C preprocessor: undefined control

[Henry Spencer]

[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 at rna.UUCP> dan at 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.
-- 
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