How do I pass a "C" string from a "C" routine to a GO function (and convert it to a GO string?) - cgo

Question

This must be something really silly and basic, but the cgo docs (and google fu) have left me stranded. Here's what I am trying to do: I want a GO function to call a "C" function using 'import "C"'. Said "C" function needs to store the address of a "C" string (malloc or constant - neither has worked for me) into an argument passed to it as *C.char. The GO function then needs to convert this to a GO string. It actually does work, except I get this:
panic: runtime error: cgo argument has Go pointer to Go pointer
If I run with GODEBUG=cgocheck=0, it all works fine. If I leave as default:
strptr = 4e1cbf ('this is a C string!')
main: yylex returned token 1
yylval.tstrptr 4e1cbf
stringval token "this is a C string!"
The problematic line seems to be:
yylval.stringval = C.GoString(yylval.tstrptr)
What little I can find about C.GoString, it left me with the impression that it allocates a GO string, and fills it in from the "C" string provided, but that seems to not be the case, or why am I getting a complaint about 'Go pointer to Go pointer'? I've tried a number of other approaches, like having the "C" function malloc the buffer and the GO function do C.free() on it. Nothing has worked (where worked == avoiding this runtime panic).
The GO source:
package main
import (
"fmt"
"unsafe"
)
// #include <stdio.h>
// int yylex (void * foo, void *tp);
import "C"
type foo_t struct {
i int32
s string
}
var foo foo_t
func main() {
var retval int
var s string
var tp *C.char
for i := 0; i < 2; i++ {
retval = int(C.yylex(unsafe.Pointer(&foo), unsafe.Pointer(&tp)))
fmt.Printf("main: yylex returned %d\n", retval)
fmt.Printf("tp = %x\n", tp)
if retval == 0 {
s = C.GoString(tp)
fmt.Printf("foo.i = %d s = %q\n", foo.i, s)
} else {
foo.s = C.GoString(tp)
fmt.Printf("foo.i = %d foo.s = %q\n", foo.i, foo.s)
}
}
}
The "C" source
#include <stdio.h>
int yylex (int * foo, char ** tp)
{
static num;
*foo = 666;
*tp = "this is a C string!";
printf ("strptr = %x ('%s')\n", *tp, *tp);
return (num++);
}
What's interesting is that if the GO func stores into foo.s first, the 2nd call to yylex bombs with the panic. If I do s and then foo.s (depending on whether I check retval as 0 or non-zero), it doesn't fail, but I'm guessing that is because the GO function exits right away and there are no subsequent calls to yylex.