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 <fcntl.h>

#include "memlib.h"

/* private variables */
static unsigned char *heap;
static unsigned char *mem_brk;
static unsigned char *mem_max_addr;

/*
 * mem_init - initialize the memory system model
 */
void mem_init(void) {
  heap = mmap((void *)0x800000000,    /* suggested start */
              MAX_HEAP,               /* length */
              PROT_WRITE,             /* permissions */
              MAP_PRIVATE | MAP_ANON, /* private or shared? */
              -1,                     /* fd */
              0);                     /* offset (dunno) */
  mem_max_addr = heap + MAX_HEAP;
  mem_brk = heap; /* heap is empty initially */
}

/*
 * mem_deinit - free the storage used by the memory system model
 */
void mem_deinit(void) {
  munmap(heap, MAX_HEAP);
}

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

/*
 * 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(long incr) {
  unsigned 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;
}

/*
 * mem_heap_lo - return address of the first heap byte
 */
void *mem_heap_lo() {
  return (void *)heap;
}

/*
 * mem_heap_hi - return address of last heap byte
 */
void *mem_heap_hi() {
  return (void *)(mem_brk - 1);
}

/*
 * mem_heapsize() - returns the heap size in bytes
 */
size_t mem_heapsize() {
  return (size_t)((void *)mem_brk - (void *)heap);
}

/*
 * mem_pagesize() - returns the page size of the system
 */
size_t mem_pagesize() {
  return (size_t)getpagesize();
}