användning av shared memory shm funktionerna

Hej uppgiften är att ändra programmet så att det gemensamma minnet ska hantera en cirkulär buffert istället för enskilda element.  Hur ska jag göra så att det fungerar med en buffer på 10 element?

Här kommer mitt försök till att få det fungera!

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdlib.h>
#define SHMSIZE 128
#define SHM_R 0400
#define SHM_W 0200
#define	SIZE_OF_BUFFER 10 //buffer size 10

float float_rand( float min, float max ) //function to get random numbers between 0.1-2
{
    float scale = rand() / (float) RAND_MAX; /* [0, 1.0] */
    return min + scale * ( max - min );      /* [min, max] */
}

int main(int argc, char **argv)
{
	struct shm_struct //struct for buffer and info about it
	{
		int buffer[SIZE_OF_BUFFER];
		int readIndex;
		int writeIndex;
		int bufferLenght;
		unsigned empty;
	};
	volatile struct shm_struct *shmp = NULL;
	char *addr = NULL;
	pid_t pid = -1;
	int var1 = 0, var2 = 0, shmid = -1;
	struct shmid_ds *shm_buf;

	/* allocate a chunk of shared memory */
	shmid = shmget(IPC_PRIVATE, SHMSIZE, IPC_CREAT | SHM_R | SHM_W);
	shmp = (struct shm_struct *) shmat(shmid, addr, 0);
	shmp->empty = 0;
	shmp->readIndex = 0;
	shmp->writeIndex = 0;
	shmp->bufferLenght = 0;

	pid = fork();
	if (pid != 0) 
	{
		/* here's the parent, acting as producer */
		while (var1 < 100) 
		{
			/* write to shmem */
			var1++;
			while (shmp->empty == 1); /* busy wait until the buffer is empty */
			printf("Sending %d\n", var1); fflush(stdout);
			shmp->buffer[shmp->writeIndex] = var1;
			sleep(float_rand(0.1,2));
			shmp->writeIndex++;

			if(shmp->writeIndex == SIZE_OF_BUFFER)
			{
				shmp->writeIndex = 0;
			}

			shmp->bufferLenght++;
			shmp->empty = 1;
		}
		shmdt(addr);
		shmctl(shmid, IPC_RMID, shm_buf);
	} 
	else 
	{
		/* here's the child, acting as consumer */
		while (var2 < 100) 
		{
			/* read from shmem */
			while (shmp->empty == 0); /* busy wait until there is something */
			var2 = shmp->buffer[shmp->readIndex];
			sleep(float_rand(0.1,2));
			shmp->bufferLenght--;
			shmp->readIndex++;

			if (shmp->readIndex == SIZE_OF_BUFFER) 
			{
				shmp->readIndex = 0;
			}

			shmp->empty = 0;
			printf("Received %d\n", var2); fflush(stdout);
		}
		shmdt(addr);
		shmctl(shmid, IPC_RMID, shm_buf);
	}
}