question on function pointers

Hello I have another question this one regards function pointers.
I have an api function that has the following prototype.

Code:

---

int sqlite3_exec(
  sqlite3*,                                  /* An open database */
  const char *sql,                          /* SQL to be evaluated */
  int (*callback)(void*,int,char**,char**),  /* Callback function */
  void *,                                    /* 1st argument to callback*/
  char **errmsg                              /* Error msg written here */
);

---

the callback function takes a void pointer for the argument and returns
an integer used for sqlite return code.


I need to be able to return values from the callback function however I
am not sure how I can do that. I have heard that u can pass a function
pointer as the argument to be able to this but I am not sure how this is
done.
If anyone can help me with this I would be greatful. I have spent a long
time trying to figure this out on my own I have to go sleep now.


thank you.

You just define your function with the same signature as the pointer:

Code:

---

int my_callback(void *ptr1, int num, char **ptr2, char **ptr3)
{
  // Code goes here
}

---

Then you just pass the name of your function in as the parameter:

Code:

---

sqlite3_exec(sqlite, "blah blah blah", my_callback, ptr1, "You messed up!");

---

I understand this, my question is that since the callback function returns an integer and I need to retrieve a struct from it, how can I do this? So basically is there a way I can pass a pointer as argument where I can save my values and be able to retrieve them from the pointer outside of the function? the argument takes a void pointer so I will have to cast it and assign it to another variable. I assume defining a global variable pointer to a struct will work but not sure.

Code:

---

static struct test *arg;

int main()
{
  struct test *q;
  q = malloc(sizeof(struct test));
  /* allocate memory for any further values needed for struct */
  sqlite3_exec(sqlite, sqlstmt, my_callback, &q, &error);
}

int my_callback(void *ptr1, int num, char **ptr2, char **ptr3)
{
  arg = (struct test *)ptr1;
  /* update arg values */
}


---

I have not tried this yet but I assume it should do the job if you have any suggestions please let me know.