Thread: Passing function pointer using message queues

Hi

A general question first: is there a way to pass a function pointer between two user
space applications in Linux, using a message queue (msgsnd, msgget pair)?

Assuming that this can be done, here is what I'm trying to do:
================================
In a header file I've defined:
================================

Code:

typedef struct msgbuf{
   int mtype;
   int (*ptr)();
   } message_buf;

int myfunc(int);

================================
In the sending application:
=================================

Code:

int myfunc(int mdata){
   /*Do something */
   return 0;   
};
.
.
.
   struct msgbuf* m_msg; /* Structure used for received messages. */
   int queue_id;         /* ID of the created queue. */
   int i;

   /* Access the public message queue. */
   queue_id = msgget(QUEUE_ID, 0);
   if (queue_id == -1) {
      perror("msgget");
      exit(1);
   }

   /* Allocate space for message */
   m_msg = (struct msgbuf*)malloc(sizeof(struct msgbuf)+MAX_MSG_SIZE);

   /* Set valid message type. */
   m_msg->mtype = VALID_TYPE;

   /* Set function. *** HOPING THIS IS RIGHT ***/
   m_msg->ptr = myfunc;

   /* Send message */
   i = msgsnd(queue_id, m_msg, sizeof(struct msgbuf)+1, 0);
   if (rc == -1) {
      perror("msgsnd");
      exit(1);
   } 

   /* Free allocated memory. */
   free(m_msg);
.
.
.

================================
In the receiving application:
=================================

Code:

  struct msgbuf * m_msg; /* Structure used for messages. */
  int i;
  int (*func)(int);
.
.

   while (1) {
      i = msgrcv(m_qid, m_msg, MAX_MSG_SIZE+1, VALID_TYPE, 0);
      if (rc == -1) {
         perror("msgrcv");
         exit(1);
      }
      func = m_msg->cb_ptr;
      /*** HERE IS THE PROBLEM. SEGMENTATION FAULT WHEN REACHED ***/
      i = func(100);
   }
.
.

Any help, even more a sample code or link, is more than welcome.

LxRz

In general a pointer from one process is meaningless in another. There is a chance, if both processes are running from the same binary, that a function pointer might work in both processes. But instead of counting on that, just pass a numeric code which indicates the function to be called, and dispatch the call with a switch statement or some other kind of lookup.

brewbuck thanx for the tip.

The method you described was my initial choice but what I'm really
after is passing a function pointer to realise a callback mechanism,
not in a local (same proccess) manner but between two applications
like this:
the first process delivers a message (including a callback pointer) to the
second, and when the second is done (asyhcronously) it replies using
the callback pointer.

Do you think this is possible? Thanx again.

LxRz

CornedBee thanx for that ...
I'll implement a message queue to the reverse path as well, and let
the first process deal with the callback.

LxRz