Re: Problem using -R in MAKEFLAGS

[John Graham-Cumming]

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
        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:

        .SUFFIXES:

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.
--
John Graham-Cumming
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