JNA to Go DLL - How do I get String returned from Go Func? - jna

Question

I have a Java program that is using JNA to call a Go Func. Here's the Interface to the Go func in Java:
public interface GPG extends Library {
// GoString class maps to: C type struct { const char *p; GoInt n; }
public class GoString extends Structure {
public static class ByValue extends GoString implements Structure.ByValue {}
public String p;
public long n;
protected List getFieldOrder(){
return Arrays.asList(new String[]{"p","n"});
}
}
// Foreign functions
public GoString.ByValue decrypt(GoString.ByValue encString, GoString.ByValue secretKeyring, GoString.ByValue passphrase);
}
The func signature in Go is:
func decrypt(encString string, secretKeyring string, passphrase string) string
The Go generated C header has:
/* Created by "go tool cgo" - DO NOT EDIT. */
/* package command-line-arguments */
#line 1 "cgo-builtin-prolog"
#include <stddef.h> /* for ptrdiff_t below */
#ifndef GO_CGO_EXPORT_PROLOGUE_H
#define GO_CGO_EXPORT_PROLOGUE_H
typedef struct { const char *p; ptrdiff_t n; } _GoString_;
#endif
/* Start of preamble from import "C" comments. */
/* End of preamble from import "C" comments. */
/* Start of boilerplate cgo prologue. */
#line 1 "cgo-gcc-export-header-prolog"
#ifndef GO_CGO_PROLOGUE_H
#define GO_CGO_PROLOGUE_H
typedef signed char GoInt8;
typedef unsigned char GoUint8;
typedef short GoInt16;
typedef unsigned short GoUint16;
typedef int GoInt32;
typedef unsigned int GoUint32;
typedef long long GoInt64;
typedef unsigned long long GoUint64;
typedef GoInt64 GoInt;
typedef GoUint64 GoUint;
typedef __SIZE_TYPE__ GoUintptr;
typedef float GoFloat32;
typedef double GoFloat64;
typedef float _Complex GoComplex64;
typedef double _Complex GoComplex128;
/*
static assertion to make sure the file is being used on architecture
at least with matching size of GoInt.
*/
typedef char _check_for_64_bit_pointer_matching_GoInt[sizeof(void*)==64/8 ? 1:-1];
typedef _GoString_ GoString;
typedef void *GoMap;
typedef void *GoChan;
typedef struct { void *t; void *v; } GoInterface;
typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;
#endif
/* End of boilerplate cgo prologue. */
#ifdef __cplusplus
extern "C" {
#endif
extern GoString decrypt(GoString p0, GoString p1, GoString p2);
#ifdef __cplusplus
}
#endif
I call the Go Func from Java using this code:
GPG gpg = (GPG) Native.loadLibrary("C:/lib/gpg.dll", GPG.class);
GPG.GoString.ByValue encString = new GPG.GoString.ByValue();
encString.p = value;
encString.n = encString.p.length();
GPG.GoString.ByValue secretKeyring = new GPG.GoString.ByValue();
secretKeyring.p = "c:/gnupg/secring.gpg";
secretKeyring.n = secretKeyring.p.length();
GPG.GoString.ByValue passphrase = new GPG.GoString.ByValue();
passphrase.p = "SecretPassPhrase";
passphrase.n = passphrase.p.length();
GPG.GoString.ByValue decValue = gpg.decrypt(encString, secretKeyring, passphrase);
Clearly the func is being called and processes up to the return of the result string. But it then produces: "panic: runtime error: cgo result has Go pointer"
How do I get a String result back from Go?
Using go version go1.10 windows/amd64, JNA 4.5.1, Java 1.8.0_152

Answer 1

Your GO function should looks like this:
//export decrypt
func decrypt(encString string, secretKeyring string, passphrase string) *C.char {
//... your code here
var str string = "returning string"
return C.CString(str)
}
Java Interface:
public String decrypt(GoString.ByValue encString, GoString.ByValue secretKeyring, GoString.ByValue passphrase);

Answer 2

Your const char * in _GoString_ should use a Pointer instead, then use Pointer.getString() with the provided offset to obtain the actual string.
If Go itself is rejecting a string return value, you'll likely have to instead populate a buffer provided by the caller.