Re: Manual: Multiple outputs

[Dmitry Goncharov]

On Mon, May 3, 2021 at 8:57 AM Paul Smith <psmith@gnu.org> wrote:
>   data.h: data.c ;
>
> (note the extra semicolon).  Now it will work.

Ofcourse, when data.c is present and data.h is missing, then make will
not recreate data.h. It'll run the empty rule.

Your original makefile
data.h data.c: data.foo
        touch data.h data.c
data.h: data.c

is the same as

data.h: data.foo
        touch data.h data.c

data.c: data.foo
        touch data.h data.c

data.h: data.c

See what this makefile tells make to do.
This makefile tells make that when data.c is missing make needs to
touch data.h data.c. Sure this creates both files.
Same for data.h.
You'll need to either rewrite the makefile to use grouped targets as
Paul suggested, or use pattern rules, as you figured out.
i'd look if it was possible to rewrite like

all: data.h data.c
data.h: data.foo
        touch $@
data.c: data.foo
        touch $@
data.c: data.h

regards, Dmitry