Re: Shorter and less error-prone rule for automatic prerequisite generation in the GNU Make manual

[Philip Guenther]

On Wed, Apr 28, 2010 at 7:12 AM, Edward Welbourne <address@hidden> wrote:
>> It's also unnecessary - you don't need a rule for %.d at all. You can
>> just generate the dependencies as a side-effect of compilation using
>> -MMD or similar.
>
> Well, if a .d file gets deleted while its .o file exists, you do need
> to regenerate it - or regenerate the .o (which may cause wasteful
> knock-on actions since it's now newer than it was, even if unchanged).
> This matters if, for example, you have a make clean-depend rule to
> purge dependency information.

Delete a "clean-depend" rule on sight, or rename it to the more
accurate "break-future-builds".  I can think of absolutely no reason
to delete dependency files without deleting the objects they describe.


>> Then use '-include' to grab all the deps if they exist.
>
> better: include only the .d files that correspond to .o files *that
> exist* - if the .o file doesn't exist, you don't need to know what it
> depends on, aside from the primary source from which it's compiled
> (which is indicated by the pattern rule that makes the .o file).

Yeah, that's true.  Nice.


>> By providing a rule for the .d files you'll cause make to
>> re-execute itself, so you'll just end up parsing the Makefile twice.
>
> Only if you include .d files that aren't really needed (which is
> common practice).

There's still the case of a .o and .d pair that both exist where the
.o is out of date to one of its dependencies.  If the .d file is a
target, then make will first rebuild the .d, re-exec, then rebuild the
.o.  The re-exec is unnecessary there; building the .d as a
side-effect of building the .o is sufficient.


Philip Guenther