How to return string from function to Ruby with CGO

Question

I have a function in Go:

func login(user *C.char) *C.char {
    cstr := C.CString("Hello World")
    defer C.free(unsafe.Pointer(cstr))
    return cstr 
}

My ruby code as below

module GoLib
  extend FFI::Library
  ffi_lib './golib.so'

  attach_function :login, [:string], :string
end

GoLib.login("User1") #=> "p\x9A\xA0\xDB\x16V"

it does not return as ruby string. How to I fix this?

Answer 1

As stated in the comments, cstr gets free'd after control has been passed to Ruby. Here is a workaround where you explicitly manage the pointer in Ruby.

package main

// #include <stdlib.h>
import "C"
import "unsafe"

//export login
func login(user *C.char) *C.char {
    return C.CString("Hello from Go")
}

//export logout
func logout(c *C.char) {
    C.free(unsafe.Pointer(c))
}

func main() {}

In Ruby:

require 'ffi'

module GoLib
  extend FFI::Library
  ffi_lib './so.so'

  attach_function :login, [:string], :strptr
  attach_function :logout, [:pointer], :void
end

s, p = GoLib.login("User1")
puts s
p = FFI::AutoPointer.new(p, GoLib.method(:logout))

Note the use of strptr as demonstrated here