Re: Confused about gram.y referencs in Makefile?

Japin Li <japinli(at)hotmail(dot)com> writes:
> I find src/backend/utils/mb/Unicode/Makefile has the following comments:

>> # Note that while each script call produces two output files, to be
>> # parallel-make safe we need to split this into two rules. (See for
>> # example gram.y for more explanation.)

> I could not find the explanation in gram.y easily. Would someone point
> it out for me? Thanks in advance!

It's referring to this bit in src/backend/parser/Makefile:

-----
# There is no correct way to write a rule that generates two files.
# Rules with two targets don't have that meaning, they are merely
# shorthand for two otherwise separate rules. If we have an action
# that in fact generates two or more files, we must choose one of them
# as primary and show it as the action's output, then make all of the
# other output files dependent on the primary, like this. Furthermore,
# the "touch" action is essential, because it ensures that gram.h is
# marked as newer than (or at least no older than) gram.c. Without that,
# make is likely to try to rebuild gram.h in subsequent runs, which causes
# failures in VPATH builds from tarballs.

gram.h: gram.c
touch $@

gram.c: BISONFLAGS += -d
gram.c: BISON_CHECK_CMD = $(PERL) $(srcdir)/check_keywords.pl $< $(top_srcdir)/src/include/parser/kwlist.h
-----

This is indeed kind of confusing, because there's no explicit
reference to gram.y here --- the last two lines just tweak
the behavior of the default .y to .c rule.

Maybe we should adjust the comment in Unicode/Makefile, but
I'm not sure what would be a better reference.

regards, tom lane