[Asterisk-Dev] "true" is -1 or 1 in boolean functions ?

[Luigi Rizzo]

On Mon, Jan 16, 2006 at 01:38:22PM -0600, Tilghman Lesher wrote:
> On Monday 16 January 2006 04:59, Luigi Rizzo wrote:
> > i notice some inconsistency in the return values for
> > functions that have a boolean result - some pieces of code
> > use -1, others use 1.
> >
> > I am totally neutral on the topic, but would like if someone
> > could put a note in the CODING-GUIDELINES on which one is
> > the recommended method, because mixing the two things is
> > confusing and error prone, as:
> >
> >  - people looking at the code might think that the difference
> >    means something other than the habit of the original coder;
> >
> >  - people using these functions might do odd things such
> >    as a bitwise AND on the result of such functions and other
> >    operands, which has different results depending on whether -1
> >    or 1 is used (i think i have seen such instances in the code,
> >    sometimes it is convenient, althoguh obfuscating);
> 
> Please show me how you can bitwise AND these two numbers and
> come up with zero.  The only thing that matters is that the result is
> always non-zero.  There simply isn't an error that can be generated
> by mixing these two numbers.

there are several error scenarios - i said "such as a bitwise AND"
because i did not want to enumerate all of them. Anyways, here
are two:

- The XOR of two booleans fails if they can be either +1 or -1.
  To make it work you have to wrap the results into functions
  that generate uniform values. No big deal, but it can be easily
  forgotten and it clutters the code.

- [bitwise and] if you know that 'in asterisk, true is -1' you may be
  tempted to write things like
	if ( (res = (ast_waitdigit(c...) & is_this_true(x...)) ) )
  where the latter is returning true or false, the former is not,
  but the masking suffices, and if the specification of "true"
  is -1 the construct is correct. I wouldn't use such a construct
  myself, but sometimes might be a convenient shorthand.

  With a mixed +1/-1 environment, the above fails.


cheers
luigi