Re: Odd make question

[Paul D. Smith]

%% "Marcus A Martin" <address@hidden> writes:

  mam> What I need to do is be able to pass only the source files that
  mam> have been changed into the compile command. So far I have only
  mam> found a way to add in all or none. The gmake structure is basicly
  mam> this this:

  mam> SOURCE = foo.c bar.c

  mam> $(OBJS): $(SOURCES)
  mam>  $(CC) $^

First, this isn't what you want.  Multiple targets on the left side of
the ":" mean that each one can be built with the same rule.  So, the
above line (assuming OBJS is "foo.o bar.o") is the same as writing:

  foo.o: foo.c bar.c
        $(CC) $^
  bar.o: foo.c bar.c
        $(CC) $^

which is clearly _not_ what you want.  This will be even slower, since
it will recompile every .o whenever _any_ .c changes.

  mam> In theory, the $^ should only get source files that have been
  mam> changed since the last compile, but it always evaluates to
  mam> null.

I don't see how that can be.

The "$^" does _NOT_ evaluate to the files that have changed, it
evaluates to _all_ the prerequisites defined for that target.  It should
always be equal to the value of $(SOURCES).  Note this assumes you're
using GNU make; some other makes don't define the automatic variables
like $<, etc. for explicit rules (for some completely incomprehensible
reason).

The variable you want is "$?", which resolves to just the files that
have changed.

See the GNU make manual section on automatic variables.

To do what you want you'll have to use a dummy file.  Something like
this might work:

  dummy.x: $(SOURCES)
        $(CC) -c $?
        touch dummy.x

  foobar.a: dummy.x
        $(SHARED_LIB_LINKER) $(OBJS) $(OUTFILE)

Obviously this is using UNIX syntax; I'm not familiar enough with
Windows to translate stuff like "touch".

-- 
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 Paul D. Smith <address@hidden>          Find some GNU make tips at:
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 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist