Re: let me rephrase that -- question about empty commands

[Paul D. Smith]

%% Noel Yap <address@hidden> writes:

  ny> Upon re-reading this portion of the paper, I think I may have
  ny> misunderstood exactly what the bug is:

  ny> Quick note: why do we have ":" as the command to be run? That's
  ny> the Bourne shell's "do nothing" operator. GNU make is actually
  ny> smart enough to notice that your rule consists of just the "do
  ny> nothing" operator, and not exec a shell. Why don't we just use an
  ny> empty command (a semicolon with nothing after it)? We should be
  ny> able to, but if you do then GNU make will print a bunch of
  ny> warnings about "nothing to do". This is actually a bug in GNU
  ny> make, but this workaround does the job until it's fixed.

Right.  This has nothing to do with anything: it's to avoid a message
that should not be printed, but there is no change in behavior with or
without it.

  ny> Anyway, the behaviour I had seen was when I had somithing like the
  ny> following rules:

  ny>   .SUFFIXES: .pm _wrap.cc
  ny>   $(build.DIR)/common/%.pm: $(build.DIR)/common/%_wrap.cc

First note that all your uses of .SUFFIXES here are useless: .SUFFIXES
is _ONLY_ used for suffix rules.  These are pattern rules, so you don't
need to set .SUFFIXES, and the value of .SUFFIXES isn't used.

  ny>   .SUFFIXES: .pm _wrap.cc
  ny>   $(build.DIR)/common/%.pm: $(build.DIR)/common/%_wrap.cc
  ny>         @:

  ny> Is this expected behaviour?

Yes, this is correct behavior.  This is a pattern rule, not an explicit
rule like the example in the whitepaper.  Pattern rules work
differently.

See the section "Canceling Implicit Rules" in the GNU make manual.

-- 
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