If it won't be simple, it simply won't be. [source code] by Miki Tebeka, CEO, 353Solutions

Tuesday, August 30, 2016

"Manual" Breakpoints in Go

When debugging, sometimes you need to set conditional breakpoints. This option is available both in gdb and delve. However sometimes when the condition is complicated, it's hard or even impossible to set it. A way around is to temporary write the condition in Go and set breakpoint "manually".

I Python we do it with pdb.set_trace(), in Go we'll need to work a little harder. The main idea is that breakpoints are special signal called SIGTRAP.

Here's the code to do this:
You'll need tell the go tool not to optimize and keep variable information:

$ go build -gcflags "-N -l" manual-bp

Then run a gdb session

$ gdb manual-bp 
(gdb) run 

 When you hit the breakpoint, you'll be in assembly code. Exit two functions to get to your code

(gdb) fin
(gdb) fin

Then you'll be in your code and can run gdb commands

(gdb) p i
$1 = 3

This scheme also works with delve

$ dlv debug manual-bp.go 
(dlv) c 

Sadly delve don't have "fin" command so you'll need to hit "n" (next) until you reach your code. 

That's it, happy debugging.

Oh - and in the very old days we did about the same trick in C code. There we manually inserted asm("int $3)" to the code. You can do with with cgo but sending a signal seems easier.
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