Re: targets in subdirectories

[Kristof Provost]

kilgore trout wrote:
> Greetings,
>  
> I am using gmake via Msys on a win xp box.
> The project that I am trying to build, has a 'build_dir' off of the
> root of the project tree which is where I am to keep all the object
> files.  I am having a problem writing a rule that will correctly
> detect if that file is missing/up-to-date.  I have, for example:
>  
> BUILD_DIR=$(PROJ_ROOT)/build_dir
>  
> $(BUILD_DIR)/%.o : %.c
>     mkdir -p $(BUILD_DIR)
>     $(CC) $(CFLAGS) $< -o $@
>     mv $@ $(BUILD_DIR)
>  
> but it will build if the .o files are missing from the local dir, and
> then not copy to the build_dir.
> If I remove the $(BUILD_DIR) from the rule, it will always rebuild the
> .o files, and copy them.
>  
> What am I missing?  Can I not include a path in a target rule?  How
> can I (properly) do this?
>  
> Any and all help appreciated!
>  
> -kt.
>  
$@ is the full target, so it already includes $(BUILD_DIR). That means
your last line expands to mv $(BUILD_DIR)/%.o $(BUILD_DIR).
If you remove $(BUILD_DIR) from the target your rule will not generate
what it promises (because it moves the output to $(BUILD_DIR)). That's
why he will regenerate the .o file every time.

Kristof