SQLite

  /* Variables related to SQLITE_CONFIG_PAGECACHE settings.  The
  ** szSlot, nSlot, pStart, pEnd, nReserve, and isInit values are all
  ** fixed at sqlite3_initialize() time and do not require mutex protection.
  ** The nFreeSlot and pFree values do require mutex protection.
  */
  int isInit;                    /* True if initialized */
  int separateCache;             /* Use a new PGroup for each PCache */
  int szSlot;                    /* Size of each free slot */
  int nSlot;                     /* The number of pcache slots */
  int nReserve;                  /* Try to keep nFreeSlot above this */
  void *pStart, *pEnd;           /* Bounds of global page cache memory */
  /* Above requires no mutex.  Use mutex below for variable that follow. */
  sqlite3_mutex *mutex;          /* Mutex for accessing the following: */
  PgFreeslot *pFree;             /* Free page blocks */

  assert( sqlite3_mutex_held(pCache->pGroup->mutex) );
  if( pCache->pFree ){
    p = pCache->pFree;
    pCache->pFree = p->pNext;
    p->pNext = 0;
  }else{
#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
    /* The group mutex must be released before pcache1Alloc() is called. This
    ** is because it might call sqlite3_release_memory(), which assumes that 
    ** this mutex is not held. */
    assert( pcache1.separateCache==0 );
    assert( pCache->pGroup==&pcache1.grp );
    pcache1LeaveMutex(pCache->pGroup);
#endif
#ifdef SQLITE_PCACHE_SEPARATE_HEADER
    pPg = pcache1Alloc(pCache->szPage);
    p = sqlite3Malloc(sizeof(PgHdr1) + pCache->szExtra);
    if( !pPg || !p ){
      pcache1Free(pPg);
      sqlite3_free(p);
      pPg = 0;
    }
#else
    pPg = pcache1Alloc(pCache->szAlloc);
    p = (PgHdr1 *)&((u8 *)pPg)[pCache->szPage];
#endif
#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
    pcache1EnterMutex(pCache->pGroup);
#endif
    if( pPg==0 ) return 0;
    p->page.pBuf = pPg;
    p->page.pExtra = &p[1];
    p->isBulkLocal = 0;
  }
  if( pCache->bPurgeable ){
    pCache->pGroup->nCurrentPage++;
  }
  return p;
}

/*
** Free a page object allocated by pcache1AllocPage().




*/
static void pcache1FreePage(PgHdr1 *p){
  PCache1 *pCache;
  assert( p!=0 );
  pCache = p->pCache;
  assert( sqlite3_mutex_held(p->pCache->pGroup->mutex) );
  if( p->isBulkLocal ){
    p->pNext = pCache->pFree;
    pCache->pFree = p;
  }else{
    pcache1Free(p->page.pBuf);
#ifdef SQLITE_PCACHE_SEPARATE_HEADER
    sqlite3_free(p);
#endif
  }
  if( pCache->bPurgeable ){
    pCache->pGroup->nCurrentPage--;

  }
}

/*
** Malloc function used by SQLite to obtain space from the buffer configured
** using sqlite3_config(SQLITE_CONFIG_PAGECACHE) option. If no such buffer
** exists, this function falls back to sqlite3Malloc().

/*
** Implementation of the sqlite3_pcache.xInit method.
*/
static int pcache1Init(void *NotUsed){
  UNUSED_PARAMETER(NotUsed);
  assert( pcache1.isInit==0 );
  memset(&pcache1, 0, sizeof(pcache1));


  /*
  ** The pcache1.separateCache variable is true if each PCache has its own
  ** private PGroup (mode-1).  pcache1.separateCache is false if the single
  ** PGroup in pcache1.grp is used for all page caches (mode-2).
  **
  **   *  Always use a unified cache (mode-2) if ENABLE_MEMORY_MANAGEMENT
  **
  **   *  Use a unified cache in single-threaded applications that have
  **      configured a start-time buffer for use as page-cache memory using
  **      sqlite3_config(SQLITE_CONFIG_PAGECACHE, pBuf, sz, N) with non-NULL 
  **      pBuf argument.
  **
  **   *  Otherwise use separate caches (mode-1)
  */
#if defined(SQLITE_ENABLE_MEMORY_MANAGEMENT)
  pcache1.separateCache = 0;
#else
  pcache1.separateCache = sqlite3GlobalConfig.pPage==0
                          || sqlite3GlobalConfig.bCoreMutex>0;
#endif

#if SQLITE_THREADSAFE
  if( sqlite3GlobalConfig.bCoreMutex ){
    pcache1.grp.mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_LRU);
    pcache1.mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PMEM);
  }
#endif
  pcache1.grp.mxPinned = 10;

** Allocate a new cache.
*/
static sqlite3_pcache *pcache1Create(int szPage, int szExtra, int bPurgeable){
  PCache1 *pCache;      /* The newly created page cache */
  PGroup *pGroup;       /* The group the new page cache will belong to */
  int sz;               /* Bytes of memory required to allocate the new cache */



















  assert( (szPage & (szPage-1))==0 && szPage>=512 && szPage<=65536 );
  assert( szExtra < 300 );

  sz = sizeof(PCache1) + sizeof(PGroup)*pcache1.separateCache;
  pCache = (PCache1 *)sqlite3MallocZero(sz);
  if( pCache ){
    if( pcache1.separateCache ){
      pGroup = (PGroup*)&pCache[1];
      pGroup->mxPinned = 10;
    }else{
      pGroup = &pcache1.grp;
    }
    pCache->pGroup = pGroup;
    pCache->szPage = szPage;
    pCache->szExtra = szExtra;
    pCache->szAlloc = szPage + szExtra + ROUND8(sizeof(PgHdr1));
    pCache->bPurgeable = (bPurgeable ? 1 : 0);
    pcache1EnterMutex(pGroup);
    pcache1ResizeHash(pCache);
    if( bPurgeable ){
      pCache->nMin = 10;
      pGroup->nMinPage += pCache->nMin;
      pGroup->mxPinned = pGroup->nMaxPage + 10 - pGroup->nMinPage;
    }
    pcache1LeaveMutex(pGroup);
    /* Try to initialize the local bulk pagecache line allocation if using
    ** separate caches and if nPage!=0 */

    if( pcache1.separateCache && sqlite3GlobalConfig.nPage!=0 ){

      int szBulk;
      char *zBulk;
      sqlite3BeginBenignMalloc();
      if( sqlite3GlobalConfig.nPage>0 ){
        szBulk = pCache->szAlloc * sqlite3GlobalConfig.nPage;
      }else{
        szBulk = -1024*sqlite3GlobalConfig.nPage;