How to cast the address to a type and print it in GDB for Golang

Question

This is the piece of code:

package main

import (
    "fmt"
)

type TestType struct {
    a int
    b int
}

func main() {
    var testType TestType = TestType{1, 2}
    fmt.Println(testType)
}

And this is the gdb debug output:

(gdb) r
Starting program: /home/bzhang/common/src/go/src/test/testBinary 

Breakpoint 1, main.main () at /home/bzhang/common/src/go/src/test/main.go:14
14              fmt.Println(testType)
(gdb) p testType
$1 = {a = 1, b = 2}
(gdb) p &testType
$2 = (main.TestType *) 0xc820059ee8
(gdb) p ('main.TestType'*) 0xc820059ee8
A syntax error in expression, near `) 0xc820059ee8'.
(gdb) p ('TestType'*) 0xc820059ee8     
A syntax error in expression, near `) 0xc820059ee8'.
(gdb) whatis testType
type = main.TestType
(gdb) 

Of course I know that I can print testType directly. But if it is a local variable, sometimes its value can not be printed out directly, and only its address is available. So I want to print its value with the indication of its type. But it seems not work correctly. Appreciate for your help!

Answer 1

The delve tool is better than gdb. https://github.com/go-delve/delve

1. install delve

go get -u github.com/go-delve/delve/cmd/dlv

2. move path on main.go

cd $GOPATH/src/YOUPROJECT/main.go

3. debug delve

$GOPATH/bin/delve debug main.go

4. break point main

Type 'help' for list of commands.
(dlv) b main.main
Breakpoint 1 set at 0x4a7bbf for main.main() main.go:12

5. run

(dlv) c
> main.main() main.go:12 (hits goroutine(1):1 total:1) (PC: 0x4a7bbf)
     7: type TestType struct {
     8:         a int
     9:         b int
    10: }
    11:
=>  12: func main() {
    13:         var testType TestType = TestType{1, 2}
    14:         fmt.Println(testType)
    15: }

6. break point 14 line

(dlv) b 14
Breakpoint 2 set at 0x4a7bf0 for main.main() main.go:14

7. continue

(dlv) c
> main.main() main.go:14 (hits goroutine(1):1 total:1) (PC: 0x4a7bf0)
     9:         b int
    10: }
    11:
    12: func main() {
    13:         var testType TestType = TestType{1, 2}
=>  14:         fmt.Println(testType)
    15: }

8. show value

(dlv) locals
testType = main.TestType {a: 1, b: 2}

Finally, using goland, vscode is more convenient for debug.