如何调用修改go内存的C函数？

// myclib.h

void modify(double* ptr, int N);

package main

// #cgo CFLAGS: -g -Wall
// #include "myclib.h"
import "C"
import (
    "fmt"
    "runtime"
)

func modifyWrapper(v []float64) {
    ptr := (*C.double)(&v[0])
    N := (C.int)(len(v))

    pinner := runtime.Pinner{}
    pinner.Pin(ptr)
    C.modify(ptr, N)
    pinner.Unpin()
}

func main() {
    v := []float64{9.0, 2.0, 1.0, 4.0, 5.0}
    modifyWrapper(v)
}

package main

/*
#cgo CFLAGS: -g -Wall
#include <stdio.h>
void modify(double* ptr, int N) {
    if (!ptr || N <= 0) {
        return;
    }
    printf("modify: %g %d\n", *ptr, N);
    *ptr = 42;
    printf("modify: %g %d\n", *ptr, N);
}
*/
import "C"

import (
    "fmt"
    "unsafe"
)

func modify(v []float64) {
    ptr := (*C.double)(unsafe.SliceData(v))
    N := (C.int)(len(v))
    C.modify(ptr, N)
}

func main() {
    v := []float64{9.0, 2.0, 1.0, 4.0, 5.0}
    fmt.Println(v)
    modify(v)
    fmt.Println(v)
}

[9 2 1 4 5]
modify: 9 5
modify: 42 5
[42 2 1 4 5]