[TUHS] When was #if introduced in C? (was: Re: Mac OS X is Unix)

Clem Cole clemc at ccc.com
Wed Jan 4 07:56:38 AEST 2017

The cpp.c code in 6th edition only supported #ifdef.   IIRC the v7 cpp had
#if, but it may have been in typesetter C.   I think mashey needed it for
SCCS (I don't have sources easy to look at PWB 1.0), so my guess is it was
developed during that transition.  The key is that  PWB 1.0 was a v6++
system.   UNIX/TS and later V7 had the newer kernel.   PWB 2.0 was based on
that one.    The typesetter compiler was developed in parallel to those
projects, which also put constraints on the language.


On Tue, Jan 3, 2017 at 4:33 PM, Michael Kjörling <michael at kjorling.se>

> On 3 Jan 2017 13:05 -0800, from charles.unix.pro at gmail.com (Charles
> Anthony):
> > I was compiling on a 32 bit int machine; the compiler flagged the '1u <<
> > 60' as a fatal error due to the size of the shift -- on this compiler the
> > expression evaluator was running before the dead code remover.
> That was my thought too; the only way to guarantee that the code is
> removed before the compiler sees it is to do so through the
> preprocessor, thus #ifdef.
> Of course, #ifdef is rather limited. The #if preprocessor directive is
> more generic, but still significantly less versatile than the if()
> language keyword.
> Which makes me curious... Does anyone here happen to know when #if was
> introduced in C? I suspect #ifdef came earlier simply by virtue of
> being (at least to a naiive first approximation) far easier to
> implement, as all that would be required would be to look at the macro
> expansion table (already required by #define) and see if that
> particular name had previously been #defined, as opposed to actually
> evaluating an expression.
> --
> Michael Kjörling • https://michael.kjorling.semichael at kjorling.se
>                  “People who think they know everything really annoy
>                  those of us who know we don’t.” (Bjarne Stroustrup)
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