How to return a string from JSON to a C caller (Golang CGO)?

Question

I'm trying to develop a routine in Go that will be called by a C++ program. The Go looks like the following:

package main

import (
    "C"
    "encoding/json"
    "log"
)

type keydata struct {
    Key   string `json:"key"`
    Error string `json:"Error"`
}

func lookupKey() string {
//simplified to remove the call to web service
    body := "{\"key\": \"blahblah\", \"Error\": \"\"}"

    k := keydata{}
    err := json.Unmarshal([]byte(body), &k)
    if err != nil {
        log.Fatal(err)
    }

    return k.Key
}

//export GetKey
func GetKey() string {
    theKey := lookupKey()
    return theKey
}

func main() {}

If I substitute some hard coded value for the return k.Key statement everything works fine and the C or C++ can call the exported GetKey function. When I try to return the decoded JSON string from k.Key or even just return the string from the variable named body - I receive an error:

runtime error: cgo result has Go pointer goroutine 17 [running, locked to thread]

I'm building this as follows:

go build -buildmode=c-archive example.go

The C++ is built as follow:

g++ -pthread test.cpp example.a -o test

What am I missing to make this work without raising a panic error? I'm digging around to find an answer but have yet to resolve this.

@JimB & @Jsor, thank you so much for your responses. Returning a *C.char certainly worked. I'm left wondering though, when I return it as a Go string behind the scenes in the auto generated header file Go actually creates and passes a C struct named GoString that contains a char array named p and the length named n. As long as I pass a hard-coded string instead of k.Key it actually works and I can interrogate the auto-generated char array in C++. When I try to return k.Key, a string it throws that exception. Is it possible to cast the Go string or add some notation to the export decoration to make it work?

I can certainly return the C.CString char array and make it work - thank you! I'm just also wanting to understand why it works when returning a hard coded string and not in the example I've posted.

Thank you both for your time and explanations.

(FYI, @ mentions only work in comments) You can't use the GoString safely from C, because it contains a pointer to memory allocated in Go. The fact that using a string literal gets missed by cgocheck is beside the point. — Mr_Pink
– Mr_Pink, Commented May 24, 2016 at 21:55

Mr_Pink · Accepted Answer · 2016-05-24 15:51:36Z

9

You can't return a Go string to a C function. If you want a C string, you can use the C.CString function to create one and return a *C.char

//export GetKey
func GetKey() *C.char {
    theKey := lookupKey()
    return C.CString(theKey)
}

The return value from this function must be explicitly freed in the C code.

If freeing the allocated buffer isn't convenient, its common to fill a buffer provided by the caller:

func GetKey(buff *C.char, n int) int

If you can allocate the memory but don't want to handle C strings, you can insert the buffer into a pointer and return the size.

func GetKey(buff **C.char) int

answered May 24, 2016 at 15:51

Mr_Pink

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1 Comment

Volatil3 Over a year ago

Can you explain this further, what is n here and how do I send from Python?

Linear · Accepted Answer · 2016-05-24 15:36:45Z

3

You need to use C.CString to convert Go strings to raw pointers to C strings. Note that C Strings are not garbage collected and must be freed by you elsewhere in the program.

This will make the return type *C.char which should be visible to C as a char array. It will also be your responsibility to return the buffer length (whether your write a separate function or a C struct to do that is up to you).

answered May 24, 2016 at 15:36

Linear

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1 Comment

Volatil3 Over a year ago

But how do I free if I am returning a CString variable?

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