SQLite

  int nHash;                  /* Size of the pager hash table */
  PgHdr **aHash;              /* Hash table to map page number to PgHdr */
#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
  Pager *pNext;               /* Doubly linked list of pagers on which */
  Pager *pPrev;               /* sqlite3_release_memory() will work */
  int iInUseMM;               /* Non-zero if unavailable to MM */
  int iInUseDB;               /* Non-zero if in sqlite3_release_memory() */
  u8 onPagerList;             /* True if part of the sqlite3PagerList */
#endif
  char *pTmpSpace;            /* Pager.pageSize bytes of space for tmp use */
  char dbFileVers[16];        /* Changes whenever database file changes */
};

/*
** The following global variables hold counters used for

** a mutex on each pager.  The mutex is recursive.
**
** This is a special-purpose mutex.  It only provides mutual exclusion
** between the Btree and the Memory Management sqlite3_release_memory()
** function.  It does not prevent, for example, two Btrees from accessing
** the same pager at the same time.  Other general-purpose mutexes in
** the btree layer handle that chore.
**
** The pagerMutexHeld(X) macro is for sanity checking.  This macro verifies
** that the database-connection mutex is held for pager X and asserts if it
** is not.
*/
#ifdef SQLITE_ENABLE_MEMORY_MANAGEMENT
  static void pagerEnter(Pager *p){
    p->iInUseDB++;
    if( p->iInUseMM && p->iInUseDB==1 ){
#ifndef SQLITE_MUTEX_NOOP
      sqlite3_mutex *mutex;
      mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_MEM2);
#endif
      p->iInUseDB = 0;
      sqlite3_mutex_enter(mutex);
      p->iInUseDB = 1;
      sqlite3_mutex_leave(mutex);
    }
    assert( p->iInUseMM==0 );
  }
  static void pagerLeave(Pager *p){
    p->iInUseDB--;
    assert( p->iInUseDB>=0 );
  }
# define pagerMutexHeld(X)  assert( (X)->iInUseDB>0 || !(X)->onPagerList )
#else
# define pagerEnter(X)
# define pagerLeave(X)
# define pagerMutexHeld(X)
#endif

/*
** Add page pPg to the end of the linked list managed by structure
** pList (pPg becomes the last entry in the list - the most recently 
** used). Argument pLink should point to either pPg->free or pPg->gfree,
** depending on whether pPg is being added to the pager-specific or

** sets the state of the pager back to what it was when it was first
** opened.  Any outstanding pages are invalidated and subsequent attempts
** to access those pages will likely result in a coredump.
*/
static void pager_reset(Pager *pPager){
  PgHdr *pPg, *pNext;
  if( pPager->errCode ) return;
  pagerMutexHeld(pPager);
  for(pPg=pPager->pAll; pPg; pPg=pNext){
    IOTRACE(("PGFREE %p %d\n", pPager, pPg->pgno));
    PAGER_INCR(sqlite3_pager_pgfree_count);
    pNext = pPg->pNextAll;
    lruListRemove(pPg);
    sqlite3_free(pPg->pData);
    sqlite3_free(pPg);

    pPager->pNext = sqlite3PagerList;
    if( sqlite3PagerList ){
      assert( sqlite3PagerList->pPrev==0 );
      sqlite3PagerList->pPrev = pPager;
    }
    pPager->pPrev = 0;
    sqlite3PagerList = pPager;
    pPager->onPagerList = 1;
    sqlite3_mutex_leave(mutex);
  }
#endif
  return SQLITE_OK;
}

/*

** to zero it and hope that we error out sanely.
*/
static void pager_truncate_cache(Pager *pPager){
  PgHdr *pPg;
  PgHdr **ppPg;
  int dbSize = pPager->dbSize;

  pagerMutexHeld(pPager);
  ppPg = &pPager->pAll;
  while( (pPg = *ppPg)!=0 ){
    if( pPg->pgno<=dbSize ){
      ppPg = &pPg->pNextAll;
    }else if( pPg->nRef>0 ){
      memset(PGHDR_TO_DATA(pPg), 0, pPager->pageSize);
      ppPg = &pPg->pNextAll;

    return rc;
  }
  if( nPage>=(unsigned)pPager->dbSize ){
    return SQLITE_OK;
  }
  if( MEMDB ){
    pPager->dbSize = nPage;
    pagerEnter(pPager);
    pager_truncate_cache(pPager);
    pagerLeave(pPager);
    return SQLITE_OK;
  }
  pagerEnter(pPager);
  rc = syncJournal(pPager);
  pagerLeave(pPager);
  if( rc!=SQLITE_OK ){
    return rc;
  }

  /* Get an exclusive lock on the database before truncating. */
  pagerEnter(pPager);
  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);

  if( rc==SQLITE_OK ){

    rc = pager_truncate(pPager, nPage);
  }
  pagerLeave(pPager);

  return rc;
}

/*
** Shutdown the page cache.  Free all memory and close all files.
**
** If a transaction was in progress when this routine is called, that

    }else{
      sqlite3PagerList = pPager->pNext;
    }
    if( pPager->pNext ){
      pPager->pNext->pPrev = pPager->pPrev;
    }
    sqlite3_mutex_leave(mutex);
    pPager->onPagerList = 0;
  }
#endif

  disable_simulated_io_errors();
  sqlite3FaultBeginBenign(-1);
  pPager->errCode = 0;
  pPager->exclusiveMode = 0;