This section shows how to install a Go debugger (delve) and how to actually do some debugging from a terminal.
For somebody coming from a C/C++ background this should feed very familiar as the GNU Debugger or GDB command line interface has similar commands. The following table illustrates just a few of the similarities.
| Function | GNU Debugger | Go debugger (dlv) |
|---|---|---|
Connect to a running program |
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Set a breakpoint |
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|
Continue after hitting a breakpoint or attaching to a program |
|
|
Show a backtrace for the current location |
|
|
More commands can be found in the Delve documentation.
1. Install debugger
2. Example debugging
In this example we’ll debug a running Auth server using delve and set a breakpoint
on the function WorkitemController.Show.
|
Note
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To see other ways to run dlv, see the usage page.
|
Let’s assume the binary ./bin/auth is running.
2.1. Attach to program
To attach the debugger to your running fabric8-auth server,run:
$ dlv attach $(pidof ./bin/auth)
The will bring you into the delve shell which looks like this:
Type 'help' for list of commands. (dlv)
The ./bin/auth program is paused right now. Once we’ve set a breakpoint we will
let it run again.
2.2. Set breakpoint
Now, set the breakpoint
on the on the WorkitemController.Show function that is defined in workitem.go:
(dlv) break WorkitemController.Show
As mentioned before the fabric8-auth server is paused and we need to bring it back into a run state. To do this, we let the program continue:
(dlv) continue
2.3. Trigger breakpoint
Open another shell and fetch an existing workitem using curl:
$ curl http://localhost:8080/api/workitems/1
(Replace the 1 with an existing workitem ID if needed)
Now, your debugger shell show something like this:
> main.(*WorkitemController).Show() /tmp/go/src/github.com/fabric8-services/fabric8-auth/workitem.go:30 (hits goroutine(11):1 total:3) (PC: 0x405633) (dlv)
The program is paused again for you to inspect it and the curl command has not returned yet.
2.4. Backtrace
To show a backtrace of how we got here in terms of stack frames, run:
(dlv) bt
The output might look similar to this but it can change over time and as development goes on:
0 0x0000000000405633 in main.(*WorkitemController).Show at /tmp/go/src/github.com/fabric8-services/fabric8-auth/workitem.go:30 1 0x0000000000520014 in github.com/fabric8-services/fabric8-auth/app.MountWorkitemController.func3 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/app/controllers.go:222 2 0x00000000005202fe in github.com/fabric8-services/fabric8-auth/app.handleWorkitemOrigin.func1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/app/controllers.go:257 3 0x0000000000540868 in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa.(*Controller).MuxHandler.func1.1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/service.go:250 4 0x00000000005d6d3e in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware.Recover.func1.1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware/recover.go:37 5 0x00000000005d3b9c in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware.ErrorHandler.func1.1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware/error_handler.go:19 6 0x00000000005d5649 in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware.LogRequest.func1.1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware/log_request.go:65 7 0x00000000005d7229 in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware.RequestIDWithHeaderAndLengthLimit.func1.1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/middleware/request_id.go:63 8 0x000000000054192c in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa.(*Controller).MuxHandler.func1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/service.go:283 9 0x000000000053faf9 in github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa.(*mux).Handle.func1 at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/goadesign/goa/mux.go:57 10 0x000000000076c3c2 in github.com/fabric8-services/fabric8-auth/vendor/github.com/dimfeld/httptreemux.(*TreeMux).ServeHTTP at /tmp/go/src/github.com/fabric8-services/fabric8-auth/vendor/github.com/dimfeld/httptreemux/router.go:247
The first incrementing number on every second line stands for the number of the stackframe. Stackframe 0 is one where we set the breakpoint earlier.
2.5. Continue with normal processing
While there are many interesting ways to inspect your program using the delve debugger, we will instead let our program continue to run and thereby proceed with delivering the workitem to the curl command.
(dlv) continue
Check the shell in which you ran the curl command to see if you have go a result.
2.6. Exit debugging
To exit the debugger you can use the exit
command or press Ctrl+d, just like you would exit any Bash for example:
(dlv) exit Would you like to kill the process? [Y/n] y
You are being asked if you want to kill the process and the answer to this question very much depends on the way you’ve started delve. If you’ve attached to a running process like we did, the answer to this question is probably no. And if you’ve just started the program under test for debugging purposes then you might as well answer y to stop it.