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From: Jason Green <news AT jgreen4 DOT fsnet DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Should I use bash as default shell in makefiles?
Date: Sun, 23 Dec 2001 20:42:48 +0000
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Eli Zaretskii <eliz AT is DOT elta DOT co DOT il> wrote:

> > > %.d: %.c
> > >         $(SHELL) -ec '$(CC) -M $(CPPFLAGS) $< \
> > >               | sed '\''s/\($*\)\.o[ :]*/\1.o $@ : /g'\'' > $@; \
> > >               [ -s $@ ] || rm -f $@'
> > 
> > I've never understood why the docs say that.  Is there a good reason for
> > all that effort to create the .d files?
> 
> The reason for the Sed command is explained in the Make manual, right
> after the command itself.  It's a valid reason, IMHO.

I've just noticed the above command is slightly different to the one I
see here, so I can't be sure we're looking at the same docs.  However, I
believe you are refering to...

>    The purpose of the `sed' command is to translate (for example):
> 
>      main.o : main.c defs.h
> 
> into:
> 
>      main.o main.d : main.c defs.h
> 
> This makes each `.d' file depend on all the source and header files
> that the corresponding `.o' file depends on.  `make' then knows it must
> regenerate the prerequisites whenever any of the source or header files
> changes.

...but when using `-MMD' the `.d' file will still be regenerated whenever
any of the source or header files changes.  I'm not disputing that sed
has some advantage here, but it's obviously too subtle for me to see.