/* shlib.c
 *
 * "Shared Memory Library" which allocates a one dimensional array of
 * characters of appropriate size.
 *
 * Copyright (c) 2012  Robert Marmorstein (marmorsteinrm@longwood.edu) Released
 * under the terms of the GPL, version 2.  See LICENSE file for details.
 *
 * For questions about the licensing of this software, contact me by mail:
 * Ruffner 329, Longwood University, 201 High Street, Farmville, VA 23909
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
	
static int shm_id;

//Destroy the shared memory as soon as the last process quits (should be the server)
void clean_shm()
{
	if (shmctl(shm_id, IPC_RMID, NULL) < 0) {
		perror("shmctl");
		exit(-1);
	}
}

//Attach to the shared memory object.  If "server" is 1, creates the shared memory block and opens it for both reading and writing.
//Otherwise, opens it only for reading.
//Returns a character array of size width*height.
char* attach_map(int server, int width, int height)
{
	char* map;
	char* path;
	key_t key;

	//Get the home directory, since it's guaranteed to exist!
	path = getenv("HOME");

	//Create an IPC key from it.
	key = ftok(path, 1);

	//Associate that key with a shared memory block
	if (server == 1) {
		shm_id = shmget(key, width*height, IPC_CREAT | S_IRUSR | S_IWUSR);
		atexit(clean_shm);
	}
	else {
		shm_id = shmget(key, width*height, S_IRUSR);
	}
	if (shm_id < 0) {
		perror("shmget");
		exit(-1);
	}
	
	map = shmat(shm_id, NULL, 0);
	if (map == (void *)-1) {
		perror("shmat");
		exit(-1);
	}
	return map;
}