How to write text file on Tizen Gear S2 local file system?

I'm developing native app on Samsung Gear S2 which intended to record data from sensors (accelerometer) on device into local file system.

I try the directory ( /opt/usr/media/documents/text.txt ) but its not creating the file on device.

I have enabled the privilege: http://tizen.org/privilege/mediastorage. Code snippet below:

static char* write_file(const char* buf)
{
    FILE *fp;
    fp = fopen("/opt/usr/media/documents/text.txt","w");
    fputs(buf,fp);
    fclose(fp);
}

static void btn_write_cb(void *data, Evas_Object *obj, void *event_info)
{
    appdata_s *ad = data;
    buf="string to write";
    write_file(buf);
}

What is the write way to do? OR Web app work for this task? Thanks

Finally I am able to write file in my Samsung Gear S2 Classic by Tizen Native App Development. With address: /opt/usr/media/text.txt. And used the privilege http://tizen.org/privilege/mediastorage. Here is my code snippet.

static void
app_get_data(const char *res_file_in, char *res_path_out, int res_path_max)
{
    char *res_path = NULL;
    res_path= "/opt/usr/media/";
    if (res_path) 
    {
        snprintf(res_path_out, res_path_max, "%s%s", res_path, res_file_in);
        //free(res_path); //Disabling this statement works for me.
    }
}

static char*
write_file(const char* filepath, const char* buf)
{
    FILE *fp;
    fp = fopen(filepath, "w"); 
    fputs(buf, fp);
    fclose(fp);
}

static void
btn_write_cb(void *data, Evas_Object *obj, void *event_info)
{
    appdata_s *ad = data;
    char *buf = NULL;
    buf="Hello";
    char filepath[PATH_MAX] = { 0, };
    app_get_data("text.txt", filepath, PATH_MAX);
    write_file(filepath, buf);
}