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 */
  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 */

  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 */
  u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
  u8 writeLock;              /* True if in a write transaction */
  u8 ckptLock;               /* True if holding a checkpoint lock */
  u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
................................................................................
    assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
    testcase( (rc&0xff)==SQLITE_IOERR );
    testcase( rc==SQLITE_PROTOCOL );
    testcase( rc==SQLITE_OK );
  }
  return rc;
}



































/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
................................................................................
    WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }
  assert( (int)pWal->szPage==szPage );





  /* Write the log file. */
  for(p=pList; p; p=p->pDirty){
    u32 nDbsize;                  /* Db-size field for frame header */
    i64 iOffset;                  /* Write offset in log file */
    void *pData;
   
    iOffset = walFrameOffset(++iFrame, szPage);
................................................................................
    nDbsize = (isCommit && p->pDirty==0) ? nTruncate : 0;
#if defined(SQLITE_HAS_CODEC)
    if( (pData = sqlite3PagerCodec(p))==0 ) return SQLITE_NOMEM;
#else
    pData = p->pData;
#endif
    walEncodeFrame(pWal, p->pgno, nDbsize, pData, aFrame);
    rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOffset);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Write the page data */
    rc = sqlite3OsWrite(pWal->pWalFd, pData, szPage, iOffset+sizeof(aFrame));
    if( rc!=SQLITE_OK ){
      return rc;
    }
    pLast = p;
  }

  /* Sync the log file if the 'isSync' flag was specified. */
................................................................................
#if defined(SQLITE_HAS_CODEC)
      if( (pData = sqlite3PagerCodec(pLast))==0 ) return SQLITE_NOMEM;
#else
      pData = pLast->pData;
#endif
      walEncodeFrame(pWal, pLast->pgno, nTruncate, pData, aFrame);
      /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
      rc = sqlite3OsWrite(pWal->pWalFd, aFrame, sizeof(aFrame), iOffset);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      iOffset += WAL_FRAME_HDRSIZE;
      rc = sqlite3OsWrite(pWal->pWalFd, pData, szPage, iOffset); 
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nLast++;
      iOffset += szPage;
    }

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 */
  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 */
  u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
  u8 writeLock;              /* True if in a write transaction */
  u8 ckptLock;               /* True if holding a checkpoint lock */
  u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
................................................................................
    assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
    testcase( (rc&0xff)==SQLITE_IOERR );
    testcase( rc==SQLITE_PROTOCOL );
    testcase( rc==SQLITE_OK );
  }
  return rc;
}

/*
** Write iAmt bytes of content into the WAL file beginning at iOffset.
**
** When crossing the boundary between the first and second sectors of the
** file, first write all of the first sector content, then fsync(), then
** continue writing content for the second sector.  This ensures that
** the WAL header is overwritten before the first commit mark.
*/
static int walWriteToLog(
  Wal *pWal,                 /* WAL to write to */
  void *pContent,            /* Content to be written */
  int iAmt,                  /* Number of bytes to write */
  sqlite3_int64 iOffset      /* Start writing at this offset */
){
  int rc;
  if( iOffset>=pWal->szFirstBlock || iOffset+iAmt<pWal->szFirstBlock ){
    /* The common and fast case.  Just write the data. */
    rc = sqlite3OsWrite(pWal->pWalFd, pContent, iAmt, iOffset);
  }else{
    /* If this write will cross the first sector boundary, it has to
    ** be split it two with a sync in between. */
    int iFirstAmt = pWal->szFirstBlock - iOffset;
    assert( iFirstAmt>0 && iFirstAmt<iAmt );
    rc = sqlite3OsWrite(pWal->pWalFd, pContent, iFirstAmt, iOffset);
    if( rc ) return rc;
    rc = sqlite3OsSync(pWal->pWalFd, SQLITE_SYNC_NORMAL);
    if( rc ) return rc;
    pContent = (void*)(iFirstAmt + (char*)pContent);
    rc = sqlite3OsWrite(pWal->pWalFd, pContent,
                        iAmt-iFirstAmt, iOffset+iFirstAmt);
  }
  return rc;
}

/* 
** Write a set of frames to the log. The caller must hold the write-lock
** on the log file (obtained using sqlite3WalBeginWriteTransaction()).
*/
int sqlite3WalFrames(
  Wal *pWal,                      /* Wal handle to write to */
................................................................................
    WALTRACE(("WAL%p: wal-header write %s\n", pWal, rc ? "failed" : "ok"));
    if( rc!=SQLITE_OK ){
      return rc;
    }
  }
  assert( (int)pWal->szPage==szPage );

  /* The size of the block containing the WAL header */
  pWal->szFirstBlock = sqlite3OsSectorSize(pWal->pWalFd);
  if( szPage>pWal->szFirstBlock ) pWal->szFirstBlock = szPage;

  /* Write the log file. */
  for(p=pList; p; p=p->pDirty){
    u32 nDbsize;                  /* Db-size field for frame header */
    i64 iOffset;                  /* Write offset in log file */
    void *pData;
   
    iOffset = walFrameOffset(++iFrame, szPage);
................................................................................
    nDbsize = (isCommit && p->pDirty==0) ? nTruncate : 0;
#if defined(SQLITE_HAS_CODEC)
    if( (pData = sqlite3PagerCodec(p))==0 ) return SQLITE_NOMEM;
#else
    pData = p->pData;
#endif
    walEncodeFrame(pWal, p->pgno, nDbsize, pData, aFrame);
    rc = walWriteToLog(pWal, aFrame, sizeof(aFrame), iOffset);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* Write the page data */
    rc = walWriteToLog(pWal, pData, szPage, iOffset+sizeof(aFrame));
    if( rc!=SQLITE_OK ){
      return rc;
    }
    pLast = p;
  }

  /* Sync the log file if the 'isSync' flag was specified. */
................................................................................
#if defined(SQLITE_HAS_CODEC)
      if( (pData = sqlite3PagerCodec(pLast))==0 ) return SQLITE_NOMEM;
#else
      pData = pLast->pData;
#endif
      walEncodeFrame(pWal, pLast->pgno, nTruncate, pData, aFrame);
      /* testcase( IS_BIG_INT(iOffset) ); // requires a 4GiB WAL */
      rc = walWriteToLog(pWal, aFrame, sizeof(aFrame), iOffset);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      iOffset += WAL_FRAME_HDRSIZE;
      rc = walWriteToLog(pWal, pData, szPage, iOffset);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      nLast++;
      iOffset += szPage;
    }