Re: Parsing binary data in flex

[Tim Van Holder]

Mohammed H Mehkri wrote:
> Is there any way to recognise bits in flex. Suppose I want to recognise
> 15bits of a particular sequence, is it possible to do so?
> 
> I couldnt find anything about it in flex manual nor I could fine
> anything useful on net.

Yes and no.  It's not possible to do this with standard flex.
But, you can override flex's I/O routines such as YYINPUT to
read bytes from the input stream and send a text representation
to flex.
Something like:

char
my_yyinput()
{
static char current_byte[9];
static int current_bit = -1;
  if (current_bit < 0) {
  int byte = fget(yyin);
    if (byte == EOF)
      memset(current_byte, 0, sizeof current_byte);
    else {
      for (int i = 0; i < 8; ++i)
        current_byte[7 - i] = ((byte & (1 << i)) ? '1' : '0');
    }
    current_bit = 0;
  }
  {
  char result = current_byte[current_bit++];
    if (current_bit > 7)
      current_bit = -1;
    return result;
  }
}

Then you can simply recognize bit patterns in flex:

"100111011001101" {
  do_something();
}

Note: this is untested, but I think it _should_ work (although
you may have to also add some mechanism to reset the statics in
my_yyinput()).