SQLite

  u8 id;                     /* Id of this connection within its unixShmNode */
#endif
};

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE      81        /* Byte offset of the first lock byte */
#define UNIX_SHM_DMS       80        /* The deadman switch lock */

#ifdef SQLITE_DEBUG
/*
** Return a pointer to a nul-terminated string in static memory that
** describes a locking mask.  The string is of the form "MSABCD" with
** each character representing a lock.  "M" for MUTEX, "S" for DMS, 

  /* Access to the unixShmNode object is serialized by the caller */
  assert( sqlite3_mutex_held(pShmNode->mutex) || pShmNode->nRef==0 );

  /* Shared locks never span more than one byte */
  assert( n==1 || lockType!=F_RDLCK );

  /* Locks are within range */
  assert( n>=1 && n<SQLITE_SHM_NLOCK );

  /* Initialize the locking parameters */
  memset(&f, 0, sizeof(f));
  f.l_type = lockType;
  f.l_whence = SEEK_SET;
  f.l_start = ofst;
  f.l_len = n;

  rc = fcntl(pShmNode->h, F_SETLK, &f);
  rc = (rc!=(-1)) ? SQLITE_OK : SQLITE_BUSY;

  /* Update the global lock state and do debug tracing */
#ifdef SQLITE_DEBUG

  unixShm *pX;                          /* For looping over all siblings */
  unixShmNode *pShmNode = p->pShmNode;  /* The underlying file iNode */
  int rc = SQLITE_OK;                   /* Result code */
  u16 mask;                             /* Mask of locks to take or release */

  assert( pShmNode==pDbFd->pInode->pShmNode );
  assert( pShmNode->pInode==pDbFd->pInode );
  assert( ofst>=0 && ofst+n<=SQLITE_SHM_NLOCK );
  assert( n>=1 );
  assert( flags==(SQLITE_SHM_LOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED)
       || flags==(SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE) );
  assert( n==1 || (flags & SQLITE_SHM_EXCLUSIVE)!=0 );

  mask = (1<<(ofst+n)) - (1<<ofst);
  assert( n>1 || mask==(1<<ofst) );
  sqlite3_mutex_enter(pShmNode->mutex);
  if( flags & SQLITE_SHM_UNLOCK ){
    u16 allMask = 0; /* Mask of locks held by siblings */

    /* See if any siblings hold this same lock */
    for(pX=pShmNode->pFirst; pX; pX=pX->pNext){
      if( pX==p ) continue;
      assert( (pX->exclMask & (p->exclMask|p->sharedMask))==0 );
      allMask |= pX->sharedMask;
    }

    /* Unlock the system-level locks */
    if( (mask & allMask)==0 ){
      rc = unixShmSystemLock(pShmNode, F_UNLCK, ofst+UNIX_SHM_BASE, n);
    }else{
      rc = SQLITE_OK;
    }

    /* Undo the local locks */
    if( rc==SQLITE_OK ){
      p->exclMask &= ~mask;

      }
      allShared |= pX->sharedMask;
    }

    /* Get shared locks at the system level, if necessary */
    if( rc==SQLITE_OK ){
      if( (allShared & mask)==0 ){
        rc = unixShmSystemLock(pShmNode, F_RDLCK, ofst+UNIX_SHM_BASE, n);
      }else{
        rc = SQLITE_OK;
      }
    }

    /* Get the local shared locks */
    if( rc==SQLITE_OK ){

      }
    }
  
    /* Get the exclusive locks at the system level.  Then if successful
    ** also mark the local connection as being locked.
    */
    if( rc==SQLITE_OK ){
      rc = unixShmSystemLock(pShmNode, F_WRLCK, ofst+UNIX_SHM_BASE, n);
      if( rc==SQLITE_OK ){
        p->sharedMask &= ~mask;
        p->exclMask |= mask;
      }
    }
  }
  sqlite3_mutex_leave(pShmNode->mutex);

  /* Only one writer allowed at a time.  Get the write lock.  Return
  ** SQLITE_BUSY if unable.
  */
  rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1);
  if( rc ){
    return rc;
  }
  pWal->writeLock = 1;

  /* If another connection has written to the database file since the
  ** time the read transaction on this connection was started, then
  ** the write is disallowed.
  */
  rc = walIndexMap(pWal, pWal->hdr.mxFrame);
  if( rc ){
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;
    return rc;
  }
  if( memcmp(&pWal->hdr, (void*)pWal->pWiData, sizeof(WalIndexHdr))!=0 ){
    walUnlockExclusive(pWal, WAL_WRITE_LOCK, 1);
    pWal->writeLock = 0;
    walIndexUnmap(pWal);
    return SQLITE_BUSY;
  }

  pInfo = walCkptInfo(pWal);
  if( pWal->readLock==0 && pInfo->nBackfill==pWal->hdr.mxFrame ){
    rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);

      pInfo->nBackfill = 0;
      memset(&pInfo->aReadMark[1], 0, sizeof(pInfo->aReadMark)-sizeof(u32));
      rc = sqlite3OsTruncate(pWal->pDbFd, 
                             ((i64)pWal->hdr.nPage*(i64)pWal->szPage));
      walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
    }
    walUnlockShared(pWal, WAL_READ_LOCK(0));
    pWal->readLock = -1;
    do{
      int notUsed;
      rc = walTryBeginRead(pWal, &notUsed, 1);
    }while( rc==WAL_RETRY );
  }
  return rc;
}