[c99, cpp]: Use fallthrough attribute

[lb90]

This uncovers a small issue when compiling with CLang:

/bin/sh ../libtool  --tag=CC   --mode=link clang  -g -O2   -o stage1flex stage1flex-scan.o stage1flex-buf.o stage1flex-ccl.o stage1flex-dfa.o stage1flex-ecs.o stage1flex-filter.o stage1flex-gen.o stage1flex-main.o stage1flex-misc.o stage1flex-nfa.o stage1flex-options.o stage1flex-parse.o stage1flex-regex.o stage1flex-scanflags.o stage1flex-scanopt.o stage1flex-skeletons.o stage1flex-sym.o stage1flex-tables.o stage1flex-tables_shared.o stage1flex-tblcmp.o stage1flex-yylex.o     -lm 
libtool: link: clang -g -O2 -o stage1flex stage1flex-scan.o stage1flex-buf.o stage1flex-ccl.o stage1flex-dfa.o stage1flex-ecs.o stage1flex-filter.o stage1flex-gen.o stage1flex-main.o stage1flex-misc.o stage1flex-nfa.o stage1flex-options.o stage1flex-parse.o stage1flex-regex.o stage1flex-scanflags.o stage1flex-scanopt.o stage1flex-skeletons.o stage1flex-sym.o stage1flex-tables.o stage1flex-tables_shared.o stage1flex-tblcmp.o stage1flex-yylex.o  -lm
( cd . && /mnt/store/flex-clang/src/stage1flex \
    -o scan.c -t scan.l ) >stage1scan.c || \
{ s=$?; rm -f stage1scan.c; exit $s; }
depbase=`echo stage1scan.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`;\
clang -DHAVE_CONFIG_H -I.  -DLOCALEDIR=\"/usr/share/locale\"   -g -O2 -MT stage1scan.o -MD -MP -MF $depbase.Tpo -c -o stage1scan.o stage1scan.c &&\
mv -f $depbase.Tpo $depbase.Po
scan.c:5240:2: error: fallthrough annotation does not directly precede switch label
 5240 |         __attribute__((fallthrough));
      |         ^

Here's the generated code:

stage1scan.c:

	case YY_STATE_EOF(OPTION):

	#ifdef __clang__
	__attribute__((fallthrough));
	#endif
	/* FALLTHROUGH */

	case YY_STATE_EOF(LINEDIR):

	#ifdef __clang__
	__attribute__((fallthrough));
	#endif
	/* FALLTHROUGH */                 /* issue: FALLTHROUGH but there's a statement below */

		yyterminate();

			case YY_END_OF_BUFFER:
			{
				/* Amount of text matched not including the EOB char. */
				int yy_amount_of_matched_text = (int) (yy_cp - (yytext_ptr)) - 1;

The error should happen also with GCC should we use [[fallthrough]] (since C++17, C23)

Fixes https://github.com/westes/flex/issues/427

[Mightyjo]

Please use the hooks instead of defining new yy* functions in C. We're in the process of making the skeleton code generation language agnostic.

You can still include the feature check in the macro definition. Quoting can be a little is all.

[lb90]

Thanks! I have also fixed the build error by not emitting EOF_STATE_CASE_FALLTHROUGH before EOF_STATE_CASE_TERMINATE: https://github.com/westes/flex/pull/709/commits/0aac7c32.

An other way is to add an explicit EOF_STATE_CASE_FALLTHROUGH_TO_TERMINATE