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Old 10-31-2013, 05:28 PM   #1
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Trouble with Makefile dependencies and recipes

Sorry if this is the wrong subforum; mods feel free to move it.

I am having trouble with make not compiling prerequisites according to the rule I've specified. Or perhaps I'm just misunderstanding something.

Suppose I have a Makefile like this:

derp: derp.c derp.h herp.o
        gcc `abunchofflags` -o derp derp.c foo.o `abunchoflibs`

foo: foo.c
        gcc `morestuff` -c foo.c
What I would expect to happen if I run "make derp" is that it first compiles foo.o according to the recipe I put in the Makefile, and then compiles derp. But what it actually does is compile foo.o with
cc     -c -o foo.o foo.c
i.e. NOT with the recipe in the Makefile, and then compiles derp properly. If I run "make foo", it does compile foo.o the way I want. Again, if foo itself has prerequisites it compiles those wrong but foo itself correctly.

Why does this happen? How do I get make to use the compiling recipe I want for prerequisites?
Old 11-02-2013, 05:07 AM   #2
Registered: Aug 2005
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There are two problems I can see with your recipes.

1. foo.o isn't listed as a dependency of derp

2. The target of the second rule is "foo" and not "foo.o". Hence, when make looks for a way of making "foo.o" it doesn't find a suitable rule so uses its own built-in rules for compiling object files from c.
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Old 11-03-2013, 07:00 PM   #3
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And it is incomplete.

derp.o depends on derp.c and presumably the include file.

so the dependency for derp is derp.o, not derp.c - it doesn't matter where the .o comes from, only that it exist.

And normally, the `abunchofflags` would be specified by a "CFLAGS=...." which may be

A `bunchoflibs` is USUALLY specified using a "LIBS=..." and then use $(LIBS) on the build command.

As a bit of configuration options, the object files needed for derp can be collected
into a definition: DERPOBS=derp.o foo.o

This would change your Makefile to:
DERPOBS=derp.o foo.o


derp: $(DERPOBS)
        gcc -oderp $(DERPOBS)

foo.o: foo.c

derp.o: derp.c derp.h
This makes a simple update as the program expands - only additional object needs for the derp program as they get added to only one line. There are even ways to generate the list of header file dependencies automatically ( as the gnu C compiler has an option (-M) to list the header dependencies...

And allow make to do what it does best.
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