malloc.c

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/* $Header: /opt/proj/minix/cvsroot/src/lib/ansi/malloc.c,v 1.1.1.1 2005/04/21 14:56:05 beng Exp $ */

/* replace undef by define */
#undef   DEBUG          /* check assertions */
#undef   SLOWDEBUG      /* some extra test loops (requires DEBUG) */

#ifndef DEBUG
#define NDEBUG
#endif

#include        <stdlib.h>
#include        <string.h>
#include        <errno.h>
#include        <assert.h>

#if _EM_WSIZE == _EM_PSIZE
#define ptrint          int
#else
#define ptrint          long
#endif

#if     _EM_PSIZE == 2
#define BRKSIZE         1024
#else
#define BRKSIZE         4096
#endif
#define PTRSIZE         ((int) sizeof(void *))
#define Align(x,a)      (((x) + (a - 1)) & ~(a - 1))
#define NextSlot(p)     (* (void **) ((p) - PTRSIZE))
#define NextFree(p)     (* (void **) (p))

/*
 * A short explanation of the data structure and algorithms.
 * An area returned by malloc() is called a slot. Each slot
 * contains the number of bytes requested, but preceeded by
 * an extra pointer to the next the slot in memory.
 * '_bottom' and '_top' point to the first/last slot.
 * More memory is asked for using brk() and appended to top.
 * The list of free slots is maintained to keep malloc() fast.
 * '_empty' points the the first free slot. Free slots are
 * linked together by a pointer at the start of the
 * user visable part, so just after the next-slot pointer.
 * Free slots are merged together by free().
 */

extern void *_sbrk(int);
extern int _brk(void *);
static void *_bottom, *_top, *_empty;

static int grow(size_t len)
{
  register char *p;

  assert(NextSlot((char *)_top) == 0);
  if ((char *) _top + len < (char *) _top
      || (p = (char *)Align((ptrint)_top + len, BRKSIZE)) < (char *) _top ) {
        errno = ENOMEM;
        return(0);
  }
  if (_brk(p) != 0)
        return(0);
  NextSlot((char *)_top) = p;
  NextSlot(p) = 0;
  free(_top);
  _top = p;
  return 1;
}

void *
malloc(size_t size)
{
  register char *prev, *p, *next, *new;
  register unsigned len, ntries;

  if (size == 0)
        return NULL;

  for (ntries = 0; ntries < 2; ntries++) {
        if ((len = Align(size, PTRSIZE) + PTRSIZE) < 2 * PTRSIZE) {
                errno = ENOMEM;
                return NULL;
        }
        if (_bottom == 0) {
                if ((p = _sbrk(2 * PTRSIZE)) == (char *) -1)
                        return NULL;
                p = (char *) Align((ptrint)p, PTRSIZE);
                p += PTRSIZE;
                _top = _bottom = p;
                NextSlot(p) = 0;
        }
#ifdef SLOWDEBUG
        for (p = _bottom; (next = NextSlot(p)) != 0; p = next)
                assert(next > p);
        assert(p == _top);
#endif
        for (prev = 0, p = _empty; p != 0; prev = p, p = NextFree(p)) {
                next = NextSlot(p);
                new = p + len;  /* easily overflows!! */
                if (new > next || new <= p)
                        continue;               /* too small */
                if (new + PTRSIZE < next) {     /* too big, so split */
                        /* + PTRSIZE avoids tiny slots on free list */
                        NextSlot(new) = next;
                        NextSlot(p) = new;
                        NextFree(new) = NextFree(p);
                        NextFree(p) = new;
                }
                if (prev)
                        NextFree(prev) = NextFree(p);
                else
                        _empty = NextFree(p);
                return p;
        }
        if (grow(len) == 0)
                break;
  }
  assert(ntries != 2);
  return NULL;
}

void *
realloc(void *oldp, size_t size)
{
  register char *prev, *p, *next, *new;
  char *old = oldp;
  register size_t len, n;

  if (old == 0)
        return malloc(size);
  if (size == 0) {
        free(old);
        return NULL;
  }
  len = Align(size, PTRSIZE) + PTRSIZE;
  next = NextSlot(old);
  n = (int)(next - old);                        /* old length */
  /*
   * extend old if there is any free space just behind it
   */
  for (prev = 0, p = _empty; p != 0; prev = p, p = NextFree(p)) {
        if (p > next)
                break;
        if (p == next) {        /* 'next' is a free slot: merge */
                NextSlot(old) = NextSlot(p);
                if (prev)
                        NextFree(prev) = NextFree(p);
                else
                        _empty = NextFree(p);
                next = NextSlot(old);
                break;
        }
  }
  new = old + len;
  /*
   * Can we use the old, possibly extended slot?
   */
  if (new <= next && new >= old) {              /* it does fit */
        if (new + PTRSIZE < next) {             /* too big, so split */
                /* + PTRSIZE avoids tiny slots on free list */
                NextSlot(new) = next;
                NextSlot(old) = new;
                free(new);
        }
        return old;
  }
  if ((new = malloc(size)) == NULL)             /* it didn't fit */
        return NULL;
  memcpy(new, old, n);                          /* n < size */
  free(old);
  return new;
}

void
free(void *ptr)
{
  register char *prev, *next;
  char *p = ptr;

  if (p == 0)
        return;

  assert(NextSlot(p) > p);
  for (prev = 0, next = _empty; next != 0; prev = next, next = NextFree(next))
        if (p < next)
                break;
  NextFree(p) = next;
  if (prev)
        NextFree(prev) = p;
  else
        _empty = p;
  if (next) {
        assert(NextSlot(p) <= next);
        if (NextSlot(p) == next) {              /* merge p and next */
                NextSlot(p) = NextSlot(next);
                NextFree(p) = NextFree(next);
        }
  }
  if (prev) {
        assert(NextSlot(prev) <= p);
        if (NextSlot(prev) == p) {              /* merge prev and p */
                NextSlot(prev) = NextSlot(p);
                NextFree(prev) = NextFree(p);
        }
  }
}

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