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makefileautotoolsautoconf

How to deal with autoconf warning "'%'-style pattern rules are a GNU make extension"

In a Makefile with 

%.o: %.c
    @echo Compiling $< ...

I'm getting the warning '%'-style pattern rules are a GNU make extension when I run autoreconf --install (of autoconf version 2.69). The warning is not very clear, but makes me think that there's something to be added to configure.ac.

I conducted searches on google.com, duckduckgo.com and yahoo.com, but they all don't seem to be able to differentiate between the large number of build logs they index (why should they...) which makes the search painful. I figured that:

  • I can silence the warning by adding AM_INIT_AUTOMAKE([-Wno-portability]) to configure.ac (found in a post of the openais mailing list) which seems not great because simply silencing a warning is generally not a good idea in a technical environment - please tell me if GNU autotools is an exception.
Solution

  • Replace

    %.o: %.c

    with

    .c.o:

    That's a suffix rule doing the same thing, and it's more portable. If your rule involves a suffix that is not known to make, list it in the prerequisites for the special .SUFFIXES target:

    .SUFFIXES: .hack .win

.hack.win:
      # build a .win file from a .hack file.

    More on how this works in detail here. They recommend to use pattern rules instead of suffix rules because they're clearer and more general, which is true, but as autoconf notes, they are indeed less portable. So if that is a worry (if you want to build on BSD/Mac OS and not install GNU make, basically), fall back on the "old-fashioned suffix rules."

    If you have a pattern rule that cannot be replaced by a suffix rule, another possible replacement that automake doesn't complain about is a static pattern rule. This is similar to a pattern rule but requires a list of targets it applies to. Instead of saying

    %.o: %.c

    You would have to say

    OBJS = foo.o bar.o baz.o # list all .o files here

$(OBJS): %.o: %.c

    Or more generally,

    target-pattern: prerequisite-pattern

    is replaced by

    target-list: target-pattern: prerequisite-pattern