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string str, str2;
str2 = str.substr (3,12);

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though, since the resulting string will automatically be null terminated in NXC while it will not be automatically null terminated in C.

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NXT_STATUS cCmdDSArrayAlloc(DS_ELEMENT_ID DSElementID, UWORD Offset, UWORD NewCount)
{
  NXT_STATUS Status = NO_ERR;
  UWORD DVIndex;
  UWORD OldCount;
  UWORD i;
#if VMProfilingCode
  ULONG enterTime= dTimerReadHiRes();
#endif
  NXT_ASSERT(cCmdIsDSElementIDSane(DSElementID));

  //Only arrays are valid here
  //!!! Recommended to upgrade NXT_ASSERT to ERR_INSTR return
  NXT_ASSERT(cCmdDSType(DSElementID) == TC_ARRAY);

  DVIndex = cCmdGetDVIndex(DSElementID, Offset);
  OldCount = DV_ARRAY[DVIndex].Count;

  if(OldCount == NewCount)
    goto allocExit;
  Status = cCmdDVArrayAlloc(DVIndex, NewCount);

  if (Status < NO_ERR)
    goto allocExit;

  if(!IS_AGGREGATE_TYPE(cCmdDSType(INC_ID(DSElementID))))
    goto allocExit;

  if (OldCount > NewCount)
  {
    //Free dope vectors for sub-arrays.
    for (i = NewCount; i < OldCount; i++)
    {
      Status = cCmdFreeSubArrayDopeVectors(INC_ID(DSElementID), ARRAY_ELEM_OFFSET(DVIndex, i));
      if (IS_ERR(Status))
        goto allocExit;
    }
  }
  else if (OldCount < NewCount)
  {
    //Alloc dope vectors for sub-arrays. Set up DVIndexes
    for (i = OldCount; i < NewCount;  i++)
    {
      Status = cCmdAllocSubArrayDopeVectors(INC_ID(DSElementID), ARRAY_ELEM_OFFSET(DVIndex, i));
      if (IS_ERR(Status))
        goto allocExit;
    }
  }

  NXT_ASSERT(cCmdVerifyMemMgr());
allocExit:
#if VMProfilingCode
  memMgrTime += dTimerReadHiRes() - enterTime;
#endif
  return Status;
}

NXT_STATUS cCmdDVArrayAlloc(DV_INDEX DVIndex, UWORD NewCount)
{
  NXT_STATUS Status = NO_ERR;
  UBYTE *pData;
  UWORD ArraySize, InplaceSize;
  UWORD NextDVIndex;
  UWORD OldCount;

  OldCount = DV_ARRAY[DVIndex].Count;

  if (OldCount == NewCount)
  {
    //Nothing to alloc. Return.
    return Status;
  }
  else if (OldCount > NewCount)
  {
    //Already have the space. Shrink inplace.
    DV_ARRAY[DVIndex].Count = NewCount;
        return Status;
  }
  else // need to grow array
  {
    //Calculate new array size
    ArraySize = NewCount * DV_ARRAY[DVIndex].ElemSize;

    //Try growing inplace
    // If the Offset == NOT_AN_OFFSET then the array has never been allocated and can't grow inplace.
    if (DV_ARRAY[DVIndex].Offset != NOT_AN_OFFSET)
    {
      //Get pointer to next dope vector in dataspace
      if (DV_ARRAY[DVIndex].Link != NOT_A_DS_ID)
      {
        NextDVIndex = DV_ARRAY[DVIndex].Link;
        InplaceSize = DV_ARRAY[NextDVIndex].Offset - DV_ARRAY[DVIndex].Offset;
      }
      else
      {
        //Last element in dataspace.
        NXT_ASSERT(DVIndex == VarsCmd.MemMgr.Tail);
        InplaceSize = VarsCmd.DataspaceSize - DV_ARRAY[DVIndex].Offset;
      }

      if (ArraySize <= InplaceSize)
      {
        DV_ARRAY[DVIndex].Count = NewCount;
            return Status;
      }
    }

    //Can't grow inplace, have to allocate new space

    //Make sure we properly align for type
    //!!! This could also overflow memory (make PoolSize > POOL_MAX_SIZE) if we're within 3 bytes of the end.
    //     I don't think it matters because if it does happend, we'll trigger the ERR_MEM below and compact.
    //     During compaction, we'll reclaim these unused bytes.
    //!!! Aligning beginning of ALL arrays to 4 byte address
    ALIGN_TO_MOD(VarsCmd.PoolSize, SIZE_ULONG);
    ALIGN_TO_MOD(VarsCmd.DataspaceSize, SIZE_ULONG);

    if (VarsCmd.PoolSize + ArraySize >= POOL_MAX_SIZE)
    {
      //Not enough memory available
      return ERR_MEM;
    }

    //Get data from end of pool
    pData = VarsCmd.Pool + VarsCmd.PoolSize;
    //Grow pool and dataspace
    VarsCmd.PoolSize += ArraySize;
    VarsCmd.DataspaceSize += ArraySize;

    //Move old Array Data to new allocation
    if(OldCount)
      memmove(pData, VarsCmd.pDataspace + DV_ARRAY[DVIndex].Offset, (UWORD)(DV_ARRAY[DVIndex].ElemSize * OldCount));
    //!!! Clear mem so old mem doesn't contain stale data. Not strictly needed.
#if WIN_DEBUG || defined(ARM_DEBUG)
    memset(VarsCmd.pDataspace + DV_ARRAY[DVIndex].Offset, 0xFF, (UWORD)(DV_ARRAY[DVIndex].ElemSize * OldCount));
#endif
    //Update dope vector
    DV_ARRAY[DVIndex].Offset = pData - VarsCmd.pDataspace;
    DV_ARRAY[DVIndex].Count = NewCount;

    //Move dope vector to end of MemMgr list
    Status = cCmdMemMgrMoveToTail(DVIndex);
    if (IS_ERR(Status))
      return Status;

    NXT_ASSERT(cCmdVerifyMemMgr());
  }

  return Status;
}

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  asm { strsubset src, src, 8, 2 }