mmap-malloc.c

/*
 * MIT License

Copyright (c) 2023 Mateusz Dukat

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

*/

#define DEBUG

#ifdef DEBUG
#include <stdio.h> // fprintf
#endif

#include <sys/mman.h> // mmap
#include <string.h> // memcpy (for realloc)
#include <pthread.h> // pthread_mutex

/*
 * We need to keep in mind what size on which address we allocated.
 * This is useful for realloc and free, since munmap() needs specified size for deallocation
 *
 * Block structure stores:
 *  - next block in line (linked list)
 *  - previous block in line
 *  - pointer we allocated
 *  - size allocated on this address
 *
 * We use NULL for first and last entry in our linked list
 *
 * Everytime we want to do an operation on our linked list, we need to lock it, so at no time two different threads try to write (or read+write)
 */

struct Block {
  struct Block *next;
  struct Block *prev;
  void* ptr;
  size_t size;
};

// Head of Block linked list, initialized in malloc() on first run
struct Block *head = NULL;

// Mutex lock for Block linked list access, initialized in malloc() on first run
pthread_mutex_t lock;

/*
 * malloc()
 */

void *malloc(size_t size)
{
  // If specified size is 0, instantly return. Nothing to do.
  if(size == 0)
    return NULL;

  // If Block linked list is initialized, lock it
  if(head != NULL)
    pthread_mutex_lock(&lock);
  
  // Allocate new block
  void *return_pointer = NULL;
  return_pointer = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);

  // if Block linked list is not initialized, initialize it
  if(head == NULL){
    head = mmap(NULL, sizeof(struct Block), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    head->next = NULL;
    head->prev = NULL;
    head->ptr = NULL;
    head->size = 0;

    // also init mutex since it's first time our code executes
    pthread_mutex_init(&lock, NULL);
    // and lock it
    pthread_mutex_lock(&lock);
  }

  struct Block *cur = head;
  
  // step to last block
  while(cur->next != NULL){
    cur = cur->next;
  }

  // make new block
  struct Block *new = mmap(NULL, sizeof(struct Block), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
  cur->next = new;
  new->prev = cur;

  // switch to next block
  cur = new;
  cur->next = NULL;
  cur->size = size;
  cur->ptr = return_pointer;
  
  pthread_mutex_unlock(&lock);
  
#ifdef DEBUG
  fprintf(stderr, "malloc(%ld) = %p\n", size, return_pointer);
#endif

  // Convert mmap error to malloc error
  if(return_pointer == (void *) -1)
    return NULL;
  return return_pointer;
}

/*
 * free()
 */

void free(void* ptr)
{
#ifdef DEBUG
  fprintf(stderr, "free(%p)\n", ptr);
#endif

  // Instantly return if ptr is NULL, nothing to do
  if(ptr == NULL)
    return;
  
  pthread_mutex_lock(&lock);

  struct Block *cur = head;
  
  // Find block with pointer we want to free()
  while(cur->ptr != ptr){
    cur = cur->next;
  }

  //   Remove this block

  // Find previous block, forget about current one, make next one, the next one
  struct Block *prev;
  if(cur->prev != NULL){
    prev = cur->prev;
    prev->next = cur->next;
  }else{
    // If we want to free() head block, simply make head the next one
    head = cur->next;
  }

  // Find next block, forget about current one, make previous one, the previous one
  struct Block *next;
  if(cur->next != NULL){
    next = cur->next;
    next->prev = cur->prev;
  }else{
    // If it's the last block, make previous one last
    prev->next = NULL;
  }
  
  pthread_mutex_unlock(&lock);

  // deallocate *ptr address and block structure
  munmap(cur->ptr, cur->size);
  munmap(cur, sizeof(struct Block));
}

/*
 * calloc()
 *
 * Fancy malloc() with number of members
 */

void *calloc(size_t nmemb, size_t size){
  // Return NULL if any of the values are equal 0
  if(nmemb == 0 || size == 0)
    return NULL;

  // Call malloc() with real size we want to allocate
  size_t realsize = nmemb * size;
  void *return_pointer = malloc(realsize);

#ifdef DEBUG
  fprintf(stderr, "calloc(%ld, %ld) = %p\n", nmemb, size, return_pointer);
#endif

  return return_pointer;
}

/*
 * realloc()
 *
 * Allocate new memory space, copy data from old space, deallocate old address
 */

void *realloc(void *ptr, size_t size){
  void* newptr = malloc(size);

#ifdef DEBUG
  fprintf(stderr, "realloc(%p, %ld) = %p\n", ptr, size, newptr);
#endif

  if(ptr == NULL)
    return newptr;
  
  pthread_mutex_lock(&lock);

  //find old block
  struct Block *old = head;
  while(old->ptr != ptr)
    old = old->next;
  
  pthread_mutex_unlock(&lock);

  // copy last block to new block
  memcpy(newptr, ptr, old->size);

  // free old block
  free(ptr);

  return newptr;
}