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Re: Shorter and less error-prone rule for automatic prerequisite genera


From: Philip Guenther
Subject: Re: Shorter and less error-prone rule for automatic prerequisite generation in the GNU Make manual
Date: Thu, 29 Apr 2010 10:26:40 -0700

On Thu, Apr 29, 2010 at 3:59 AM, Edward Welbourne <address@hidden> wrote:
>> Delete a "clean-depend" rule on sight,
>
> I cannot agree.
> If I write a rule to make something, I also write a rule to get rid of
> it.  It's just basic hygiene ...

I propose the following guideline: If you have a target that generates
A (and B as a side-effect), then a 'clean*' rule that deletes B should
also delete A (and vice versa).

So, I do have a rule to delete *.d files, it's called "clean".  Since
I don't have rules for building .d files other than with .o files, it
Just Works.


>> or rename it to the more accurate "break-future-builds".
>
> If you have a sensible rule to generate .d files when needed, you
> haven't broken your builds - you've just obliged yourself to
> regenerate .d files.  Which may be wasteful, but see below.

Okay, so if you have a rule to delete .d files without deleting .o
files, you need rules to build .d files.  In my experience, the only
reason to have *either* of those is "because that's how the Makefile
was originally written and the current behavior doesn't hurt enough
for me to spend the time to fix it".


...
> Speaking of the subtleties of dependency tracking: do an update in
> your version control system, watch some header go away - and all files
> that used to reference it drop those references.  Your .d files claim
> a bunch of stuff depends on this missing file; but you have no rule to
> regenerate it.  So make will not even try to compile anything (even
> though everything *would* compile fine) because your .d file say that
> all the .o files that need recompiled depend on a file that doesn't
> exist any more; make clean-depend fixes that.

The fix for that has been documented for years on Paul's webpage, and
is most easily done now with gcc's -MP option.


> If generating .d as a side-effect, don't listen to the manual's advice
> that says you need to sed its content to claim that the .d depends on
> the same things the .o does.  If those things have changed, the .o
> shall be regenerated and hence so shall the .d; and you don't need
> this updated version of the .d file to discover that the .o needs
> rebuilt.  Changes to .h files consequently never trigger re-exec.

Ah, it looks like your comments are addressed at just what's in the
GNU make info pages and not the advanced method on Paul's webpage.  I
agree that what's in the info pages has many of the problems you
mention...which is why this thread is about updating what's there.


Philip Guenther




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