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bug#40421: 26.2; (And 27.0.90) Inconsistent fontifying in c-mode

From: Alan Mackenzie
Subject: bug#40421: 26.2; (And 27.0.90) Inconsistent fontifying in c-mode
Date: 4 Apr 2020 09:09:19 -0000
User-agent: tin/2.4.3-20181224 ("Glen Mhor") (UNIX) (FreeBSD/11.3-RELEASE-p3 (amd64))

Hello, Rolf.

In article <mailman.96.1585962545.2644.bug-gnu-emacs@gnu.org> you wrote:

> Recipt:

> emacs -Q

> Insert:

> /* 11 */
> EXTERN int              XML_GetCurrentByteCount(XML_Parser parser);
> /* 12 */
> EXTERN enum XML_Status  XML_SetBase(XML_Parser parser, const XML_Char *base);
> /* 13 */
> EXTERN const XML_Char *  XML_GetBase(XML_Parser parser);
> /* 14 */
> EXTERN int              XML_GetSpecifiedAttributeCount(XML_Parser parser);
> /* 15 */

> and M-x c-mode. The font-look seems randomly. Some EXTERN colered, some
> not, some function names black, some blue.

Yes.  The problem here is that these declarations aren't syntactically
correct C.  To C Mode, "EXTERN" just looks like an ordinary identifier,
not a macro which (presumably) expands to "extern".

I would recommend you to use CC Mode's "noise macro" facility here.
(See the CC Mode manual.)  With this, you would tell CC Mode that
"EXTERN" just "expands to nothing", and thus won't interfere with the
syntactic analysis.  One way to do this is to set c-noise-macro-names in
a hook function, probably c-mode-common-hook.  Another way (if you don't
have many source files) would be to set that variable in a file local
variables section (see the Emacs manual) - for example, put the
following at the end of your C file:

    /* Local Variables: */
    /* c-noise-macro-names: ("EXTERN") */
    /* END: */

.  Then restart C Mode with M-x c-mode.  The fontification should now
look OK.

> The whole (small) file is here
> http://tdom.org/index.html/raw/generic/tdomDecls.h?name=cccb9227. With
> this syntactically correct file (at least gcc and clang consume it
> without complain) there is similiar font-lock flicker around the lines
> commented with 12, 13, 14.

Thanks for taking the trouble to reduce the problem to a minimal test
case.  That's appreciated.

> I report this (and see it) on my working-place emacs-26.2 but have
> verified, that a freshly build emacs-27.0.90 shows the same behaviour.

Alan Mackenzie (Nuremberg, Germany).

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