processes communicate with shared memory in golang

reader.go

package main

// #include <stdlib.h> 
// #include "wrapper.c"
import "C"
import "unsafe"
import "fmt"

func read(filename string) string {
	f := C.CString(filename)
	defer C.free(unsafe.Pointer(f))
	s := C.my_shm_read(f)
	defer C.free(unsafe.Pointer(s))
	return C.GoString(s)
}

func main() {
	fmt.Println(read("/tmp"))
}

wrapper.c

#include <stdlib.h> 
#include <string.h>
#include <sys/shm.h>
#include <sys/types.h>

int my_shm_open(char* filename, int open_flag){
	int shm_id;
	key_t key;
	key = ftok(filename, 0x03);
	if(key == -1){
		return -1;
	}
	if(open_flag)
		shm_id = shmget(key, 4096, IPC_CREAT|IPC_EXCL|0600);
	else
		shm_id = shmget(key, 0, 0);
	if(shm_id == -1){
		return -1;
	}
	return shm_id;
}

int my_shm_update(int shm_id, char* content){
	char* addr;
	addr = (char*)shmat(shm_id, NULL, 0);
	if(addr == (char*)-1){
		return -1;
	}
	if(strlen(content) > 4095)
		return -1;
	strcpy(addr, content);
	shmdt(addr);
	return 0;
}

int my_shm_close(int shm_id){
	shmctl(shm_id, IPC_RMID, NULL);
	return 0;
}

char* my_shm_read(char* filename){
	int shm_id;
	char* addr;
	char* s;
	shm_id = my_shm_open(filename, 0);
	if(shm_id == -1)
		return NULL;
	addr = (char*)shmat(shm_id, NULL, 0);
	if(addr == (char*)-1){
		return NULL;
	}
	s = (char*)malloc(strlen(addr) + 1);
	strcpy(s, addr);
	shmdt(addr);
	return s;
}

writer.go

package main

// #include <stdlib.h> 
// #include "wrapper.c"
import "C"
import "unsafe"

import (
	"log"
	"time"
)

type errorString struct {
	s string
}

func (e *errorString) Error() string {
	return e.s
}

func open(file string) (int, error) {
	f := C.CString(file)
	defer C.free(unsafe.Pointer(f))
	r := int(C.my_shm_open(f, C.int(1)))
	if r == -1 {
		return 0, &errorString{"error"}
	}
	return r, nil
}

func update(shm_id int, content string) error {
	c := C.CString(content)
	defer C.free(unsafe.Pointer(c))
	r := int(C.my_shm_update(C.int(shm_id), c))
	if r == -1 {
		return &errorString{"update error"}
	}
	return nil
}

func close(shm_id int) error {
	C.my_shm_close(C.int(shm_id))
	return nil
}

func main() {
	id, err := open("/tmp")
	if err != nil {
		log.Fatal(err)
	}
	defer close(id)
	err = update(id, "hello world")
	if err != nil {
		log.Fatal(err)
	}
	time.Sleep(1e9 * 100)
}