way, I think it's two separate approaches and I
| think in different circumstances there are definitely
| benefits to one or the other. In most of my Python
| scripts it's faster to add a `print(f"variable is
| {variable}")` than it would be to attach a debugger; also
| for a small script that's only going to run a few times,
| or run very occasionally, writing unit tests is often
| overkill because often the entire script is effectively a
| unit.
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