SQLite

    sizeof(pPager) +                     /* Space to hold a pointer */
    4 +                                  /* Database prefix */
    nPathname + 1 +                      /* database filename */
    nUriByte +                           /* query parameters */
    nPathname + 8 + 1 +                  /* Journal filename */
#ifndef SQLITE_OMIT_WAL
    nPathname + 4 + 1 +                  /* WAL filename */

#endif
    3                                    /* Terminator */
  );
  assert( EIGHT_BYTE_ALIGNMENT(SQLITE_INT_TO_PTR(journalFileSize)) );
  if( !pPtr ){
    sqlite3DbFree(0, zPathname);
    return SQLITE_NOMEM_BKPT;

    pPager->zWal = (char*)pPtr;
    memcpy(pPtr, zPathname, nPathname);   pPtr += nPathname;
    memcpy(pPtr, "-wal", 4);              pPtr += 4 + 1;
#ifdef SQLITE_ENABLE_8_3_NAMES
    sqlite3FileSuffix3(zFilename, pPager->zWal);
    pPtr = (u8*)(pPager->zWal + sqlite3Strlen30(pPager->zWal)+1);
#endif


  }else{
    pPager->zWal = 0;
  }
#endif

  if( nPathname ) sqlite3DbFree(0, zPathname);
  pPager->pVfs = pVfs;

  i64 mxWalSize,                  /* Truncate WAL to this size on reset */
  int bWal2,                      /* True to open in wal2 mode */
  Wal **ppWal                     /* OUT: Allocated Wal handle */
){
  int rc;                         /* Return Code */
  Wal *pRet;                      /* Object to allocate and return */
  int flags;                      /* Flags passed to OsOpen() */
  int nWalName;                   /* Length of zWalName in bytes */
  int nByte;                      /* Bytes of space to allocate */

  assert( zWalName && zWalName[0] );
  assert( pDbFd );

  /* In the amalgamation, the os_unix.c and os_win.c source files come before
  ** this source file.  Verify that the #defines of the locking byte offsets
  ** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
  ** For that matter, if the lock offset ever changes from its initial design
  ** value of 120, we need to know that so there is an assert() to check it.
  */
  assert( 120==WALINDEX_LOCK_OFFSET );
  assert( 136==WALINDEX_HDR_SIZE );
#ifdef WIN_SHM_BASE
  assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif
#ifdef UNIX_SHM_BASE
  assert( UNIX_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif

  nWalName = sqlite3Strlen30(zWalName);
  nByte = sizeof(Wal) + pVfs->szOsFile*2 + nWalName+2;

  /* Allocate an instance of struct Wal to return. */
  *ppWal = 0;
  pRet = (Wal*)sqlite3MallocZero(nByte);
  if( !pRet ){
    return SQLITE_NOMEM_BKPT;
  }

  pRet->pVfs = pVfs;
  pRet->apWalFd[0] = (sqlite3_file*)((char*)pRet+sizeof(Wal));
  pRet->apWalFd[1] = (sqlite3_file*)((char*)pRet+sizeof(Wal)+pVfs->szOsFile);
  pRet->pDbFd = pDbFd;
  pRet->readLock = WAL_LOCK_NONE;
  pRet->mxWalSize = mxWalSize;
  pRet->zWalName = zWalName;
  pRet->syncHeader = 1;
  pRet->padToSectorBoundary = 1;
  pRet->exclusiveMode = (bNoShm ? WAL_HEAPMEMORY_MODE: WAL_NORMAL_MODE);
  pRet->bWal2 = bWal2;

  pRet->zWalName2 = (char*)pRet + sizeof(Wal) + 2*pVfs->szOsFile;
  memcpy(pRet->zWalName2, zWalName, nWalName);
  pRet->zWalName2[nWalName] = '2';
  pRet->zWalName2[nWalName+1] = '\0';

  /* Open a file handle on the first write-ahead log file. */
  flags = (SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE|SQLITE_OPEN_WAL);
  rc = sqlite3OsOpen(pVfs, zWalName, pRet->apWalFd[0], flags, &flags);
  if( rc==SQLITE_OK && flags&SQLITE_OPEN_READONLY ){
    pRet->readOnly = WAL_RDONLY;
  }