How to watch the init of the go map

Question

I use the delve to debug galang code in vscode. I want to use the delve to see the the creation of the golang map,and I create a breakpoint when use the make function to init the map,but it just cannot step into.

Answer 1

You can use delve to debug the runtime, however you will need to get familiar with the dlv command line interface(without using a GUI).

I will be using the following program as an example:

package main

func main() {
    abc := make(map[string]int)
    abc["a"] = 1
    abc["b"] = 2
    abc["c"] = 2
    fmt.Println(abc)
}

We can ask delve to build and debug our program with the dlv debug command. This will drop us into the interactive debugger.

One of the things that makes maps "fun" to debug is that the compiler can create maps in different ways. We first need to know which functions the compiler is using to create a map(can be several depending on map size). We will ask delve to disassemble the main func with the disassemble -l main.main command:

(dlv) disassemble -l main.main
TEXT main.main(SB) /home/caveman/Downloads/test/main.go
        main.go:7       0x4b6860        64488b0c25f8ffffff              mov rcx, qword ptr fs:[0xfffffff8]
        main.go:7       0x4b6869        488d4424f0                      lea rax, ptr [rsp-0x10]
        main.go:7       0x4b686e        483b4110                        cmp rax, qword ptr [rcx+0x10]
        main.go:7       0x4b6872        0f8668010000                    jbe 0x4b69e0
        main.go:7       0x4b6878        4881ec90000000                  sub rsp, 0x90
        main.go:7       0x4b687f        4889ac2488000000                mov qword ptr [rsp+0x88], rbp
        main.go:7       0x4b6887        488dac2488000000                lea rbp, ptr [rsp+0x88]
        main.go:8       0x4b688f        e84c86f5ff                      call $runtime.makemap_small
        main.go:8       0x4b6894        488b0424                        mov rax, qword ptr [rsp]
        main.go:8       0x4b6898        4889442430                      mov qword ptr [rsp+0x30], rax
        ...

Now this looks complicated, but we just have to look for calls to the runtime. In this case call $runtime.makemap_small a call to the makemap_small function.

Now that we know this, we need to make a breakpoint in this runtime function. We do this using the break mkSmallMap runtime.makemap_small command:

(dlv) break mkSmallMap runtime.makemap_small
Breakpoint mkSmallMap set at 0x40eeef for runtime.makemap_small() /usr/local/go/src/runtime/map.go:292

Now we can start our program by executing the c(continue) command:

(dlv) c
> [mkSmallMap] runtime.makemap_small() /usr/local/go/src/runtime/map.go:292 (hits goroutine(1):1 total:1) (PC: 0x40eeef)
Warning: debugging optimized function
   287: }
   288:
   289: // makemap_small implements Go map creation for make(map[k]v) and
   290: // make(map[k]v, hint) when hint is known to be at most bucketCnt
   291: // at compile time and the map needs to be allocated on the heap.
=> 292: func makemap_small() *hmap {
   293:         h := new(hmap)
   294:         h.hash0 = fastrand()
   295:         return h
   296: }
   297:

We break in the runtime function itself. The help command will help you get started with all the commands you need for debugging. A few basic ones:

• continue (alias: c) --------- Run until breakpoint or program termination.
• next (alias: n) ------------- Step over to next source line.
• step (alias: s) ------------- Single step through program.
• stepout (alias: so) --------- Step out of the current function.
• list (alias: ls | l) ------- Show source code.
• args ----------------- Print function arguments.
• locals --------------- Print local variables.

If we change the map creation to abc := make(map[string]int, 100000) you will see in the disassembly that the runtime function has changed to runtime.makemap which is likely of more interest to you.

I hope this gets you going, please leave a comment if you need clarification on anything.