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View of /sml/trunk/src/runtime/memory/malloc.c

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Revision 651 - (download) (as text) (annotate)
Thu Jun 1 18:34:03 2000 UTC (19 years, 3 months ago) by monnier
File size: 34778 byte(s)
bring revisions from the vendor branch to the trunk
/* 
  A version of malloc/free/realloc written by Doug Lea and released to the 
  public domain. 

  VERSION 2.5

* History:
    Based loosely on libg++-1.2X malloc. (It retains some of the overall 
       structure of old version,  but most details differ.)
    trial version Fri Aug 28 13:14:29 1992  Doug Lea  (dl at g.oswego.edu)
    fixups Sat Aug  7 07:41:59 1993  Doug Lea  (dl at g.oswego.edu)
      * removed potential for odd address access in prev_chunk
      * removed dependency on getpagesize.h
      * misc cosmetics and a bit more internal documentation
      * anticosmetics: mangled names in macros to evade debugger strangeness
      * tested on sparc, hp-700, dec-mips, rs6000 
          with gcc & native cc (hp, dec only) allowing
          Detlefs & Zorn comparison study (to appear, SIGPLAN Notices.)

* Overview

  This malloc, like any other, is a compromised design. 

  Chunks of memory are maintained using a `boundary tag' method as
  described in e.g., Knuth or Standish.  The size of the chunk is
  stored both in the front of the chunk and at the end.  This makes
  consolidating fragmented chunks into bigger chunks very fast.  The
  size field also hold a bit representing whether a chunk is free or
  in use.

  Malloced chunks have space overhead of 8 bytes: The preceding and
  trailing size fields.  When a chunk is freed, 8 additional bytes are
  needed for free list pointers. Thus, the minimum allocatable size is
  16 bytes.  8 byte alignment is currently hardwired into the design.
  This seems to suffice for all current machines and C compilers.
  Calling memalign will return a chunk that is both 8-byte aligned
  and meets the requested (power of two) alignment.

  It is assumed that 32 bits suffice to represent chunk sizes.  The
  maximum size chunk is 2^31 - 8 bytes.  malloc(0) returns a pointer
  to something of the minimum allocatable size.  Requests for negative
  sizes (when size_t is signed) or with the highest bit set (when
  unsigned) will also return a minimum-sized chunk.

  Available chunks are kept in doubly linked lists. The lists are
  maintained in an array of bins using a power-of-two method, except
  that instead of 32 bins (one for each 1 << i), there are 128: each
  power of two is split in quarters.  Chunk sizes up to 128 are
  treated specially; they are categorized on 8-byte boundaries.  This
  both better distributes them and allows for special faster
  processing.

  The use of very fine bin sizes closely approximates the use of one
  bin per actually used size, without necessitating the overhead of
  locating such bins. It is especially desirable in common
  applications where large numbers of identically-sized blocks are
  malloced/freed in some dynamic manner, and then later are all freed.
  The finer bin sizes make finding blocks fast, with little wasted
  overallocation. The consolidation methods ensure that once the
  collection of blocks is no longer useful, fragments are gathered
  into bigger chunks awaiting new roles.

  The bins av[i] serve as heads of the lists. Bins contain a dummy
  header for the chunk lists. Each bin has two lists. The `dirty' list
  holds chunks that have been returned (freed) and not yet either
  re-malloc'ed or consolidated. (A third free-standing list contains
  returned chunks that have not yet been processed at all.) The `clean'
  list holds split-off fragments and consolidated space. All
  procedures maintain the invariant that no clean chunk physically
  borders another clean chunk. Thus, clean chunks never need to be
  scanned during consolidation.

* Algorithms

  Malloc:

    This is a very heavily disguised first-fit algorithm.
    Most of the heuristics are designed to maximize the likelihood
    that a usable chunk will most often be found very quickly,
    while still minimizing fragmentation and overhead.

    The allocation strategy has several phases:

      0. Convert the request size into a usable form. This currently
         means to add 8 bytes overhead plus possibly more to obtain
         8-byte alignment. Call this size `nb'.

      1. Check if the last returned (free()'d) or preallocated chunk
         is of the exact size nb. If so, use it.  `Exact' means no
         more than MINSIZE (currently 16) bytes larger than nb. This
         cannot be reduced, since a chunk with size < MINSIZE cannot
         be created to hold the remainder.

         This check need not fire very often to be effective.  It
         reduces overhead for sequences of requests for the same
         preallocated size to a dead minimum.

      2. Look for a dirty chunk of exact size in the bin associated
         with nb.  `Dirty' chunks are those that have never been
         consolidated.  Besides the fact that they, but not clean
         chunks require scanning for consolidation, these chunks are
         of sizes likely to be useful because they have been
         previously requested and then freed by the user program.

         Dirty chunks of bad sizes (even if too big) are never used
         without consolidation. Among other things, this maintains the
         invariant that split chunks (see below) are ALWAYS clean.

         2a If there are dirty chunks, but none of the right size,
             consolidate them all, as well as any returned chunks
             (i.e., the ones from step 3). This is all a heuristic for
             detecting and dealing with excess fragmentation and
             random traversals through memory that degrade
             performance especially when the user program is running
             out of physical memory.

      3. Pull other requests off the returned chunk list, using one if
         it is of exact size, else distributing into the appropriate
         bins.

      4. Try to use the last chunk remaindered during a previous
         malloc. (The ptr to this chunk is kept in var last_remainder,
         to make it easy to find and to avoid useless re-binning
         during repeated splits.  The code surrounding it is fairly
         delicate. This chunk must be pulled out and placed in a bin
         prior to any consolidation, to avoid having other pointers
         point into the middle of it, or try to unlink it.) If
         it is usable, proceed to step 9.

      5. Scan through clean chunks in the bin, choosing any of size >=
         nb. Split later (step 9) if necessary below.  (Unlike in step
         2, it is good to split here, because it creates a chunk of a
         known-to-be-useful size out of a fragment that happened to be
         close in size.)

      6. Scan through the clean lists of all larger bins, selecting
         any chunk at all. (It will surely be big enough since it is
         in a bigger bin.)  The scan goes upward from small bins to
         large.  It would be faster downward, but could lead to excess
         fragmentation. If successful, proceed to step 9.

      7. Consolidate chunks in other dirty bins until a large enough
         chunk is created. Break out to step 9 when one is found.

         Bins are selected for consolidation in a circular fashion
         spanning across malloc calls. This very crudely approximates
         LRU scanning -- it is an effective enough approximation for
         these purposes.

      8. Get space from the system using sbrk.

         Memory is gathered from the system (via sbrk) in a way that
         allows chunks obtained across different sbrk calls to be
         consolidated, but does not require contiguous memory. Thus,
         it should be safe to intersperse mallocs with other sbrk
         calls.

      9. If the selected chunk is too big, then:

         9a If this is the second split request for nb bytes in a row,
             use this chunk to preallocate up to MAX_PREALLOCS
             additional chunks of size nb and place them on the
             returned chunk list.  (Placing them here rather than in
             bins speeds up the most common case where the user
             program requests an uninterrupted series of identically
             sized chunks. If this is not true, the chunks will be
             binned in step 3 next time.)

         9b Split off the remainder and place in last_remainder.
             Because of all the above, the remainder is always a
             `clean' chunk.

     10.  Return the chunk.


  Free: 
    Deallocation (free) consists only of placing the chunk on a list
    of returned chunks. free(0) has no effect.  Because freed chunks
    may be overwritten with link fields, this malloc will often die
    when freed memory is overwritten by user programs.  This can be
    very effective (albeit in an annoying way) in helping users track
    down dangling pointers.

  Realloc:
    Reallocation proceeds in the usual way. If a chunk can be extended,
    it is, else a malloc-copy-free sequence is taken. 

  Memalign, valloc:
    memalign arequests more than enough space from malloc, finds a spot
    within that chunk that meets the alignment request, and then
    possibly frees the leading and trailing space. Overreliance on
    memalign is a sure way to fragment space.

* Other implementation notes

  This malloc is NOT designed to work in multiprocessing applications.
  No semaphores or other concurrency control are provided to ensure
  that multiple malloc or free calls don't run at the same time, which
  could be disasterous. A single semaphore could be used across malloc,
  realloc, and free. It would be hard to obtain finer granularity.

  The implementation is in straight, hand-tuned ANSI C.  Among other
  consequences, it uses a lot of macros. These would be nicer as
  inlinable procedures, but using macros allows use with non-inlining
  compilers, and also makes it a bit easier to control when they
  should be expanded out by selectively embedding them in other macros
  and procedures. (According to profile information, it is almost, but
  not quite always best to expand.)

*/



/* TUNABLE PARAMETERS */

/* 
  SBRK_UNIT is a good power of two to call sbrk with It should
  normally be system page size or a multiple thereof.  If sbrk is very
  slow on a system, it pays to increase this.  Otherwise, it should
  not matter too much.
*/

#define SBRK_UNIT 8192

/* 
  MAX_PREALLOCS is the maximum number of chunks to preallocate.  The
  actual number to prealloc depends on the available space in a
  selected victim, so typical numbers will be less than the maximum.
  Because of this, the exact value seems not to matter too much, at
  least within values from around 1 to 100.  Since preallocation is
  heuristic, making it too huge doesn't help though. It may blindly
  create a lot of chunks when it turns out not to need any more, and
  then consolidate them all back again immediatetly. While this is
  pretty fast, it is better to avoid it.
*/

#define MAX_PREALLOCS 5




/* preliminaries */

#include <stddef.h>   /* for size_t */
#include <stdio.h>    /* needed for malloc_stats */

#ifdef __cplusplus
extern "C" {
#endif

extern void*     sbrk(size_t);

/* mechanics for getpagesize; adapted from bsd/gnu getpagesize.h */

#if defined(BSD) || defined(DGUX) || defined(sun) || defined(HAVE_GETPAGESIZE)
   extern size_t getpagesize();
#  define malloc_getpagesize getpagesize()
#else
#  include <sys/param.h>
#  ifdef EXEC_PAGESIZE
#    define malloc_getpagesize EXEC_PAGESIZE
#  else
#    ifdef NBPG
#      ifndef CLSIZE
#        define malloc_getpagesize NBPG
#      else
#        define malloc_getpagesize (NBPG * CLSIZE)
#      endif
#    else 
#      ifdef NBPC
#        define malloc_getpagesize NBPC
#      else
#        define malloc_getpagesize SBRK_UNIT    /* just guess */
#      endif
#    endif 
#  endif
#endif 

#ifdef __cplusplus
};  /* end of extern "C" */
#endif



/*  CHUNKS */


struct malloc_chunk
{
  size_t size;               /* Size in bytes, including overhead. */
                             /* Or'ed with INUSE if in use. */

  struct malloc_chunk* fd;   /* double links -- used only if free. */
  struct malloc_chunk* bk;

};

typedef struct malloc_chunk* mchunkptr;

/*  sizes, alignments */

#define SIZE_SZ              (sizeof(size_t))
#define MALLOC_MIN_OVERHEAD  (SIZE_SZ + SIZE_SZ)
#define MALLOC_ALIGN_MASK    (MALLOC_MIN_OVERHEAD - 1)
#define MINSIZE              (sizeof(struct malloc_chunk) + SIZE_SZ)


/* pad request bytes into a usable size */

#define request2size(req) \
  (((long)(req) <= 0) ?  MINSIZE : \
    (((req) + MALLOC_MIN_OVERHEAD + MALLOC_ALIGN_MASK) \
      & ~(MALLOC_ALIGN_MASK)))


/* Check if m has acceptable alignment */

#define aligned_OK(m)    (((size_t)((m)) & (MALLOC_ALIGN_MASK)) == 0)


/* Check if a chunk is immediately usable */

#define exact_fit(ptr, req) ((unsigned)((ptr)->size - (req)) < MINSIZE)

/* maintaining INUSE via size field */

#define INUSE  0x1     /* size field is or'd with INUSE when in use */
                       /* INUSE must be exactly 1, so can coexist with size */

#define inuse(p)       ((p)->size & INUSE)
#define set_inuse(p)   ((p)->size |= INUSE)
#define clear_inuse(p) ((p)->size &= ~INUSE)



/* Physical chunk operations  */

/* Ptr to next physical malloc_chunk. */

#define next_chunk(p)\
  ((mchunkptr)( ((char*)(p)) + ((p)->size & ~INUSE) ))

/* Ptr to previous physical malloc_chunk */

#define prev_chunk(p)\
  ((mchunkptr)( ((char*)(p)) - ( *((size_t*)((char*)(p) - SIZE_SZ)) & ~INUSE)))

/* place size at front and back of chunk */

#define set_size(P, Sz)														  \
{ 																			  \
  size_t Sss = (Sz);														  \
  (P)->size = *((size_t*)((char*)(P) + Sss - SIZE_SZ)) = Sss;				  \
}																			  \


/* conversion from malloc headers to user pointers, and back */

#define chunk2mem(p)   ((void*)((char*)(p) + SIZE_SZ))
#define mem2chunk(mem) ((mchunkptr)((char*)(mem) - SIZE_SZ))




/* BINS */

struct malloc_bin
{
  struct malloc_chunk dhd;   /* dirty list header */
  struct malloc_chunk chd;   /* clean list header */
};

typedef struct malloc_bin* mbinptr;


/* field-extraction macros */

#define clean_head(b)  (&((b)->chd))
#define first_clean(b) ((b)->chd.fd)
#define last_clean(b)  ((b)->chd.bk)

#define dirty_head(b)  (&((b)->dhd))
#define first_dirty(b) ((b)->dhd.fd)
#define last_dirty(b)  ((b)->dhd.bk)




/* The bins, initialized to have null double linked lists */

#define NBINS     128             /* for 32 bit addresses */
#define LASTBIN   (&(av[NBINS-1]))
#define FIRSTBIN  (&(av[2]))      /* 1st 2 bins unused but simplify indexing */

/* Bins < MAX_SMALLBIN_OFFSET are special-cased since they are 8 bytes apart */

#define MAX_SMALLBIN_OFFSET  18
#define MAX_SMALLBIN_SIZE   144  /* Max size for which small bin rules apply */

/* Helper macro to initialize bins */
#define IAV(i)\
  {{ 0, &(av[i].dhd),  &(av[i].dhd) }, { 0, &(av[i].chd),  &(av[i].chd) }}

static struct malloc_bin  av[NBINS] = 
{
  IAV(0),   IAV(1),   IAV(2),   IAV(3),   IAV(4), 
  IAV(5),   IAV(6),   IAV(7),   IAV(8),   IAV(9),
  IAV(10),  IAV(11),  IAV(12),  IAV(13),  IAV(14), 
  IAV(15),  IAV(16),  IAV(17),  IAV(18),  IAV(19),
  IAV(20),  IAV(21),  IAV(22),  IAV(23),  IAV(24), 
  IAV(25),  IAV(26),  IAV(27),  IAV(28),  IAV(29),
  IAV(30),  IAV(31),  IAV(32),  IAV(33),  IAV(34), 
  IAV(35),  IAV(36),  IAV(37),  IAV(38),  IAV(39),
  IAV(40),  IAV(41),  IAV(42),  IAV(43),  IAV(44), 
  IAV(45),  IAV(46),  IAV(47),  IAV(48),  IAV(49),
  IAV(50),  IAV(51),  IAV(52),  IAV(53),  IAV(54), 
  IAV(55),  IAV(56),  IAV(57),  IAV(58),  IAV(59),
  IAV(60),  IAV(61),  IAV(62),  IAV(63),  IAV(64), 
  IAV(65),  IAV(66),  IAV(67),  IAV(68),  IAV(69),
  IAV(70),  IAV(71),  IAV(72),  IAV(73),  IAV(74), 
  IAV(75),  IAV(76),  IAV(77),  IAV(78),  IAV(79),
  IAV(80),  IAV(81),  IAV(82),  IAV(83),  IAV(84), 
  IAV(85),  IAV(86),  IAV(87),  IAV(88),  IAV(89),
  IAV(90),  IAV(91),  IAV(92),  IAV(93),  IAV(94), 
  IAV(95),  IAV(96),  IAV(97),  IAV(98),  IAV(99),
  IAV(100), IAV(101), IAV(102), IAV(103), IAV(104), 
  IAV(105), IAV(106), IAV(107), IAV(108), IAV(109),
  IAV(110), IAV(111), IAV(112), IAV(113), IAV(114), 
  IAV(115), IAV(116), IAV(117), IAV(118), IAV(119),
  IAV(120), IAV(121), IAV(122), IAV(123), IAV(124), 
  IAV(125), IAV(126), IAV(127)
};



/* 
  Auxiliary lists 

  Even though they use bk/fd ptrs, neither of these are doubly linked!
  They are null-terminated since only the first is ever accessed.
  returned_list is just singly linked for speed in free().
  last_remainder currently has length of at most one.

*/

static mchunkptr returned_list = 0;  /* List of (unbinned) returned chunks */
static mchunkptr last_remainder = 0; /* last remaindered chunk from malloc */



/* 
  Indexing into bins 
  
  Funny-looking mechanics: 
    For small bins, the index is just size/8.
    For others, first find index corresponding to the power of 2
        closest to size, using a variant of standard base-2 log
        calculation that starts with the first non-small index and
        adjusts the size so that zero corresponds with it. On each
        iteration, the index is incremented across the four quadrants
        per power of two. (This loop runs a max of 27 iterations;
        usually much less.) After the loop, the remainder is quartered
        to find quadrant. The offsets for loop termination and
        quartering allow bins to start, not end at powers.
*/


#define findbin(Sizefb, Bfb)												  \
{																			  \
  size_t Sfb = (Sizefb);													  \
  if (Sfb < MAX_SMALLBIN_SIZE)												  \
    (Bfb) = (av + (Sfb >> 3));												  \
  else																		  \
  {																			  \
    /* Offset wrt small bins */												  \
    size_t Ifb = MAX_SMALLBIN_OFFSET;										  \
    Sfb >>= 3;																  \
	/* find power of 2 */													  \
    while (Sfb >= (MINSIZE * 2)) { Ifb += 4; Sfb >>= 1; }					  \
	/* adjust for quadrant */												  \
    Ifb += (Sfb - MINSIZE) >> 2;                            				  \
    (Bfb) = av + Ifb;														  \
  }																			  \
}																			  \




/* Keep track of the maximum actually used clean bin, to make loops faster */
/* (Not worth it to do the same for dirty ones) */

static mbinptr maxClean = FIRSTBIN;

#define reset_maxClean														  \
{																			  \
  while (maxClean > FIRSTBIN && clean_head(maxClean)==last_clean(maxClean))	  \
    --maxClean;																  \
}																			  \


/* Macros for linking and unlinking chunks */

/* take a chunk off a list */

#define unlink(Qul)															  \
{																			  \
  mchunkptr Bul = (Qul)->bk;												  \
  mchunkptr Ful = (Qul)->fd;												  \
  Ful->bk = Bul;  Bul->fd = Ful;											  \
}																			  \


/* place a chunk on the dirty list of its bin */

#define dirtylink(Qdl)														  \
{																			  \
  mchunkptr Pdl = (Qdl);													  \
  mbinptr   Bndl; 															  \
  mchunkptr Hdl, Fdl;														  \
																			  \
  findbin(Pdl->size, Bndl);													  \
  Hdl  = dirty_head(Bndl); 													  \
  Fdl  = Hdl->fd; 															  \
																			  \
  Pdl->bk = Hdl;  Pdl->fd = Fdl;  Fdl->bk = Hdl->fd = Pdl;					  \
}																			  \



/* Place a consolidated chunk on a clean list */

#define cleanlink(Qcl)														  \
{																			  \
  mchunkptr Pcl = (Qcl);													  \
  mbinptr Bcl; 																  \
  mchunkptr Hcl, Fcl;														  \
																			  \
  findbin(Qcl->size, Bcl);													  \
  Hcl  = clean_head(Bcl); 													  \
  Fcl  = Hcl->fd; 															  \
  if (Hcl == Fcl && Bcl > maxClean) maxClean = Bcl;							  \
																			  \
  Pcl->bk = Hcl;  Pcl->fd = Fcl;  Fcl->bk = Hcl->fd = Pcl;					  \
}																			  \



/* consolidate one chunk */

#define consolidate(Qc)														  \
{																			  \
  for (;;)																	  \
  {																			  \
    mchunkptr Pc = prev_chunk(Qc);											  \
    if (!inuse(Pc))															  \
    {																		  \
      unlink(Pc);															  \
      set_size(Pc, Pc->size + (Qc)->size);									  \
      (Qc) = Pc;															  \
    }																		  \
    else break;																  \
  }																			  \
  for (;;)																	  \
  {																			  \
    mchunkptr Nc = next_chunk(Qc);											  \
    if (!inuse(Nc))															  \
    {																		  \
      unlink(Nc);															  \
      set_size((Qc), (Qc)->size + Nc->size);								  \
    }																		  \
    else break;																  \
  }																			  \
}																			  \



/* Place the held remainder in its bin */
/* This MUST be invoked prior to ANY consolidation */

#define clear_last_remainder												  \
{																			  \
  if (last_remainder != 0)													  \
  {																			  \
    cleanlink(last_remainder);												  \
    last_remainder = 0;														  \
  }																			  \
}																			  \


/* Place a freed chunk on the returned_list */

#define return_chunk(Prc)													  \
{																			  \
  (Prc)->fd = returned_list;												  \
  returned_list = (Prc); 													  \
}																			  \



/* Misc utilities */

/* A helper for realloc */

static void free_returned_list()
{
  clear_last_remainder;
  while (returned_list != 0)
  {
    mchunkptr p = returned_list;
    returned_list = p->fd;
    clear_inuse(p);
    dirtylink(p);
  }
}

/* Utilities needed below for memalign */
/* Standard greatest common divisor algorithm */

static size_t gcd(size_t a, size_t b)
{
  size_t tmp;
  
  if (b > a)
  {
    tmp = a; a = b; b = tmp;
  }
  for(;;)
  {
    if (b == 0)
      return a;
    else if (b == 1)
      return b;
    else
    {
      tmp = b;
      b = a % b;
      a = tmp;
    }
  }
}

static size_t  lcm(size_t x, size_t y)
{
  return x / gcd(x, y) * y;
}





/* Dealing with sbrk */
/* This is one step of malloc; broken out for simplicity */

static size_t sbrked_mem = 0; /* Keep track of total mem for malloc_stats */

static mchunkptr malloc_from_sys(size_t nb)
{

  /* The end of memory returned from previous sbrk call */
  static size_t* last_sbrk_end = 0; 

  mchunkptr p;            /* Will hold a usable chunk */
  size_t*   ip;           /* to traverse sbrk ptr in size_t units */
  
  /* Find a good size to ask sbrk for.  */
  /* Minimally, we need to pad with enough space */
  /* to place dummy size/use fields to ends if needed */

  size_t sbrk_size = ((nb + SBRK_UNIT - 1 + SIZE_SZ + SIZE_SZ) 
                       / SBRK_UNIT) * SBRK_UNIT;

  ip = (size_t*)(sbrk(sbrk_size));
  if ((char*)ip == (char*)(-1)) /* sbrk returns -1 on failure */
    return 0;

  sbrked_mem += sbrk_size;

  if (last_sbrk_end != &ip[-1]) /* Is this chunk continguous with last? */
  {                             
    /* It's either first time through or someone else called sbrk. */
    /* Arrange end-markers at front & back */

    /* Shouldn't be necessary, but better to be safe */
    while (!aligned_OK(ip)) { ++ip; sbrk_size -= SIZE_SZ; }

    /* Mark the front as in use to prevent merging. (End done below.) */
    /* Note we can get away with only 1 word, not MINSIZE overhead here */

    *ip++ = SIZE_SZ | INUSE;
    
    p = (mchunkptr)ip;
    set_size(p,sbrk_size - (SIZE_SZ + SIZE_SZ)); 
    
  }
  else 
  {
    mchunkptr l;  

    /* We can safely make the header start at end of prev sbrked chunk. */
    /* We will still have space left at the end from a previous call */
    /* to place the end marker, below */

    p = (mchunkptr)(last_sbrk_end);
    set_size(p, sbrk_size);

    /* Even better, maybe we can merge with last fragment: */

    l = prev_chunk(p);
    if (!inuse(l))  
    {
      unlink(l);
      set_size(l, p->size + l->size);
      p = l;
    }

  }

  /* mark the end of sbrked space as in use to prevent merging */

  last_sbrk_end = (size_t*)((char*)p + p->size);
  *last_sbrk_end = SIZE_SZ | INUSE;

  return p;
}




/* Consolidate dirty chunks until create one big enough for current req. */
/* Call malloc_from_sys if can't create one. */
/* This is just one phase of malloc, but broken out for sanity */

static mchunkptr malloc_find_space(size_t nb)
{
  /* Circularly traverse bins so as not to pick on any one too much */
  static mbinptr rover = LASTBIN;    /* Circular roving ptr */

  mbinptr origin = rover;
  mbinptr b      = rover;

  /* Preliminaries.  */
  clear_last_remainder;
  reset_maxClean;

  do
  {
    mchunkptr p;

    while ( (p = last_dirty(b)) != dirty_head(b))
    {
      unlink(p);
      consolidate(p);

      if (p->size >= nb)
      {
        rover = b;
        return p;
      }
      else
        cleanlink(p);
    }

    b = (b == FIRSTBIN)? LASTBIN : b - 1;      /* circularly sweep down */

  } while (b != origin);

  /* If no return above, chain to the next step of malloc */
  return  malloc_from_sys(nb);
}


/* Clear out dirty chunks from a bin, along with the free list. */
/* Invoked from malloc when things look too fragmented */

static void malloc_clean_bin(mbinptr bin)
{
  mchunkptr p;

  clear_last_remainder;
  
  while ( (p = last_dirty(bin)) != dirty_head(bin))
  {
    unlink(p);
    consolidate(p);
    cleanlink(p);
  }

  while (returned_list != 0)
  {
    p = returned_list;
    returned_list = p->fd;
    clear_inuse(p);
    consolidate(p);
    cleanlink(p);
  }
}




/*   Finally, the user-level functions  */


void* malloc(size_t bytes)
{
  static size_t previous_request = 0;  /* To control preallocation */

  size_t    nb = request2size(bytes);  /* padded request size */
  mbinptr   bin;                       /* corresponding bin */
  mchunkptr victim;                    /* will hold selected chunk */

  /* ----------- Peek at returned_list; hope for luck */

  if ((victim = returned_list) != 0 && 
      exact_fit(victim, nb)) /* size check works even though INUSE set */
  {
    returned_list = victim->fd;
    return chunk2mem(victim);
  }
  
  findbin(nb, bin);  /*  Need to know bin for other traversals */

  /* ---------- Scan dirty list of own bin */

     /* Code for small bins special-cased out since */
     /* no size check or traversal needed and */
     /* clean bins are exact matches so might as well test now */

  if (nb < MAX_SMALLBIN_SIZE)
  {
    if ( ((victim = first_dirty(bin)) != dirty_head(bin)) ||
         ((victim = last_clean(bin))  != clean_head(bin)))
    {
      unlink(victim);
      set_inuse(victim);
      return chunk2mem(victim);
    }
  }
  else
  {
    if ( (victim = first_dirty(bin)) != dirty_head(bin))
    {
      do
      {
        if (exact_fit(victim, nb))
        {
          unlink(victim);
          set_inuse(victim);
          return chunk2mem(victim);
        }
        else victim = victim->fd;
      } while (victim != dirty_head(bin));
      
      /* If there were chunks in there but none matched, then */
      /* consolidate all chunks in this bin plus those on free list */
      /* to prevent further traversals and fragmentation. */
      
      malloc_clean_bin(bin);
    }
  }
    
  /* ------------ Search free list */

  if ( (victim = returned_list) != 0)
  {
    do
    {
      mchunkptr next = victim->fd;
      if (exact_fit(victim, nb))
      {
        returned_list = next;
        return chunk2mem(victim);
      }
      else    /* Place unusable chunks in their bins  */
      {
        clear_inuse(victim);
        dirtylink(victim);
        victim = next;
      }
    } while (victim != 0);
    returned_list = 0;
  }

  /* -------------- Try the remainder from last successful split */

  if ( (victim = last_remainder) != 0 && victim->size >= nb)
  {
    last_remainder = 0; /* reset for next time */
    goto split;
  }

  /* -------------- Scan through own clean bin */

      /* (Traversing back-to-front tends to choose `old' */
      /*  chunks that could not be further consolidated.) */

  for (victim = last_clean(bin); victim != clean_head(bin); victim=victim->bk)
  {
    if (victim->size >= nb)
    {
      unlink(victim); 
      goto split;
    }
  }

  /* -------------- Try all bigger clean bins */

      /* (Scanning upwards is slower but prevents fragmenting big */
      /* chunks that we might need later. If there aren't any smaller */
      /* ones, most likely we got a big one from last_remainder anyway.) */

  {
    mbinptr b;

    for (b = bin + 1; b <= maxClean; ++b)
    {
      if ( (victim = last_clean(b)) != clean_head(b) ) 
      {
        unlink(victim);
        goto split;
      }
    }
  }

  /* -------------  Consolidate or sbrk */

  victim =  malloc_find_space(nb);

  if (victim == 0)  /* propagate failure */
    return 0; 

  /* -------------- Possibly split victim chunk */

 split:  
  {
    size_t room = victim->size - nb;
    if (room >= MINSIZE)     
    {
      mchunkptr v = victim;  /* Hold so can break up in prealloc */
      
      set_size(victim, nb);  /* Adjust size of chunk to be returned */
      
      /* ---------- Preallocate */
      
          /* Try to preallocate some more of this size if */
          /* last (split) req was of same size */
      
      if (previous_request == nb)
      {
        int i;
        
        for (i = 0; i < MAX_PREALLOCS && room >= nb + MINSIZE; ++i)
        {
          room -= nb;
           
          v = (mchunkptr)((char*)(v) + nb); 
          set_size(v, nb);
          set_inuse(v);     /* free-list chunks must have inuse set */
          return_chunk(v);  /* add to free list */
        } 
      }

      previous_request = nb;  /* record for next time */

      /* ---------- Create remainder chunk  */
      
      /* Get rid of the old one first */
      if (last_remainder != 0) cleanlink(last_remainder);
      
      last_remainder = (mchunkptr)((char*)(v) + nb);
      set_size(last_remainder, room);
    }

    set_inuse(victim);
    return chunk2mem(victim);
  }
}




void free(void* mem)
{
  if (mem != 0)
  {
    mchunkptr p = mem2chunk(mem);
    return_chunk(p);
  }
}

 


void* realloc(void* mem, size_t bytes)
{
  if (mem == 0) 
    return malloc(bytes);
  else
  {
    size_t       nb      = request2size(bytes);
    mchunkptr    p       = mem2chunk(mem);
    size_t       oldsize;
    long         room;
    mchunkptr    nxt;

    if (p == returned_list) /* support realloc-last-freed-chunk idiocy */
       returned_list = returned_list->fd;

    clear_inuse(p);
    oldsize = p->size;

    /* try to expand (even if already big enough), to clean up chunk */

    free_returned_list(); /* make freed chunks available to consolidate */

    while (!inuse(nxt = next_chunk(p))) /* Expand the chunk forward */
    {
      unlink(nxt);
      set_size(p, p->size + nxt->size);
    }

    room = p->size - nb;
    if (room >= 0)          /* Successful expansion */
    {
      if (room >= MINSIZE)  /* give some back if possible */
      {
        mchunkptr remainder = (mchunkptr)((char*)(p) + nb);
        set_size(remainder, room);
        cleanlink(remainder);
        set_size(p, nb);
      }
      set_inuse(p);
      return chunk2mem(p);
    }
    else /* Could not expand. Get another chunk and copy. */
    {
      void* newmem;
      size_t count;
      size_t* src;
      size_t* dst;

      set_inuse(p);    /* don't let malloc consolidate us yet! */
      newmem = malloc(nb);

      /* Copy -- we know that alignment is at least `size_t' */

      src = (size_t*) mem;
      dst = (size_t*) newmem;
      count = (oldsize - SIZE_SZ) / sizeof(size_t);
      while (count-- > 0) *dst++ = *src++;

      free(mem);
      return newmem;
    }
  }
}



/* Return a pointer to space with at least the alignment requested */
/* Alignment argument should be a power of two */

void* memalign(size_t alignment, size_t bytes)
{
  mchunkptr p;
  size_t    nb = request2size(bytes);
  size_t    room;

  /* find an alignment that both we and the user can live with: */
  /* least common multiple guarantees mutual happiness */
  size_t    align = lcm(alignment, MALLOC_MIN_OVERHEAD);

  /* call malloc with worst case padding to hit alignment; */
  /* we will give back extra */

  size_t req = nb + align + MINSIZE;
  void*  m = malloc(req);

  if (m == 0) return 0; /* propagate failure */

  p = mem2chunk(m);
  clear_inuse(p);


  if (((size_t)(m) % align) != 0) /* misaligned */
  {

    /* find an aligned spot inside chunk */

    mchunkptr ap = (mchunkptr)((((size_t)(m) + align-1) & -align) - SIZE_SZ);

    size_t gap = (size_t)(ap) - (size_t)(p);

    /* we need to give back leading space in a chunk of at least MINSIZE */

    if (gap < MINSIZE)
    {
      /* This works since align >= MINSIZE */
      /* and we've malloc'd enough total room */

      ap = (mchunkptr)( (size_t)(ap) + align );
      gap += align;    
    }

    room = p->size - gap;

    /* give back leader */
    set_size(p, gap);
    dirtylink(p); /* Don't really know if clean or dirty; be safe */

    /* use the rest */
    p = ap;
    set_size(p, room);
  }

  /* also give back spare room at the end */

  room = p->size - nb;
  if (room >= MINSIZE)
  {
    mchunkptr remainder = (mchunkptr)((char*)(p) + nb);
    set_size(remainder, room);
    dirtylink(remainder); /* Don't really know; be safe */
    set_size(p, nb);
  }

  set_inuse(p);
  return chunk2mem(p);

}



/* Derivatives */

void* valloc(size_t bytes)
{
  /* Cache result of getpagesize */
  static size_t malloc_pagesize = 0;

  if (malloc_pagesize == 0) malloc_pagesize = malloc_getpagesize;
  return memalign (malloc_pagesize, bytes);
}


void* calloc(size_t n, size_t elem_size)
{
  size_t sz = n * elem_size;
  void* p = malloc(sz);
  char* q = (char*) p;
  while (sz-- > 0) *q++ = 0;
  return p;
}

void cfree(void *mem)
{
  free(mem);
}

size_t malloc_usable_size(void* mem)
{
  if (mem == 0)
    return 0;
  else
  {
    mchunkptr p = (mchunkptr)((char*)(mem) - SIZE_SZ); 
    size_t sz = p->size & ~(INUSE);
    /* report zero if not in use or detectably corrupt */
    if (p->size == sz || sz != *((size_t*)((char*)(p) + sz - SIZE_SZ)))
      return 0;
    else
      return sz - MALLOC_MIN_OVERHEAD;
  }
}
    

void malloc_stats()
{

  /* Traverse through and count all sizes of all chunks */

  size_t avail = 0;
  size_t malloced_mem;

  mbinptr b;

  free_returned_list();

  for (b = FIRSTBIN; b <= LASTBIN; ++b)
  {
    mchunkptr p;

    for (p = first_dirty(b); p != dirty_head(b); p = p->fd)
      avail += p->size;

    for (p = first_clean(b); p != clean_head(b); p = p->fd)
      avail += p->size;
  }

  malloced_mem = sbrked_mem - avail;

  fprintf(stderr, "total mem = %10u\n", sbrked_mem);
  fprintf(stderr, "in use    = %10u\n", malloced_mem);

}


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