Thread: Passing a memory location back to main

I am working on an assignment in which I can not modify main. I am trying to create a ringbuffer module so that the main program can call multiple ringbuffers at a time (it is actually multi-threaded, but that doesn't matter yet.) I am tring to create a ringbuf_alloc function that initializes the ringbuf-t struct and passes back the pointer to the ringbuf_t struct. However I am getting stuck on passing the pointer. I can't make it a local variable, because it changes each time I call it, and I can't make it a static variable because it changes each time a new buffer calls it. Appreciate any help or any ideas:

Code:

Code:
Ringbuf_t Typedef:
typedef struct ringbuf {
    int           size;	/* # of valid elements in the buffer */
    int           head;		/* head pointer */
    int           tail;		/* tail poiner */
    int	   capacity;
    int	  *buf;
} ringbuf_t;

Main:
ringbuf_t  *ringbuf1 = NULL;
ringbuf_t  *ringbuf2 = NULL;
n = 10;
    if((ringbuf1 = ringbuf_alloc(n)) == NULL)
        syserr("test 1 ringbuf_alloc 2");
    n = 20;
    if((ringbuf2 = ringbuf_alloc(n)) == NULL)
        syserr("test 1 ringbuf_alloc 3");
    if(ringbuf_get_capacity(ringbuf1) != 10 || 
	    ringbuf_get_capacity(ringbuf2) != 20)
        printf("logic error  7\n");

Ringbuf_alloc function
static ringbuf_t p, rb;

ringbuf_t 
*ringbuf_alloc(int capacity)
{
  if (pthread_mutex_init(&mtx, NULL))
	  syserr("pthread_mutex_init");
  p.capacity = capacity;
  p.size = 0;
  p.head = 0;
  p.tail = 0;
  p.buf=(int *) malloc(sizeof(int)*capacity);
  return &p;
}

This gives me an error: Invalid conversion from 'void *' to 'ringbuf_t *'

When I add that to the code:
[

Code:

ringbuf_t 
*ringbuf_alloc(int capacity)
{
  if (pthread_mutex_init(&mtx, NULL))
	  syserr("pthread_mutex_init");
  ringbuf_t *p = malloc( sizeof *p );
  p->capacity = capacity;
  p->size = 0;
  p->head = 0;
  p->tail = 0;
  p->buf=(int *) malloc(sizeof(int)*capacity);
  return p;
}

Thanks for your help. I was using g++ as my prof had suggested it for catching when I forget to include necessary libraries.