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Re: Problem using -R in MAKEFLAGS

From: John Graham-Cumming
Subject: Re: Problem using -R in MAKEFLAGS
Date: Mon, 06 Nov 2006 17:17:01 +0100
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Philip Guenther wrote:
You can't affect how make processes a makefile by setting MAKEFLAGS in
the makefile itself.  Currently, make sets all the built-in variables
and rules before parsing the makefile, and that can be tested by the
makefile itself, so changing it would create all sorts of problems.

That's not actually totally true. Page 50 of the GNU Make manual states: 'You can also set MAKEFLAGS in a makefile, to specify additional flags that should be in effect for that makefile.'

For example,

        MAKEFLAGS += -n

        .PHONY: all
                @echo Doing all

will print 'echo Doing all' because the -n is in effect for that Makefile. Unfortunately, as you do state, there are some flags that will not work when MAKEFLAGS is set inside a Makefile. The source of GNU Make shows that MAKEFLAGS is actually parsed twice: once before handling the Makefile and once after:

/* Decode switches again, in case the variables were set by the makefile. */
  decode_env_switches (STRING_SIZE_TUPLE ("MAKEFLAGS"));

The flag you want to set for no built in rules is -r (note that the original message errorneously talked about -R which is for built in variables although it does force -r). This translates to the no_builtin_rules_flag variable in GNU Make's source.

Unfortunately, this flag is used to control the setting up of the built in rules BEFORE the Makefiles are parsed (as you state) and hence you can't add -r to MAKEFLAGS in a Makefile and have it affect that Makefile. This is done so that new rules in the Makefile can override the default rules as necessary.

Most of the command-line switches will work if specified in MAKEFLAGS in the Makefile and affect that Makefile (you can try out silly examples like -v and -p to see the effect).

Now, to actually solve your problem (disabling default rules in a Makefile), there's a simple technique: you clear the default suffix list for built-in rules. This will prevent any of the built-in rules from being searched and you can go ahead and define your own pattern rules.

So simply write:


in the Makefile where you want to disable built-in rules. This won't kill the built-in variables, but they are probably less of a worry.

John Graham-Cumming


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