src/alloc.c


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/*
 * src/alloc.c
 *
 * Primary implementation of allocator logic for liballoc
 * CISC 301 -- Operating Systems, Project 3
 *
 * Copyright (C) 2025  Douglas B. Rumbaugh <dbrumbaugh@harrisburgu.edu>
 * 
 * Distributed under the Modified BSD License
 *
 */
#include "alloc.h"
#include "constants.h"
#include "free_list.h"

typedef struct header {
  size_t size;
  size_t magic_number;
} header;

static_assert(sizeof(header) % ALIGNMENT == 0, "Header improperly aligned");

static void *heap_start = 0;
static void *heap_end = 0;
free_nd *free_list = NULL;

static void initialize() {
  heap_start = sbrk(0);

  if (heap_start == (void *)-1) {
    perror("liballoc initialization:");
    exit(EXIT_FAILURE);
  }

  size_t padding = (size_t)heap_start % ALIGNMENT;
  if (padding) {
    heap_start = sbrk(padding);
    if (heap_start == (void *)-1) {
      perror("liballoc initialization:");
      exit(EXIT_FAILURE);
    }
  }

  assert((size_t)heap_start % ALIGNMENT == 0);
  heap_end = heap_start;
}

void *allocate(size_t size) {
  if (!heap_start) {
    initialize();
  }

  /*
   * pad the requested size to ensure alignment using
   * the alignment one-liner we discussed in class
   */
  size = (size + ALIGNMENT - 1) & ~(ALIGNMENT - 1);

  /* first check for a suitable memory block on the free list */
  free_nd *nd = fl_find_first_fit(free_list, size);
  if (nd) {
    void *return_region = fl_split_node(free_list, nd, size);
  }

  /*
   * if there aren't any blocks on the free list, we need to allocate
   * from scratch
   */
  void *new_region = sbrk(size + sizeof(header));

  /* out of memory */
  if (new_region == (void *)-1) {
    return NULL;
  }

  heap_end = new_region + size + sizeof(header);

  ((header *)new_region)->size = size;
  ((header *)new_region)->magic_number = MAGIC_NUMBER;

  return new_region + sizeof(header);
}

void release(void *ptr) {
  fl_add_node(free_list, ptr);
  fl_coalesce_nodes(free_list);
}