golang get char* as return value from dll

Question

I'm using golang to call a Dll function like char* fn(), the dll is not written by myself and I cannot change it. Here's my code:

package main

import (
    "fmt"
    "syscall"
    "unsafe"
)

func main() {
    dll := syscall.MustLoadDLL("my.dll")
    fn := dll.MustFindProc("fn")

    r, _, _ := fn.Call()

    p := (*byte)(unsafe.Pointer(r))
    // define a slice to fill with the p string
    data := make([]byte, 0)

    // loop until find '\0'
    for *p != 0 {
        data = append(data, *p)        // append 1 byte
        r += unsafe.Sizeof(byte(0))    // move r to next byte
        p = (*byte)(unsafe.Pointer(r)) // get the byte value
    }
    name := string(data) // convert to Golang string
    fmt.Println(name)
}

I have some questions:

1. Is there any better way of doing this? There're hundred of dll functions like this, I'll have to write the loop for all functions.
2. For very-long-string like 100k+ bytes, will append() cause performance issue?
3. Solved. the unsafe.Pointer(r) causes linter govet shows warning possible misuse of unsafe.Pointer, but the code runs fine, how to avoid this warning? Solution: This can be solved by adding -unsafeptr=false to govet command line, for vim-ale, add let g:ale_go_govet_options = '-unsafeptr=false'.

Answer 1

Casting uintptr as upointer is haram. You must read the rules: https://golang.org/pkg/unsafe/#Pointer

But there's hacky way, that shouldn't produce warning:

//go:linkname gostringn     runtime.gostringn
func gostringn(p uintptr, l int) string

//go:linkname findnull     runtime.findnull
//go:nosplit
func findnull(s uintptr) int

// ....

name := gostringn(r, findnull(r))

Functions takes pointer, but we link them from runtime as uintptr because they have same sizeof.

Might work in theory. But is also frowned upon.

Getting back to your code, as JimB said, you could do it one line with:

name := C.GoString((*C.char)(unsafe.Pointer(r)))