memlib.c

/*
 * memlib.c - a module that simulates the memory system.  Needed because it 
 *            allows us to interleave calls from the student's malloc package 
 *            with the system's malloc package in libc.
 */
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>

#include "memlib.h"
#include "config.h"

/* private variables */
static char *mem_start_brk;  /* points to first byte of heap */
static char *mem_brk;        /* points to last byte of heap */
static char *mem_max_addr;   /* largest legal heap address */ 

/* 
 * mem_init - initialize the memory system model
 */
void mem_init(void)
{
    /* allocate the storage we will use to model the available VM */
    if ((mem_start_brk = (char *)malloc(MAX_HEAP)) == NULL) {
	fprintf(stderr, "mem_init_vm: malloc error\n");
	exit(1);
    }

    mem_max_addr = mem_start_brk + MAX_HEAP;  /* max legal heap address */
    mem_brk = mem_start_brk;                  /* heap is empty initially */
}

/* 
 * mem_deinit - free the storage used by the memory system model
 */
void mem_deinit(void)
{
    free(mem_start_brk);
}

/*
 * mem_reset_brk - reset the simulated brk pointer to make an empty heap
 */
void mem_reset_brk()
{
    mem_brk = mem_start_brk;
}

/* 
 * mem_sbrk - simple model of the sbrk function. Extends the heap 
 *    by incr bytes and returns the start address of the new area. In
 *    this model, the heap cannot be shrunk.
 */
void *mem_sbrk(int incr) 
{
    char *old_brk = mem_brk;

    if ( (incr < 0) || ((mem_brk + incr) > mem_max_addr)) {
	errno = ENOMEM;
	fprintf(stderr, "ERROR: mem_sbrk failed. Ran out of memory...\n");
	return (void *)-1;
    }
    mem_brk += incr;
    return (void *)old_brk;
}