Hi Anton - in this echo, just ask "All", there's heaps of us out

here...



 AM> Now, what I need to know is how you break a variable down into

 AM> individual bits, or small groups of bits that can be individually

 AM> accessed as though they were part of a structure - the original

 AM> variable. I read somewhere that this can be done so that each part

 AM> doesn't necessarily have to belong to a specific named variable - it

 AM> may have been done with UNIONS (which I don't know much about).



C doesn't seem to support binary variables directly, like the way

it handles decimal or hex values. But here's a small example from

something I'm working on at the moment:



My program has just received an 8 bit byte over a serial link from

a microprocessor sprinkler controller. This byte contains the

active days for a particular solenoid in the lower 7 bits of this

byte, eg if every day was active, the byte would be 01111111

binary, or if only Monday and Tuesday were active, it would be

00110000b. My program wants to turn the byte into a human readable

string, like "SMTWTFS" for the first byte, and " MT    " for the

second one.



(In the following, I name the 8 bits in a byte 0 to 7. The least

significant bit (right hand end) is bit 0, the most significant bit

(left end) is bit 7.)



------



  unsigned char k;  /* could be char or int, uchar is tidier... */

  int j;

  char days[] = "SMTWTFS"; /* string I'll pinch day initials from */

  char outstring[8];       /* destination string */



  k = events[i].pump_days;  /* the byte with the days in it from

                               the comms section of the program.

                               events[] is an array of structures,

                               irrelevant here. */

  for(j=0;j<7;j++)          /* loop through 7 times */

     {

      k = k << 1;           /* shift k left one bit, so bit 6 will be

                               in bit 7's place */

      if(k & 0x80)          /* logical AND k with 10000000 binary,

                               which = 80 hex. Result will be true

                               if bit 7 in k is a 1. */

        { outstring[j] = days[j]} /* do this on true result */

      else

        { outstring[j] = " "}  /* do this on false result */

     }

  outstring[7] = '\0';      /* gotta terminate the string */



------



The gory longhand version, without the left shift loop,

runs something like



------

 if(k & 0x40)

   outstring[0]="S";

 else

   outstring[0]=" ";

 if(k & 0x20)

   outstring[0]="M";

 else

   outstring[0]=" ";

 if(k & 0x10)

   outstring[0]="T";

 else

   outstring[0]=" ";

 if(k & 0x08)

   outstring[0]="W";

 else

   outstring[0]=" ";

 if(k & 0x04)

   outstring[0]="T";

 else

   outstring[0]=" ";

 if(k & 0x02)

   outstring[0]="F";

 else

   outstring[0]=" ";

 if(k & 0x01)

   outstring[0]="S";

 else

   outstring[0]=" ";



 outstring[7] = " ";

------



(note I didn't start with if(k & 0x80) here, cos I haven't left

shifted k at all.)



The way I see it, "one bit" variables aren't needed here, cos it's

quite easy to detect and/or change the state of any bit in a char

or integer variable.



Cheers



--- PPoint 1.88