如何在Go中返回切片并从C调用？

Question

I am trying to use cgo to use Go package in C code. Following is a piece of my code:

func LinearTransformToUInt8(frame []int64, winWidth int, winCenter int) []uint8 {
    var transformed []uint8
    // my cool code
    return transformed
}

However, when calling from C, it says

panic: runtime error: cgo result has Go pointer

I believe the problem is the returned []uint8 is a Go type, which should be replaced by a C type. However, I don't know how to achieve it. Please help!

Answer 1

main.go

package main

import (
    "C"
    "unsafe"
)
import (
    "reflect"
)

func main() {
}

//export phew
func phew() uintptr {
    res := make([]uint8, 2)
    for i := 0; i < 2; i++ {
        res[i] = uint8(i + 1)
    }
    hdr := (*reflect.SliceHeader)(unsafe.Pointer(&res))
    return hdr.Data
}

main.c

#include <stdio.h>
#include <inttypes.h>

#include "libtemp.h"

int main(){
    uintptr_t resPtr = phew();
    uint8_t *res = (uint8_t*)resPtr;

    for (int i = 0; i < 2; i++){
        printf("%d
", res[i]);
    }

    printf("Exiting gracefully
");
}

You cannot pass a Go pointer which contains other Go Pointer, slice,string,channel,function, interface, map contain pointers.

So one cannot pass them around, rules to passing around pointers are documented here and go's representation of basic types is documented here.

But some Go contributors were saying, one shouldn't return a Go pointer to C code in the first place.