fread(realimbuf,1,163840,rdfile) data wrong problem

hello,

 my program  some like that:

unsigned  short realimbuf[163840];

...

FILE * rdfile;

rdfile=fopen("....","r");// read an image data

unsigned int  filesize;

filesize=fread(realimbuf,1,163840,rdfile);

...

but 

debug  :     find   filesize value is   149180 ；

 memory  : realimbuf    only have a part of  image data   ,  some  data have not read in this memory  ,   so  what  is the problem ?

i  hope some people  tell  me  the reason.

thank  all!

thank!

                                                      luo