*************************************************************************
** This is the implementation of the page cache subsystem.
** 
** The page cache is used to access a database file.  The pager journals
** all writes in order to support rollback.  Locking is used to limit
** access to one or more reader or one writer.
**
** @(#) $Id: pager.c,v 1.9 2001/06/22 19:15:00 drh Exp $
*/
#include "sqliteInt.h"
#include "pager.h"
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <assert.h>
................................................................................
}

/*
** Increment the reference count for a page.  If the page is
** currently on the freelist (the reference count is zero) then
** remove it from the freelist.
*/
int sqlitepager_ref(void *pData){
  PgHdr *pPg = DATA_TO_PGHDR(pData);
  if( pPg->nRef==0 ){
    /* The page is currently on the freelist.  Remove it. */
    if( pPg->pPrevFree ){
      pPg->pPrevFree->pNextFree = pPg->pNextFree;
    }else{
      pPg->pPager->pFirst = pPg->pNextFree;
    }
................................................................................
      pPg->pNextFree->pPrevFree = pPg->pPrevFree;
    }else{
      pPg->pPager->pLast = pPg->pPrevFree;
    }
    pPg->pPager->nRef++;
  }
  pPg->nRef++;









  return SQLITE_OK;
}

/*
** Acquire a page.
**
** A read lock is obtained for the first page acquired.  The lock
................................................................................
    }
    if( pPager->nExtra>0 ){
      memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
    }
  }else{
    /* The requested page is in the page cache. */
    pPager->nHit++;
    sqlitepager_ref(pPg);
  }
  *ppPage = PGHDR_TO_DATA(pPg);
  return SQLITE_OK;
}

/*
** Acquire a page if it is already in the in-memory cache.  Do
................................................................................
    return 0;
  }
  if( pPager->nRef==0 ){
    return 0;
  }
  pPg = pager_lookup(pPager, pgno);
  if( pPg==0 ) return 0;
  sqlitepager_ref(pPg);
  return PGHDR_TO_DATA(pPg);
}

/*
** Release a page.
**
** If the number of references to the page drop to zero, then the

*************************************************************************
** This is the implementation of the page cache subsystem.
** 
** The page cache is used to access a database file.  The pager journals
** all writes in order to support rollback.  Locking is used to limit
** access to one or more reader or one writer.
**
** @(#) $Id: pager.c,v 1.10 2001/06/23 11:36:20 drh Exp $
*/
#include "sqliteInt.h"
#include "pager.h"
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <assert.h>
................................................................................
}

/*
** Increment the reference count for a page.  If the page is
** currently on the freelist (the reference count is zero) then
** remove it from the freelist.
*/
static void page_ref(PgHdr *pPg){

  if( pPg->nRef==0 ){
    /* The page is currently on the freelist.  Remove it. */
    if( pPg->pPrevFree ){
      pPg->pPrevFree->pNextFree = pPg->pNextFree;
    }else{
      pPg->pPager->pFirst = pPg->pNextFree;
    }
................................................................................
      pPg->pNextFree->pPrevFree = pPg->pPrevFree;
    }else{
      pPg->pPager->pLast = pPg->pPrevFree;
    }
    pPg->pPager->nRef++;
  }
  pPg->nRef++;
}

/*
** Increment the reference count for a page.  The input pointer is
** a reference to the page data.
*/
int sqlitepager_ref(void *pData){
  PgHdr *pPg = DATA_TO_PGHDR(pData);
  page_ref(pPg);
  return SQLITE_OK;
}

/*
** Acquire a page.
**
** A read lock is obtained for the first page acquired.  The lock
................................................................................
    }
    if( pPager->nExtra>0 ){
      memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
    }
  }else{
    /* The requested page is in the page cache. */
    pPager->nHit++;
    page_ref(pPg);
  }
  *ppPage = PGHDR_TO_DATA(pPg);
  return SQLITE_OK;
}

/*
** Acquire a page if it is already in the in-memory cache.  Do
................................................................................
    return 0;
  }
  if( pPager->nRef==0 ){
    return 0;
  }
  pPg = pager_lookup(pPager, pgno);
  if( pPg==0 ) return 0;
  page_ref(pPg);
  return PGHDR_TO_DATA(pPg);
}

/*
** Release a page.
**
** If the number of references to the page drop to zero, then the