Re: bitfields

From: Pink Floyd (
Date: 11/26/95

   In message <> you said:
   > ...
   > Also, you can put in many blanks for future expansion by doing this:
   > ...
   >  aff_hide : 1
   >  extras   : 100
   > }
   >  then to add a new one, simply:
   > ...
   >  aff_hide : 1
   >  aff_new  : 1
   >  extras   : 99  /* pfile is not corrupted */
   > }
   >  The size of the bitfield in bytes will be the number of bits allocated, 
   > divided by 8, rounded up.

   This is a false assumption.  From Kernighan & Ritchie, "All aspects of
   the implementation of bitfields are undefined."  That may not be an exact
   quote as I don't have my copy in front of me, but you get the idea.  PFor
   all you know, the compiler may be allocating an entire byte for each
   bitfield variable!  Granted, I am proposing an extreme case, but you never
   know.  For that same reason, you can not assume that the two structures
   listed above will be structured identically in memory.

I was under the impression that you can do the following with a bitfield:
Say you want to read in a status byte from a piece of hardware, but are only
interested in, say, the 5th and 6th bytes.  You can:
struct status_byte {
  unsigned buf      :   4;  /* this will hold the first 4 bytes */
  unsigned status_5 :   1;
  unsigned status_6 :   1;
  unsigned buf2     :   2;
This will accurately allow you to get just the 5th and 6th bytes, or at least
it should, otherwise bitfields wouldn't be nearly as useful! So I guess I'm
not sure, will it?

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