Xref: utzoo comp.lang.c:17050 comp.bugs.4bsd:1231
Newsgroups: comp.lang.c,comp.bugs.4bsd
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: C preprocessor: undefined control
Message-ID: <1989Mar20.190946.23190@utzoo.uucp>
Organization: U of Toronto Zoology
References: <435@rna.UUCP>
Date: Mon, 20 Mar 89 19:09:46 GMT

[I am disregarding Dan's request to reply by mail, as I think it is of
general interest and hasn't really been covered before.]

In article <435@rna.UUCP> dan@rna.UUCP (Dan Ts'o) writes:
>  ...	The 4.3BSD complains about the #pragma, even though it should be
>excluded entirely since the #ifdef MSDOS is not satisfied:
>
>#ifdef MSDOS
>	... C code ...
>#pragma loop_opt(off)
>	while(...)
>	...
>#endif
>
>	I naturally see how this could happen, although I do consider it a
>bug (what does ANSI say about this sort of thing). Is there any way around
>this ? ...

As I read the fine print in the Oct. draft, an unrecognized thing that
looks like a directive, in a section that is being skipped, is not in
fact a directive and therefore should not draw a complaint, except perhaps
a warning message from an excessively-clever compiler.

An ANSI-compatible compiler will ignore a #pragma in that position (well,
probably... the old "can #pragma change the rules?" question can make
trouble here).

As to how to get around it, try putting white space in front of the #
that starts the #pragma.  An ANSI-compatible compiler will recognize
it even so, and with luck your MeSsDOS compiler will too, but the old
compilers won't see it unless the # is first on the line.
-- 
Welcome to Mars!  Your         |     Henry Spencer at U of Toronto Zoology
passport and visa, comrade?    | uunet!attcan!utzoo!henry henry@zoo.toronto.edu