foo.go

// filename: cmd/foo/foo.go

package main

import simple "simple"

func main() {
	d := simple.GetData()
	simple.PutData(d)
}

output.txt

*data=hello
data=0x42045000
data=0x442086fb8
*data=

simple.c

#include <runtime.h>

int8*
simple·GetData() {
    int8 *data = runtime·malloc(6);
    runtime·memmove(data, "hello", 6);
    runtime·printf("*data=%s\n", data);
    runtime·printf("data=%p\n", data);
    return data;
}

void
simple·PutData(int8* data) {
    runtime·printf("data=%p\n", data);
    runtime·printf("*data=%s\n", data);
    runtime·free(data);
}

simple.go

package simple

func GetData() *byte
func PutData(b *byte)