SQLite

            iOff+=pgszSrc
          ){
            PgHdr *pSrcPg = 0;
            const Pgno iSrcPg = (Pgno)((iOff/pgszSrc)+1);
            rc = sqlite3PagerGet(pSrcPager, iSrcPg, &pSrcPg);
            if( rc==SQLITE_OK ){
              u8 *zData = sqlite3PagerGetData(pSrcPg);
              rc = sqlite3PagerWriteData(pDestPager, zData, pgszSrc, iOff);
              rc = sqlite3OsWrite(pFile, zData, pgszSrc, iOff);
            }
            sqlite3PagerUnref(pSrcPg);
          }
          if( rc==SQLITE_OK ){
            rc = backupTruncateFile(pFile, iSize);
          }

  int pgsz,
  int bExtend,                    /* True to extend file if necessary */
  void volatile **pp              /* OUT: Pointer to mapping */
){
  DO_OS_MALLOC_TEST(id);
  return id->pMethods->xShmMap(id, iPage, pgsz, bExtend, pp);
}
int sqlite3OsMremap(
  sqlite3_file *id,               /* Database file handle */

int sqlite3OsFetch(sqlite3_file *id, i64 iOff, int iAmt, void **pp){
  int flags,                      /* SQLITE_MREMAP_XXX flags */
  i64 iOff,                       /* Offset at which mapping(s) start */
  return id->pMethods->xFetch(id, iOff, iAmt, pp);
  i64 nOld,                       /* Size of old mapping */
  i64 nNew,                       /* Size of requested mapping */
  void **pp                       /* IN/OUT: Pointer to mapped region */
}
int sqlite3OsUnfetch(sqlite3_file *id, void *p){
){
  return id->pMethods->xMremap(id, flags, iOff, nOld, nNew, pp);
  return id->pMethods->xUnfetch(id, p);
}

/*
** The next group of routines are convenience wrappers around the
** VFS methods.
*/
int sqlite3OsOpen(

#define SQLITE_FCNTL_DB_UNCHANGED 0xca093fa0
int sqlite3OsSectorSize(sqlite3_file *id);
int sqlite3OsDeviceCharacteristics(sqlite3_file *id);
int sqlite3OsShmMap(sqlite3_file *,int,int,int,void volatile **);
int sqlite3OsShmLock(sqlite3_file *id, int, int, int);
void sqlite3OsShmBarrier(sqlite3_file *id);
int sqlite3OsShmUnmap(sqlite3_file *id, int);
int sqlite3OsMremap(sqlite3_file *id, int, i64, i64, i64, void **);
int sqlite3OsFetch(sqlite3_file *id, i64, int, void **);
int sqlite3OsUnfetch(sqlite3_file *, void *);


/* 
** Functions for accessing sqlite3_vfs methods 
*/
int sqlite3OsOpen(sqlite3_vfs *, const char *, sqlite3_file*, int, int *);
int sqlite3OsDelete(sqlite3_vfs *, const char *, int);

  ** occur if a file is updated without also updating the transaction
  ** counter.  This test is made to avoid new problems similar to the
  ** one described by ticket #3584. 
  */
  unsigned char transCntrChng;   /* True if the transaction counter changed */
  unsigned char dbUpdate;        /* True if any part of database file changed */
  unsigned char inNormalWrite;   /* True if in a normal write operation */

#endif
  sqlite3_int64 mmapSize;        /* Size of xMremap() */
  void *pMapRegion;              /* Area memory mapped */
#endif
  sqlite3_int64 mmapSize;         /* Usable size of mapping at pMapRegion */
  sqlite3_int64 mmapOrigsize;     /* Actual size of mapping at pMapRegion */
  sqlite3_int64 mmapLimit;        /* Configured FCNTL_MMAP_SIZE value */
  void *pMapRegion;               /* Memory mapped region */
  int nFetchOut;                  /* Number of outstanding xFetch refs */

#ifdef SQLITE_TEST
  /* In test mode, increase the size of this structure a bit so that 
  ** it is larger than the struct CrashFile defined in test6.c.
  */
  char aPadding[32];
#endif
};

** If the locking level of the file descriptor is already at or below
** the requested locking level, this routine is a no-op.
*/
static int unixUnlock(sqlite3_file *id, int eFileLock){
  return posixUnlock(id, eFileLock, 0);
}

static int unixMapfile(unixFile *pFd, i64 nByte);
static void unixUnmapfile(unixFile *pFd);

/*
** This function performs the parts of the "close file" operation 
** common to all locking schemes. It closes the directory and file
** handles, if they are valid, and sets all fields of the unixFile
** structure to 0.
**
** It is *not* necessary to hold the mutex when this routine is called,
** even on VxWorks.  A mutex will be acquired on VxWorks by the
** vxworksReleaseFileId() routine.
*/
static int closeUnixFile(sqlite3_file *id){
  unixFile *pFile = (unixFile*)id;
  unixUnmapfile(pFile);
  if( pFile->h>=0 ){
    robust_close(pFile, pFile->h, __LINE__);
    pFile->h = -1;
  }
#if OS_VXWORKS
  if( pFile->pId ){
    if( pFile->ctrlFlags & UNIXFILE_DELETE ){

  void *pBuf, 
  int amt,
  sqlite3_int64 offset
){
  unixFile *pFile = (unixFile *)id;
  int got;
  assert( id );
  assert( offset>=pFile->mmapSize );  /* Never read from the mmapped region */

  /* If this is a database file (not a journal, master-journal or temp
  ** file), the bytes in the locking range should never be read or written. */
#if 0
  assert( pFile->pUnused==0
       || offset>=PENDING_BYTE+512
       || offset+amt<=PENDING_BYTE 
  );
#endif

  /* Deal with as much of this write request as possible by transfering
  ** data to the memory mapping using memcpy().  */
  if( offset<pFile->mmapSize ){
    if( offset+amt <= pFile->mmapSize ){
      memcpy(pBuf, &((u8 *)(pFile->pMapRegion))[offset], amt);
      return SQLITE_OK;
    }else{
      int nCopy = pFile->mmapSize - offset;
      memcpy(pBuf, &((u8 *)(pFile->pMapRegion))[offset], nCopy);
      pBuf = &((u8 *)pBuf)[nCopy];
      amt -= nCopy;
      offset += nCopy;
    }
  }

  got = seekAndRead(pFile, offset, pBuf, amt);
  if( got==amt ){
    return SQLITE_OK;
  }else if( got<0 ){
    /* lastErrno set by seekAndRead */
    return SQLITE_IOERR_READ;

  int amt,
  sqlite3_int64 offset 
){
  unixFile *pFile = (unixFile*)id;
  int wrote = 0;
  assert( id );
  assert( amt>0 );
  assert( offset>=pFile->mmapSize );   /* Never write into the mmapped region */

  /* If this is a database file (not a journal, master-journal or temp
  ** file), the bytes in the locking range should never be read or written. */
#if 0
  assert( pFile->pUnused==0
       || offset>=PENDING_BYTE+512
       || offset+amt<=PENDING_BYTE 

      SimulateIOErrorBenign(0);
      if( rc!=4 || memcmp(oldCntr, &((char*)pBuf)[24-offset], 4)!=0 ){
        pFile->transCntrChng = 1;  /* The transaction counter has changed */
      }
    }
  }
#endif

  /* Deal with as much of this write request as possible by transfering
  ** data from the memory mapping using memcpy().  */
  if( offset<pFile->mmapSize ){
    if( offset+amt <= pFile->mmapSize ){
      memcpy(&((u8 *)(pFile->pMapRegion))[offset], pBuf, amt);
      return SQLITE_OK;
    }else{
      int nCopy = pFile->mmapSize - offset;
      memcpy(&((u8 *)(pFile->pMapRegion))[offset], pBuf, nCopy);
      pBuf = &((u8 *)pBuf)[nCopy];
      amt -= nCopy;
      offset += nCopy;
    }
  }

  while( amt>0 && (wrote = seekAndWrite(pFile, offset, pBuf, amt))>0 ){
    amt -= wrote;
    offset += wrote;
    pBuf = &((char*)pBuf)[wrote];
  }
  SimulateIOError(( wrote=(-1), amt=1 ));

    ** that effectively updates the change counter.  This might happen
    ** when restoring a database using the backup API from a zero-length
    ** source.
    */
    if( pFile->inNormalWrite && nByte==0 ){
      pFile->transCntrChng = 1;
    }
#endif

    /* If the file was just truncated to a size smaller than the currently
    ** mapped region, reduce the effective mapping size as well. SQLite will
    ** use read() and write() to access data beyond this point from now on.  
    */
    if( nByte<pFile->mmapSize ){
      pFile->mmapSize = nByte;
    }
#endif

    return SQLITE_OK;
  }
}

/*
** Determine the current size of a file in bytes

        int nWrite = seekAndWrite(pFile, iWrite, "", 1);
        if( nWrite!=1 ) return SQLITE_IOERR_WRITE;
        iWrite += nBlk;
      }
#endif
    }
  }

  if( pFile->mmapLimit>0 ){
    int rc;
    if( pFile->szChunk<=0 ){
      if( robust_ftruncate(pFile->h, nByte) ){
        pFile->lastErrno = errno;
        return unixLogError(SQLITE_IOERR_TRUNCATE, "ftruncate", pFile->zPath);
      }
    }

    rc = unixMapfile(pFile, nByte);
    return rc;
  }

  return SQLITE_OK;
}

/*
** If *pArg is inititially negative then this is a query.  Set *pArg to
** 1 or 0 depending on whether or not bit mask of pFile->ctrlFlags is set.

      char *zTFile = sqlite3_malloc( pFile->pVfs->mxPathname );
      if( zTFile ){
        unixGetTempname(pFile->pVfs->mxPathname, zTFile);
        *(char**)pArg = zTFile;
      }
      return SQLITE_OK;
    }
    case SQLITE_FCNTL_GETFD: {
      *(int*)pArg = pFile->h;
    case SQLITE_FCNTL_MMAP_SIZE: {
      pFile->mmapLimit = *(i64*)pArg;
      return SQLITE_OK;
    }
#ifdef SQLITE_DEBUG
    /* The pager calls this method to signal that it has done
    ** a rollback and that the database is therefore unchanged and
    ** it hence it is OK for the transaction change counter to be
    ** unchanged.

**
**     ROUNDUP(0,  8) ->  0
**     ROUNDUP(13, 8) -> 16
**     ROUNDUP(32, 8) -> 32
*/
#define ROUNDUP(x,y)     (((x)+y-1)&~(y-1))

/*
** Map, remap or unmap part of the database file.
*/
static int unixMremap(
static void unixUnmapfile(unixFile *pFd){
  assert( pFd->nFetchOut==0 );
  if( pFd->pMapRegion ){
    munmap(pFd->pMapRegion, pFd->mmapOrigsize);
    pFd->pMapRegion = 0;
    pFd->mmapSize = 0;
    pFd->mmapOrigsize = 0;
  }
}

static int unixMapfile(unixFile *pFd, i64 nByte){
  sqlite3_file *fd,               /* Main database file */
  int flags,                      /* Mask of SQLITE_MREMAP_XXX flags */
  sqlite3_int64 iOff,             /* Offset to start mapping at */
  sqlite3_int64 nOld,             /* Size of old mapping, or zero */
  i64 nMap = nByte;
  sqlite3_int64 nNew,             /* Size of new mapping, or zero */
  void **ppMap                    /* IN/OUT: Old/new mappings */
){
  unixFile *p = (unixFile *)fd;   /* The underlying database file */
  int rc = SQLITE_OK;             /* Return code */
  int rc;
  void *pNew = 0;                 /* New mapping */
  i64 nNewRnd;                    /* nNew rounded up */
  i64 nOldRnd;                    /* nOld rounded up */

  assert( iOff==0 );
  assert( nMap>=0 || pFd->nFetchOut==0 );
  /* assert( p->mmapSize==nOld ); */
  assert( p->pMapRegion==0 || p->pMapRegion==(*ppMap) );
  if( pFd->nFetchOut>0 ) return SQLITE_OK;

  /* If the SQLITE_MREMAP_EXTEND flag is set, then the size of the requested 
  ** mapping (nNew bytes) may be greater than the size of the database file.
  ** If this is the case, extend the file on disk using ftruncate().  */
  assert( nNew>0 || (flags & SQLITE_MREMAP_EXTEND)==0 );
  if( flags & SQLITE_MREMAP_EXTEND ){
  if( nMap<0 ){
    struct stat statbuf;          /* Low-level file information */
    rc = osFstat(p->h, &statbuf);
    if( rc==SQLITE_OK && nNew>statbuf.st_size ){
      rc = robust_ftruncate(p->h, nNew);
    rc = osFstat(pFd->h, &statbuf);
    if( rc!=SQLITE_OK ){
      return SQLITE_IOERR_FSTAT;
    }
    if( rc!=SQLITE_OK ) return rc;
    nMap = statbuf.st_size;
  }
  if( nMap>pFd->mmapLimit ){
    nMap = pFd->mmapLimit;

  /* According to some sources, the effect of changing the size of the
  }

  ** underlying file on mapped regions that correspond to the added or
  ** removed pages is undefined. However, there is reason to believe that
  ** on modern platforms like Linux or OSX, things just work. For example,
  ** it is possible to create a mapping larger than the file on disk and
  ** extend the file on disk later on.
  **
  ** Exploit this on Linux and OSX to reduce the number of munmap()/mmap() 
  if( nMap!=pFd->mmapSize ){
    void *pNew;
    unixUnmapfile(pFd);

    if( nMap>0 ){
  ** calls required if the file size is changing. In this case all mappings 
  ** are rounded up to the nearest 4MB. And if a new mapping is requested 
  ** that has the same rounded size as an old mapping, the old mapping can 
  ** be reused as is. */
      void *pNew;
#if defined(__APPLE__) || defined(__linux__)
  nNewRnd = ROUNDUP(nNew, 4096*1024);
  nOldRnd = ROUNDUP(nOld, 4096*1024);
#else
  nNewRnd = ROUNDUP(nNew, 4096*1);
  nOldRnd = ROUNDUP(nOld, 4096*1);
#endif
      int flags = PROT_READ;
      if( (pFd->ctrlFlags & UNIXFILE_RDONLY)==0 ) flags |= PROT_WRITE;
      pNew = mmap(0, ROUNDUP(nMap, 4096), flags, MAP_SHARED, pFd->h, 0);
      if( pNew==MAP_FAILED ){
        return SQLITE_IOERR_MREMAP;
      }

      pFd->pMapRegion = pNew;
      pFd->mmapOrigsize = pFd->mmapSize = nMap;
    }
  }

  return SQLITE_OK;
}

  /* On OSX or Linux, reuse the old mapping if it is the right size. */
#if defined(__APPLE__) || defined(__linux__)
  if( nNewRnd==nOldRnd ){
    VVA_ONLY( p->mmapSize = nNew; )
    return SQLITE_OK;
  }
#endif

static int unixFetch(sqlite3_file *fd, i64 iOff, int nAmt, void **pp){
  unixFile *pFd = (unixFile *)fd;   /* The underlying database file */
  *pp = 0;

  if( pFd->mmapLimit>0 ){
    if( pFd->pMapRegion==0 ){
      int rc = unixMapfile(pFd, -1);
      if( rc!=SQLITE_OK ) return rc;
    }
    if( pFd->mmapSize >= iOff+nAmt ){
      *pp = &((u8 *)pFd->pMapRegion)[iOff];
      pFd->nFetchOut++;
    }
  /* If we get this far, unmap any old mapping. */
  if( nOldRnd!=0 ){
    void *pOld = *ppMap;
    munmap(pOld, nOldRnd);
    VVA_ONLY( p->mmapSize = 0; p->pMapRegion = 0; );
  }
  return SQLITE_OK;

  /* And, if required, use mmap() to create a new mapping. */
}

  if( nNewRnd>0 ){
    int flags = PROT_READ;
    if( (p->ctrlFlags & UNIXFILE_RDONLY)==0 ) flags |= PROT_WRITE;
    pNew = mmap(0, nNewRnd, flags, MAP_SHARED, p->h, iOff);
    if( pNew==MAP_FAILED ){
static int unixUnfetch(sqlite3_file *fd, void *p){
  unixFile *pFd = (unixFile *)fd;   /* The underlying database file */

  assert( (p==0)==(pFd->nFetchOut==0) );

  if( p ){
      pNew = 0;
      VVA_ONLY( p->mmapSize = 0; p->pMapRegion = 0; )
      rc = SQLITE_IOERR_MREMAP;
    }else{
      VVA_ONLY( p->mmapSize = nNew; p->pMapRegion = pNew; )
    }
  }

    pFd->nFetchOut--;
  }else{
    unixUnmapfile(pFd);
  }

  assert( pFd->nFetchOut>=0 );
  *ppMap = pNew;
  return rc;
  return SQLITE_OK;
}

/*
** Here ends the implementation of all sqlite3_file methods.
**
********************** End sqlite3_file Methods *******************************
******************************************************************************/

   unixFileControl,            /* xFileControl */                            \
   unixSectorSize,             /* xSectorSize */                             \
   unixDeviceCharacteristics,  /* xDeviceCapabilities */                     \
   unixShmMap,                 /* xShmMap */                                 \
   unixShmLock,                /* xShmLock */                                \
   unixShmBarrier,             /* xShmBarrier */                             \
   unixShmUnmap,               /* xShmUnmap */                               \
   unixFetch,                  /* xFetch */                                  \
   unixMremap,                 /* xMremap */                                 \
   unixUnfetch,                /* xUnfetch */                                \
};                                                                           \
static const sqlite3_io_methods *FINDER##Impl(const char *z, unixFile *p){   \
  UNUSED_PARAMETER(z); UNUSED_PARAMETER(p);                                  \
  return &METHOD;                                                            \
}                                                                            \
static const sqlite3_io_methods *(*const FINDER)(const char*,unixFile *p)    \
    = FINDER##Impl;

  assert( zFilename!=0 || (ctrlFlags & UNIXFILE_NOLOCK)!=0 );

  OSTRACE(("OPEN    %-3d %s\n", h, zFilename));
  pNew->h = h;
  pNew->pVfs = pVfs;
  pNew->zPath = zFilename;
  pNew->ctrlFlags = (u8)ctrlFlags;
  VVA_ONLY( pNew->mmapSize = 0; )
  if( sqlite3_uri_boolean(((ctrlFlags & UNIXFILE_URI) ? zFilename : 0),
                           "psow", SQLITE_POWERSAFE_OVERWRITE) ){
    pNew->ctrlFlags |= UNIXFILE_PSOW;
  }
  if( strcmp(pVfs->zName,"unix-excl")==0 ){
    pNew->ctrlFlags |= UNIXFILE_EXCL;
  }

  i64 journalOff;             /* Current write offset in the journal file */
  i64 journalHdr;             /* Byte offset to previous journal header */
  sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
  PagerSavepoint *aSavepoint; /* Array of active savepoints */
  int nSavepoint;             /* Number of elements in aSavepoint[] */
  char dbFileVers[16];        /* Changes whenever database file changes */

  void *pMap;                 /* Memory mapped prefix of database file */
  i64 nMap;                   /* Size of mapping at pMap in bytes */ 
  u8 bUseFetch;               /* True to use xFetch() */
  i64 nMapValid;              /* Bytes at pMap known to be valid */
  i64 nMapLimit;              /* Maximum permitted mapping size */
  int nMapCfgLimit;           /* Configured limit value */
  int nMmapOut;               /* Number of mmap pages currently outstanding */
  PgHdr *pFree;               /* List of free mmap page headers (pDirty) */
  int bMapResize;             /* Check if the mapping should be resized */
  /*
  ** End of the routinely-changing class members
  ***************************************************************************/

  u16 nExtra;                 /* Add this many bytes to each in-memory page */
  i16 nReserve;               /* Number of unused bytes at end of each page */
  u32 vfsFlags;               /* Flags for sqlite3_vfs.xOpen() */

                           (int)pPager->nReserve);
  }
}
#else
# define pagerReportSize(X)     /* No-op if we do not support a codec */
#endif

/*
** Write nBuf bytes of data from buffer pBuf to offset iOff of the 
** database file. If this part of the database file is memory mapped,
** use memcpy() to do so. Otherwise, call sqlite3OsWrite().
**
** Return SQLITE_OK if successful, or an SQLite error code if an error 
** occurs.
*/
int sqlite3PagerWriteData(Pager *pPager, const void *pBuf, int nBuf, i64 iOff){
  int rc = SQLITE_OK;
  if( pPager->nMapValid>=(iOff+nBuf) ){
    memcpy(&((u8 *)(pPager->pMap))[iOff], pBuf, nBuf);
  }else{
    rc = sqlite3OsWrite(pPager->fd, pBuf, nBuf, iOff);
  }
  return rc;
}

/*
** Read a single page from either the journal file (if isMainJrnl==1) or
** from the sub-journal (if isMainJrnl==0) and playback that page.
** The page begins at offset *pOffset into the file. The *pOffset
** value is increased to the start of the next page in the journal.
**
** The main rollback journal uses checksums - the statement journal does 

  if( isOpen(pPager->fd)
   && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN)
   && isSynced
  ){
    i64 ofst = (pgno-1)*(i64)pPager->pageSize;
    testcase( !isSavepnt && pPg!=0 && (pPg->flags&PGHDR_NEED_SYNC)!=0 );
    assert( !pagerUseWal(pPager) );
    rc = sqlite3PagerWriteData(pPager, aData, pPager->pageSize, ofst);
    rc = sqlite3OsWrite(pPager->fd, (u8 *)aData, pPager->pageSize, ofst);
    if( pgno>pPager->dbFileSize ){
      pPager->dbFileSize = pgno;
    }
    if( pPager->pBackup ){
      CODEC1(pPager, aData, pgno, 3, rc=SQLITE_NOMEM);
      sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)aData);
      CODEC2(pPager, aData, pgno, 7, rc=SQLITE_NOMEM, aData);

    assert( pPager->eLock==EXCLUSIVE_LOCK );
    /* TODO: Is it safe to use Pager.dbFileSize here? */
    rc = sqlite3OsFileSize(pPager->fd, &currentSize);
    newSize = szPage*(i64)nPage;
    if( rc==SQLITE_OK && currentSize!=newSize ){
      if( currentSize>newSize ){
        rc = sqlite3OsTruncate(pPager->fd, newSize);
        if( newSize<pPager->nMapValid ){
          pPager->nMapValid = newSize;
        }
      }else if( (currentSize+szPage)<=newSize ){
        char *pTmp = pPager->pTmpSpace;
        memset(pTmp, 0, szPage);
        testcase( (newSize-szPage) == currentSize );
        testcase( (newSize-szPage) >  currentSize );
        rc = sqlite3OsWrite(pPager->fd, pTmp, szPage, newSize-szPage);
      }

  }

  if( iFrame ){
    /* Try to pull the page from the write-ahead log. */
    rc = sqlite3WalReadFrame(pPager->pWal, iFrame, pgsz, pPg->pData);
  }else{
    i64 iOffset = (pgno-1)*(i64)pPager->pageSize;
    if( pPager->pMap && pPager->nMapValid>=iOffset+pPager->pageSize ){
      memcpy(pPg->pData, &((u8 *)(pPager->pMap))[iOffset], pPager->pageSize);
    }else{
      rc = sqlite3OsRead(pPager->fd, pPg->pData, pgsz, iOffset);
      if( rc==SQLITE_IOERR_SHORT_READ ){
        rc = SQLITE_OK;
    rc = sqlite3OsRead(pPager->fd, pPg->pData, pgsz, iOffset);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
      }
    }
  }

  if( pgno==1 ){
    if( rc ){
      /* If the read is unsuccessful, set the dbFileVers[] to something
      ** that will never be a valid file version.  dbFileVers[] is a copy

  ** the duplicate call is harmless.
  */
  sqlite3WalEndReadTransaction(pPager->pWal);

  rc = sqlite3WalBeginReadTransaction(pPager->pWal, &changed);
  if( rc!=SQLITE_OK || changed ){
    pager_reset(pPager);
    if( pPager->bUseFetch ) sqlite3OsUnfetch(pPager->fd, 0);
  }

  return rc;
}
#endif

/*

** Change the maximum number of in-memory pages that are allowed.
*/
void sqlite3PagerSetCachesize(Pager *pPager, int mxPage){
  sqlite3PcacheSetCachesize(pPager->pPCache, mxPage);
}

/*
** Set Pager.nMapLimit, the maximum permitted mapping size, based on the
** current values of Pager.nMapCfgLimit and Pager.pageSize.
** Invoke SQLITE_FCNTL_MMAP_SIZE based on the current value of nMapCfgLimit.
**
** If this connection should not use mmap at all, set nMapLimit to zero.
*/
static void pagerFixMaplimit(Pager *pPager){
  sqlite3_file *fd = pPager->fd;
  if( isOpen(pPager->fd)==0 
   || pPager->fd->pMethods->iVersion<3 
  if( isOpen(fd) ){
    pPager->bUseFetch = (fd->pMethods->iVersion>=3) && pPager->nMapCfgLimit!=0;
   || pPager->fd->pMethods->xMremap==0 
   || pPager->tempFile 
    if( pPager->bUseFetch ){
  ){
    pPager->nMapLimit = 0;
  }else if( pPager->nMapCfgLimit<0 ){
    pPager->nMapLimit = (i64)pPager->nMapCfgLimit * -1024;
  }else{
    pPager->nMapLimit = (i64)pPager->nMapCfgLimit * pPager->pageSize;
      void *p;
      i64 nMapLimit;
      if( pPager->nMapCfgLimit<0 ){
        nMapLimit = (i64)pPager->nMapCfgLimit * -1024;
      }else{
        nMapLimit = (i64)pPager->nMapCfgLimit * pPager->pageSize;
      }

      p = (void *)&nMapLimit;
      sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_MMAP_SIZE, p);
    }
  }
}

/*
** Change the maximum size of any memory mapping made of the database file.
*/
void sqlite3PagerSetMmapsize(Pager *pPager, int nMap){

  }
  if( rc==SQLITE_OK ){
    rc = sqlite3OsFileSize(pPager->jfd, &pPager->journalHdr);
  }
  return rc;
}

/*
** Unmap any memory mapping of the database file.
*/
static int pagerUnmap(Pager *pPager){
  assert( pPager->nMmapOut==0 );
  if( pPager->pMap ){
    sqlite3OsMremap(pPager->fd, 0, 0, pPager->nMap, 0, &pPager->pMap);
    pPager->nMap = 0;
    pPager->nMapValid = 0;
  }
  return SQLITE_OK;
}

/*
** Create, or recreate, the memory mapping of the database file.
*/
static int pagerMap(Pager *pPager, int bExtend){
  int rc = SQLITE_OK;             /* Return code */
  Pgno nPg;                       /* Size of mapping to request in pages */
  i64 sz;                         /* Size of mapping to request in bytes */

  assert( isOpen(pPager->fd) && pPager->tempFile==0 );
  assert( pPager->pMap==0 || pPager->nMap>0 );
  /* assert( pPager->eState>=1 ); */
  assert( pPager->nMmapOut==0 );
  assert( pPager->nMapLimit>0 );

  /* Figure out how large a mapping to request. Set variable sz to this 
  ** value in bytes. */
  nPg = (pPager->eState==1) ? pPager->dbSize : pPager->dbFileSize;
  sz = (i64)nPg * pPager->pageSize;
  if( sz>pPager->nMapLimit ) sz = pPager->nMapLimit;

  if( sz!=pPager->nMapValid ){
    int flags = (bExtend ? SQLITE_MREMAP_EXTEND : 0);
    rc = sqlite3OsMremap(pPager->fd, flags, 0, pPager->nMap, sz, &pPager->pMap);
    if( rc==SQLITE_OK ){
      assert( pPager->pMap!=0 );
      pPager->nMap = sz;
    }else{
      assert( pPager->pMap==0 );
      pPager->nMap = 0;
    }
    pPager->nMapValid = pPager->nMap;
  }
  pPager->bMapResize = 0;

  return rc;
}

/*
** Obtain a reference to a memory mapped page object for page number pgno. 
** The caller must ensure that page pgno lies within the currently mapped 
** region. If successful, set *ppPage to point to the new page reference
** The new object will use the pointer pData, obtained from xFetch().
** If successful, set *ppPage to point to the new page reference
** and return SQLITE_OK. Otherwise, return an SQLite error code and set
** *ppPage to zero.
**
** Page references obtained by calling this function should be released
** by calling pagerReleaseMapPage().
*/
static int pagerAcquireMapPage(Pager *pPager, Pgno pgno, PgHdr **ppPage){
static int pagerAcquireMapPage(
  Pager *pPager,                  /* Pager object */
  Pgno pgno,                      /* Page number */
  void *pData,                    /* xFetch()'d data for this page */
  PgHdr **ppPage                  /* OUT: Acquired page object */
){
  PgHdr *p;                       /* Memory mapped page to return */

  if( pPager->pFree ){
    *ppPage = p = pPager->pFree;
    pPager->pFree = p->pDirty;
    p->pDirty = 0;
    memset(p->pExtra, 0, pPager->nExtra);

  assert( p->pExtra==(void *)&p[1] );
  assert( p->pPage==0 );
  assert( p->flags==PGHDR_MMAP );
  assert( p->pPager==pPager );
  assert( p->nRef==1 );

  p->pData = &((u8 *)pPager->pMap)[(i64)(pgno-1) * pPager->pageSize];
  p->pgno = pgno;
  p->pData = pData;
  pPager->nMmapOut++;

  return SQLITE_OK;
}

/*
** Release a reference to page pPg. pPg must have been returned by an 
** earlier call to pagerAcquireMapPage().
*/
static void pagerReleaseMapPage(PgHdr *pPg){
  Pager *pPager = pPg->pPager;
  pPager->nMmapOut--;
  pPg->pDirty = pPager->pFree;
  pPager->pFree = pPg;

  assert( pPager->fd->pMethods->iVersion>=3 );
  sqlite3OsUnfetch(pPager->fd, pPg->pData);
}

/*
** Free all PgHdr objects stored in the Pager.pFree list.
*/
static void pagerFreeMapHdrs(Pager *pPager){
  PgHdr *p;

*/
int sqlite3PagerClose(Pager *pPager){
  u8 *pTmp = (u8 *)pPager->pTmpSpace;

  assert( assert_pager_state(pPager) );
  disable_simulated_io_errors();
  sqlite3BeginBenignMalloc();
  pagerUnmap(pPager);
  pagerFreeMapHdrs(pPager);
  /* pPager->errCode = 0; */
  pPager->exclusiveMode = 0;
#ifndef SQLITE_OMIT_WAL
  sqlite3WalClose(pPager->pWal, pPager->ckptSyncFlags, pPager->pageSize, pTmp);
  pPager->pWal = 0;
#endif

  */
  sqlite3PcacheClearSyncFlags(pPager->pPCache);
  pPager->eState = PAGER_WRITER_DBMOD;
  assert( assert_pager_state(pPager) );
  return SQLITE_OK;
}

/*
** This is called by the wal.c module at the start of a checkpoint. If the
** checkpoint runs to completion, it will set the database file size to
** szReq bytes. This function performs two tasks:
**
**   * If the file is currently less than szReq bytes in size, an
**     xFileControl(SQLITE_FNCTL_SIZE_HINT) is issued to inform the OS
**     layer of the expected file size, and
**
**   * If mmap is being used, then the mapping is extended to szReq
**     bytes in size.
**
** SQLITE_OK is returned if successful, or an error code if an error occurs.
*/
int sqlite3PagerSetFilesize(Pager *pPager, i64 szReq){
  int rc;
  i64 sz;                         /* Size of file on disk in bytes */

  assert( pPager->eState==PAGER_OPEN );
  assert( pPager->nMmapOut==0 );

  rc = sqlite3OsFileSize(pPager->fd, &sz);
  if( rc==SQLITE_OK ){
    if( sz>szReq ){
      sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_SIZE_HINT, &sz);
    }
  }


  if( rc==SQLITE_OK ){
    i64 szMap = (szReq > pPager->nMapLimit) ? pPager->nMapLimit : szReq;
    if( pPager->nMapValid!=pPager->nMap || szMap!=pPager->nMap ){
      pPager->dbFileSize = (szReq / pPager->pageSize);
      rc = pagerMap(pPager, 1);
    }
  }

  return rc;
}

/*
** The argument is the first in a linked list of dirty pages connected
** by the PgHdr.pDirty pointer. This function writes each one of the
** in-memory pages in the list to the database file. The argument may
** be NULL, representing an empty list. In this case this function is
** a no-op.
**

  assert( rc!=SQLITE_OK || isOpen(pPager->fd) );
  if( rc==SQLITE_OK 
   && (pList->pDirty ? pPager->dbSize : pList->pgno+1)>pPager->dbHintSize 
  ){
    sqlite3_int64 szFile = pPager->pageSize * (sqlite3_int64)pPager->dbSize;
    sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_SIZE_HINT, &szFile);
    pPager->dbHintSize = pPager->dbSize;

    if( pPager->nMmapOut==0 && pPager->nMapLimit>0 ){
      pPager->dbFileSize = pPager->dbSize;
      rc = pagerMap(pPager, 1);
    }
  }

  while( rc==SQLITE_OK && pList ){
    Pgno pgno = pList->pgno;

    /* If there are dirty pages in the page cache with page numbers greater
    ** than Pager.dbSize, this means sqlite3PagerTruncateImage() was called to

      assert( (pList->flags&PGHDR_NEED_SYNC)==0 );
      if( pList->pgno==1 ) pager_write_changecounter(pList);

      /* Encode the database */
      CODEC2(pPager, pList->pData, pgno, 6, return SQLITE_NOMEM, pData);

      /* Write out the page data. */
      rc = sqlite3PagerWriteData(pPager, pData, pPager->pageSize, offset);
      rc = sqlite3OsWrite(pPager->fd, pData, pPager->pageSize, offset);

      /* If page 1 was just written, update Pager.dbFileVers to match
      ** the value now stored in the database file. If writing this 
      ** page caused the database file to grow, update dbFileSize. 
      */
      if( pgno==1 ){
        memcpy(&pPager->dbFileVers, &pData[24], sizeof(pPager->dbFileVers));

           || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
      );
    }

    if( !pPager->tempFile && (
        pPager->pBackup 
     || sqlite3PcachePagecount(pPager->pPCache)>0 
     || pPager->pMap
     || pPager->bUseFetch /* TODO: Currently required for xUnfetch(0) only. */
    )){
      /* The shared-lock has just been acquired on the database file
      ** and there are already pages in the cache (from a previous
      ** read or write transaction).  Check to see if the database
      ** has been modified.  If the database has changed, flush the
      ** cache.
      **

      */
      Pgno nPage = 0;
      char dbFileVers[sizeof(pPager->dbFileVers)];

      rc = pagerPagecount(pPager, &nPage);
      if( rc ) goto failed;

      if( nPage>0 || pPager->pMap ){
      if( nPage>0 ){
        IOTRACE(("CKVERS %p %d\n", pPager, sizeof(dbFileVers)));
        if( pPager->pMap ){
          memcpy(&dbFileVers, &((u8 *)(pPager->pMap))[24], sizeof(dbFileVers));
        }else{
          rc = sqlite3OsRead(pPager->fd, &dbFileVers, sizeof(dbFileVers), 24);
        rc = sqlite3OsRead(pPager->fd, &dbFileVers, sizeof(dbFileVers), 24);
        }
        if( rc!=SQLITE_OK ){
          goto failed;
        }
      }else{
        memset(dbFileVers, 0, sizeof(dbFileVers));
      }

      if( memcmp(pPager->dbFileVers, dbFileVers, sizeof(dbFileVers))!=0 ){
        pager_reset(pPager);

        /* Unmap the database file. It is possible that external processes
        ** may have truncated the database file and then extended it back
        ** to its original size while this process was not holding a lock.
        ** In this case there may exist a Pager.pMap mapping that appears
        ** to be the right size but is not actually valid. Avoid this
        ** possibility by unmapping the db here. */
        pagerUnmap(pPager);
      }else if( pPager->pMap ){
        pPager->bMapResize = 1;
        if( pPager->bUseFetch ){
          sqlite3OsUnfetch(pPager->fd, 0);
        }
      }
    }

    /* If there is a WAL file in the file-system, open this database in WAL
    ** mode. Otherwise, the following function call is a no-op.
    */
    rc = pagerOpenWalIfPresent(pPager);

  u32 iFrame = 0;                 /* Frame to read from WAL file */
  const int noContent = (flags & PAGER_ACQUIRE_NOCONTENT);

  /* It is acceptable to use a read-only (mmap) page for any page except
  ** page 1 if there is no write-transaction open or the ACQUIRE_READONLY
  ** flag was specified by the caller. And so long as the db is not a 
  ** temporary or in-memory database.  */
  const int bMmapOk = (pPager->nMapLimit>0 && pgno!=1
  const int bMmapOk = (pgno!=1 && pPager->bUseFetch
   && (pPager->eState==PAGER_READER || (flags & PAGER_ACQUIRE_READONLY))
  );

  assert( pPager->eState>=PAGER_READER );
  assert( assert_pager_state(pPager) );
  assert( noContent==0 || bMmapOk==0 );

    if( bMmapOk && pagerUseWal(pPager) ){
      rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
      if( rc!=SQLITE_OK ) goto pager_acquire_err;
    }

    if( iFrame==0 && bMmapOk ){
      void *pData = 0;
      if( pPager->pMap==0 || (pPager->bMapResize && pPager->nMmapOut==0) ){
        rc = pagerMap(pPager, 0);
      }
      if( rc==SQLITE_OK && pPager->nMap>=((i64)pgno * pPager->pageSize) ){

      rc = sqlite3OsFetch(pPager->fd, 
          (i64)(pgno-1) * pPager->pageSize, pPager->pageSize, &pData
      );

      if( rc==SQLITE_OK && pData ){
        if( pPager->eState>PAGER_READER ){
          (void)sqlite3PcacheFetch(pPager->pPCache, pgno, 0, &pPg);
        }
        if( pPg==0 ){
          rc = pagerAcquireMapPage(pPager, pgno, &pPg);
          rc = pagerAcquireMapPage(pPager, pgno, pData, &pPg);
        }else{
          sqlite3OsUnfetch(pPager->fd, pData);
        }
        if( pPg ){
          assert( rc==SQLITE_OK );
          *ppPage = pPg;
          return SQLITE_OK;
        }
      }

    rc = pagerExclusiveLock(pPager);
  }

  /* Open the connection to the log file. If this operation fails, 
  ** (e.g. due to malloc() failure), return an error code.
  */
  if( rc==SQLITE_OK ){
    rc = sqlite3WalOpen(pPager->pVfs, pPager,
    rc = sqlite3WalOpen(pPager->pVfs,
        pPager->fd, pPager->zWal, pPager->exclusiveMode,
        pPager->journalSizeLimit, &pPager->pWal
    );
  }
  pagerFixMaplimit(pPager);

  return rc;

  assert( assert_pager_state(pPager) );
  assert( pPager->eState==PAGER_OPEN   || pbOpen );
  assert( pPager->eState==PAGER_READER || !pbOpen );
  assert( pbOpen==0 || *pbOpen==0 );
  assert( pbOpen!=0 || (!pPager->tempFile && !pPager->pWal) );

  pagerUnmap(pPager);

  if( !pPager->tempFile && !pPager->pWal ){
    if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;

    /* Close any rollback journal previously open */
    sqlite3OsClose(pPager->jfd);

    rc = pagerOpenWal(pPager);

void sqlite3PagerClearCache(Pager *);
int sqlite3SectorSize(sqlite3_file *);

/* Functions used to truncate the database file. */
void sqlite3PagerTruncateImage(Pager*,Pgno);
int sqlite3PagerSetFilesize(Pager *, i64);

/* Write data to the database file */
int sqlite3PagerWriteData(Pager *pPager, const void *pBuf, int nBuf, i64 iOff);

#if defined(SQLITE_HAS_CODEC) && !defined(SQLITE_OMIT_WAL)
void *sqlite3PagerCodec(DbPage *);
#endif

/* Functions to support testing and debugging. */
#if !defined(NDEBUG) || defined(SQLITE_TEST)
  Pgno sqlite3PagerPagenumber(DbPage*);

** to xWrite().
**
** If xRead() returns SQLITE_IOERR_SHORT_READ it must also fill
** in the unread portions of the buffer with zeros.  A VFS that
** fails to zero-fill short reads might seem to work.  However,
** failure to zero-fill short reads will eventually lead to
** database corruption.
**
** Assuming parameter nNew is non-zero, the xMremap method should attempt
** to memory map a region nNew bytes in size starting at offset iOffset
** of the file.  If successful, it should set *ppMap to point to the
** mapping and return SQLITE_OK. If the file is opened for read-write
** access, then the mapping should also be read-write.
**
** If nOld is non-zero, then the initial value of *ppMap points to a
** mapping returned by a previous call to xMremap. The existing mapping
** is nOld bytes in size and starts at offset iOffset of the file. In
** this case the xMremap method is expected to unmap the existing mapping
** and overwrite *ppMap with the pointer to the new mapping. If nOld is
** zero, then the initial value of *ppMap is undefined.
**
** If nNew is zero, then no new mapping should be created. Any old
** mapping must still be unmapped if nOld is non-zero. If the nOld
** parameter is non-zero, then the existing mapping is always unmapped -
** even if an error occurs.
*/
typedef struct sqlite3_io_methods sqlite3_io_methods;
struct sqlite3_io_methods {
  int iVersion;
  int (*xClose)(sqlite3_file*);
  int (*xRead)(sqlite3_file*, void*, int iAmt, sqlite3_int64 iOfst);
  int (*xWrite)(sqlite3_file*, const void*, int iAmt, sqlite3_int64 iOfst);

  int (*xDeviceCharacteristics)(sqlite3_file*);
  /* Methods above are valid for version 1 */
  int (*xShmMap)(sqlite3_file*, int iPg, int pgsz, int, void volatile**);
  int (*xShmLock)(sqlite3_file*, int offset, int n, int flags);
  void (*xShmBarrier)(sqlite3_file*);
  int (*xShmUnmap)(sqlite3_file*, int deleteFlag);
  /* Methods above are valid for version 2 */
  int (*xMremap)(sqlite3_file *fd, int flags,
      sqlite3_int64 iOff, sqlite3_int64 nOld, sqlite3_int64 nNew, void **ppMap);
  int (*xFetch)(sqlite3_file*, sqlite3_int64 iOfst, int iAmt, void **pp);
  int (*xUnfetch)(sqlite3_file*, void *p);
  /* Methods above are valid for version 3 */
  /* Additional methods may be added in future releases */
};

#define SQLITE_MREMAP_EXTEND  0x0001        /* xMremap call may extend file */

/*
** CAPI3REF: Standard File Control Opcodes
**
** These integer constants are opcodes for the xFileControl method
** of the [sqlite3_io_methods] object and for the [sqlite3_file_control()]
** interface.
**

** ^Application can invoke this file-control to have SQLite generate a
** temporary filename using the same algorithm that is followed to generate
** temporary filenames for TEMP tables and other internal uses.  The
** argument should be a char** which will be filled with the filename
** written into memory obtained from [sqlite3_malloc()].  The caller should
** invoke [sqlite3_free()] on the result to avoid a memory leak.
**
** <li>[[SQLITE_FCNTL_MMAP_SIZE]]
** The argument is assumed to point to a value of type sqlite3_int64. An
** advisory maximum amount of this file to memory map in bytes.
**
** </ul>
*/
#define SQLITE_FCNTL_LOCKSTATE               1
#define SQLITE_GET_LOCKPROXYFILE             2
#define SQLITE_SET_LOCKPROXYFILE             3
#define SQLITE_LAST_ERRNO                    4
#define SQLITE_FCNTL_SIZE_HINT               5
#define SQLITE_FCNTL_CHUNK_SIZE              6
#define SQLITE_FCNTL_FILE_POINTER            7
#define SQLITE_FCNTL_SYNC_OMITTED            8
#define SQLITE_FCNTL_WIN32_AV_RETRY          9
#define SQLITE_FCNTL_PERSIST_WAL            10
#define SQLITE_FCNTL_OVERWRITE              11
#define SQLITE_FCNTL_VFSNAME                12
#define SQLITE_FCNTL_POWERSAFE_OVERWRITE    13
#define SQLITE_FCNTL_PRAGMA                 14
#define SQLITE_FCNTL_BUSYHANDLER            15
#define SQLITE_FCNTL_TEMPFILENAME           16
#define SQLITE_FCNTL_GETFD                  17
#define SQLITE_FCNTL_MMAP_SIZE              18

/*
** CAPI3REF: Mutex Handle
**
** The mutex module within SQLite defines [sqlite3_mutex] to be an
** abstract type for a mutex object.  The SQLite core never looks
** at the internal representation of an [sqlite3_mutex].  It only

** An open write-ahead log file is represented by an instance of the
** following object.
*/
struct Wal {
  sqlite3_vfs *pVfs;         /* The VFS used to create pDbFd */
  sqlite3_file *pDbFd;       /* File handle for the database file */
  sqlite3_file *pWalFd;      /* File handle for WAL file */
  Pager *pPager;             /* Pager object */
  u32 iCallback;             /* Value to pass to log callback (or 0) */
  i64 mxWalSize;             /* Truncate WAL to this size upon reset */
  int nWiData;               /* Size of array apWiData */
  int szFirstBlock;          /* Size of first block written to WAL file */
  volatile u32 **apWiData;   /* Pointer to wal-index content in memory */
  u32 szPage;                /* Database page size */
  i16 readLock;              /* Which read lock is being held.  -1 for none */

**
** If the log file is successfully opened, SQLITE_OK is returned and 
** *ppWal is set to point to a new WAL handle. If an error occurs,
** an SQLite error code is returned and *ppWal is left unmodified.
*/
int sqlite3WalOpen(
  sqlite3_vfs *pVfs,              /* vfs module to open wal and wal-index */
  Pager *pPager,                  /* Pager object handle */
  sqlite3_file *pDbFd,            /* The open database file */
  const char *zWalName,           /* Name of the WAL file */
  int bNoShm,                     /* True to run in heap-memory mode */
  i64 mxWalSize,                  /* Truncate WAL to this size on reset */
  Wal **ppWal                     /* OUT: Allocated Wal handle */
){
  int rc;                         /* Return Code */

  pRet->pWalFd = (sqlite3_file *)&pRet[1];
  pRet->pDbFd = pDbFd;
  pRet->readLock = -1;
  pRet->mxWalSize = mxWalSize;
  pRet->zWalName = zWalName;
  pRet->syncHeader = 1;
  pRet->padToSectorBoundary = 1;
  pRet->pPager = pPager;
  pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);

  /* Open file handle on the write-ahead log file. */
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
  rc = sqlite3OsOpen(pVfs, zWalName, pRet->pWalFd, flags, &flags);
  if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
    pRet->readOnly = WAL_RDONLY;

    u32 nBackfill = pInfo->nBackfill;

    /* Sync the WAL to disk */
    if( sync_flags ){
      rc = sqlite3OsSync(pWal->pWalFd, sync_flags);
    }

    /* If the database file is currently smaller than mxPage pages in size,
    ** the call below issues an SQLITE_FCNTL_SIZE_HINT to the OS layer to
    /* If the database may grow as a result of this checkpoint, hint
    ** about the eventual size of the db file to the VFS layer.
    ** inform it that it is likely to grow to that size.
    **
    */
    ** Additionally, if the pager is using mmap(), then the call to 
    ** SetFilesize() guarantees that the mapping is not larger than mxPage
    ** pages. This makes the sqlite3OsTruncate() call below safe - no pages
    ** that are part of the mapped region will be truncated away.  */
    if( rc==SQLITE_OK ){
      i64 nReq = ((i64)mxPage * szPage);
      rc = sqlite3PagerSetFilesize(pWal->pPager, nReq);
    }
      rc = sqlite3OsFileSize(pWal->pDbFd, &nSize);
      if( rc==SQLITE_OK && nSize<nReq ){
        sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_SIZE_HINT, &nReq);
      }
    }


    /* Iterate through the contents of the WAL, copying data to the db file. */
    while( rc==SQLITE_OK && 0==walIteratorNext(pIter, &iDbpage, &iFrame) ){
      i64 iOffset;
      assert( walFramePgno(pWal, iFrame)==iDbpage );
      if( iFrame<=nBackfill || iFrame>mxSafeFrame || iDbpage>mxPage ) continue;
      iOffset = walFrameOffset(iFrame, szPage) + WAL_FRAME_HDRSIZE;
      /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL file */
      rc = sqlite3OsRead(pWal->pWalFd, zBuf, szPage, iOffset);
      if( rc!=SQLITE_OK ) break;
      iOffset = (iDbpage-1)*(i64)szPage;
      testcase( IS_BIG_INT(iOffset) );
      rc = sqlite3PagerWriteData(pWal->pPager, zBuf, szPage, iOffset);
      rc = sqlite3OsWrite(pWal->pDbFd, zBuf, szPage, iOffset);
      if( rc!=SQLITE_OK ) break;
    }

    /* If work was actually accomplished... */
    if( rc==SQLITE_OK ){
      if( mxSafeFrame==walIndexHdr(pWal)->mxFrame ){
        i64 szDb = pWal->hdr.nPage*(i64)szPage;

/* Connection to a write-ahead log (WAL) file. 
** There is one object of this type for each pager. 
*/
typedef struct Wal Wal;

/* Open and close a connection to a write-ahead log. */
int sqlite3WalOpen(
  sqlite3_vfs*, Pager *, sqlite3_file*, const char *, int, i64, Wal**);
int sqlite3WalOpen(sqlite3_vfs*, sqlite3_file*, const char *, int, i64, Wal**);
int sqlite3WalClose(Wal *pWal, int sync_flags, int, u8 *);

/* Set the limiting size of a WAL file. */
void sqlite3WalLimit(Wal*, i64);

/* Used by readers to open (lock) and close (unlock) a snapshot.  A 
** snapshot is like a read-transaction.  It is the state of the database