Multi tool use

How gdb print address of a var in go program?

I success install gdb 8.0.1 and make it run in mac os x. When debug this program I don't see the address of key.

key

package main

func main(){
m := map[string]int{
"abc":123,
}

key := byte("abc")
x, ok := m[string(key)]
println(x, ok)
}


Here is what I have done with gdb:

go build -gcflags "-N" test_append.go
gdb test_append
(gdb) b 9
Breakpoint 1 at 0x104d4b4: file /Users/jiamo/go/src/test/test_append.go, line 9.
(gdb) c
The program is not being run.
(gdb) run
Starting program: /Users/jiamo/go/src/test/test_append
Thread 3 hit Breakpoint 1, main.main () at /Users/jiamo/go/src/test/test_append.go:9
9 x, ok := m[string(key)]
(gdb) info locals
key = uint8 = {97 'a', 98 'b', 99 'c'}
m = map[string]int = {["abc"] = 123}
ok = false
x = 17195648
(gdb) p key
$1 = uint8 * = {97 'a', 98 'b', 99 'c'}
(gdb) p &key
$2 = uint8 * = {97 'a', 98 'b', 99 'c'}


I give a look at lldb. (lldb need b on main.main then b on line)

main.main

(lldb) b main.main
Breakpoint 1: where = test_append`main.main + 50 at test_append.go:4, address = 0x000000000104d372
(lldb) run
(lldb) b 9
(lldb) c
(lldb) fr v
(uint8) key = (len 3, cap 32) {
[0] = 97
[1] = 98
[2] = 99
}
# no address too

(lldb) p key
(uint8) key = (len 3, cap 32) {
[0] = 97
[1] = 98
[2] = 99
}
(lldb) p &key
(*uint8) = 0x000000c420055e10 (len 0, cap 0)
# now it can show the address,
# And I am not sure why it becomes (len 0, cap 0)


And my question is how can show the address of key in gdb?

key

1 Answer
1

You can either disable the Python pretty-printers for Go, then you will get this:

(gdb) print key
$1 = {array = 0xc42003de10 "abc", len = 3, cap = 32}


Or you can temporarily switch to the C language, like this:

(gdb) set language c
Warning: the current language does not match this frame.
(gdb) print key
$1 = uint8 = {97 'a', 98 'b', 99 'c'}
(gdb) print (char *)key
$2 = 0xc420043e10 "abc"
(gdb)


This assumes that Go arrays are interpreted by GDB in a certain way in C mode, but it seems to be to work in this case.

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