#pragma STRUCT_ALIGN usage

Hi...

I read the document about SPRU187Q.pdf..

In the PDF #pragma STRUCT_ALIGN( type, expression)...

I made a one stuct type like below

typedef  struct RCX_FILE_DOWNLOAD_REQ_DATA_Ttag
{
 unsigned  int                          ulXferType;             
  unsigned  int                          ulMaxBlockSize;         
unsigned  int                            ulFileLength;           
unsigned  int                             ulChannelNo;            
unsigned  short                        usFileNameLength;       
  }  RCX_FILE_DOWNLOAD_REQ_DATA_T;

#pragma STRUCT_ALIGN (RCX_FILE_DOWNLOAD_REQ_DATA_T, 2); 

and then...

Sizeof( RCX_FILE_DOWNLOAD_REQ_DATA_T)  ==>  Value is 20...

I expected it may be the 18...

What's wrong with that..?...