SQLite

**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file contains the C functions that implement a memory
** allocation subsystem for use by SQLite. 
**
** This version of the memory allocation subsystem omits all
** use of malloc(). The application gives SQLite a block of memory
** before calling sqlite3_initialize() from which allocations
** are made and returned by the xMalloc() and xRealloc() 
** implementations. Once sqlite3_initialize() has been called,
** the amount of memory available to SQLite is fixed and cannot
** be changed.
**
** This version of the memory allocation subsystem is included
** in the build only if SQLITE_ENABLE_MEMSYS5 is defined.
**
** This memory allocator uses the following algorithm:
**
**   1.  All memory allocations sizes are rounded up to a power of 2.
**
**   2.  To adjacent and aligned free blocks are coalesced into a single
**       block of the next larger size.
**
**   3.  New memory is allocated from the first available free block.
**
** This algorithm is described in: J. M. Robson. "Bounds for Some Functions
** Concerning Dynamic Storage Allocation". Journal of the Association for
** Computing Machinery, Volume 21, Number 8, July 1974, pages 491-499.
** 
** Let n be the size of the largest allocation divided by the minimum
** allocation size (after rounding all sizes up to a power of 2.)  Let M
** be the maximum amount of memory ever outstanding at one time.  Let
** N be the total amount of memory available for allocation.  Robson
** proved that this memory allocator will never breakdown due to 
** fragmentation as long as the following constraint holds:
**
**      N >=  M*(1 + log2(n)/2) - n + 1
**
** The sqlite3_status() logic tracks the maximum values of n and M so
** that an application can, at any time, verify this constraint.
*/
#include "sqliteInt.h"

/*
** This version of the memory allocator is used only when 
** SQLITE_ENABLE_MEMSYS5 is defined.
*/
#ifdef SQLITE_ENABLE_MEMSYS5

/*
** A minimum allocation is an instance of the following structure.
** Larger allocations are an array of these structures where the
** size of the array is a power of 2.
**
** The size of this object must be a power of two.  That fact is
** verified in memsys5Init().
*/
typedef struct Mem5Link Mem5Link;
struct Mem5Link {
  int next;       /* Index of next free chunk */
  int prev;       /* Index of previous free chunk */
};

/*
** Maximum size of any allocation is ((1<<LOGMAX)*mem5.nAtom). Since
** mem5.nAtom is always at least 8 and 32-bit integers are used,
** it is not actually possible to reach this limit.
*/
#define LOGMAX 30

/*
** Masks used for mem5.aCtrl[] elements.
*/
#define CTRL_LOGSIZE  0x1f    /* Log2 Size of this block relative to POW2_MIN */

*/
static SQLITE_WSD struct Mem5Global {
  /*
  ** Memory available for allocation
  */
  int nAtom;       /* Smallest possible allocation in bytes */
  int nBlock;      /* Number of nAtom sized blocks in zPool */
  u8 *zPool;       /* Memory available to be allocated */
  
  /*
  ** Mutex to control access to the memory allocation subsystem.
  */
  sqlite3_mutex *mutex;

  /*

  /*
  ** Space for tracking which blocks are checked out and the size
  ** of each block.  One byte per block.
  */
  u8 *aCtrl;

} mem5 = { 0 };

/*
** Access the static variable through a macro for SQLITE_OMIT_WSD
*/
#define mem5 GLOBAL(struct Mem5Global, mem5)

/*
** Assuming mem5.zPool is divided up into an array of Mem5Link
** structures, return a pointer to the idx-th such lik.
*/
#define MEM5LINK(idx) ((Mem5Link *)(&mem5.zPool[(idx)*mem5.nAtom]))

/*
** Unlink the chunk at mem5.aPool[i] from list it is currently
** on.  It should be found on mem5.aiFreelist[iLogsize].
*/
static void memsys5Unlink(int i, int iLogsize){

  }
  memsys5Unlink(iFirst, iLogsize);
  return iFirst;
}

/*
** Return a block of memory of at least nBytes in size.
** Return NULL if unable.  Return NULL if nBytes==0.
**
** The caller guarantees that nByte positive.
**
** The caller has obtained a mutex prior to invoking this
** routine so there is never any chance that two or more
** threads can be in this routine at the same time.
*/
static void *memsys5MallocUnsafe(int nByte){
  int i;           /* Index of a mem5.aPool[] slot */
  int iBin;        /* Index into mem5.aiFreelist[] */
  int iFullSz;     /* Size of allocation rounded up to power of 2 */
  int iLogsize;    /* Log2 of iFullSz/POW2_MIN */

  /* nByte must be a positive */
  assert( nByte>0 );

  /* Keep track of the maximum allocation request.  Even unfulfilled
  ** requests are counted */
  if( (u32)nByte>mem5.maxRequest ){
    mem5.maxRequest = nByte;
  }

  /* Abort if the size is too great */
  if( nByte > 0x40000000 ){
    return 0;
  }

  /* Round nByte up to the next valid power of two */
  for(iFullSz=mem5.nAtom, iLogsize=0; iFullSz<nByte; iFullSz *= 2, iLogsize++){}

  /* Make sure mem5.aiFreelist[iLogsize] contains at least one free
  ** block.  If not, then split a block of the next larger power of
  ** two in order to create a new free block of size iLogsize.

}

/*
** Free an outstanding memory allocation.
*/
static void memsys5FreeUnsafe(void *pOld){
  u32 size, iLogsize;
  int iBlock;

  /* Set iBlock to the index of the block pointed to by pOld in 
  ** the array of mem5.nAtom byte blocks pointed to by mem5.zPool.
  */
  iBlock = ((u8 *)pOld-mem5.zPool)/mem5.nAtom;

  /* Check that the pointer pOld points to a valid, non-free block. */

    memsys5Leave();
  }
  return (void*)p; 
}

/*
** Free memory.
**
** The outer layer memory allocator prevents this routine from
** being called with pPrior==0.
*/
static void memsys5Free(void *pPrior){

  assert( pPrior!=0 );


  memsys5Enter();
  memsys5FreeUnsafe(pPrior);
  memsys5Leave();  
}

/*
** Change the size of an existing memory allocation.
**
** The outer layer memory allocator prevents this routine from
** being called with pPrior==0.
*/
static void *memsys5Realloc(void *pPrior, int nBytes){
  int nOld;
  void *p;
  assert( pPrior!=0 );


  if( nBytes<=0 ){
    memsys5Free(pPrior);
    return 0;
  }
  nOld = memsys5Size(pPrior);
  if( nBytes<=nOld ){
    return pPrior;
  }
  memsys5Enter();
  p = memsys5MallocUnsafe(nBytes);
  if( p ){
    memcpy(p, pPrior, nOld);
    memsys5FreeUnsafe(pPrior);
  }
  memsys5Leave();
  return p;
}

/*
** Round up a request size to the next valid allocation size.  If
** the allocation is too large to be handled by this allocation system,
** return 0.
*/
static int memsys5Roundup(int n){
  int iFullSz;
  if( n > 0x40000000 ) return 0;
  for(iFullSz=mem5.nAtom; iFullSz<n; iFullSz *= 2);
  return iFullSz;
}

/*
** Return the logarithm base 2 of iValue.
*/
static int memsys5Log(int iValue){
  int iLog;
  for(iLog=0; (1<<iLog)<iValue; iLog++);
  return iLog;
}

/*