Re: Circular dependency with order-only dependencies

[Eli Zaretskii]

> From: Paul Smith <address@hidden>
> Cc: address@hidden
> Date: Wed, 23 Jan 2019 13:55:49 -0500
> 
> That's not how it works.  Order-only prerequisites are considered
> identically to normal prerequisites in every way except one: after all
> the prerequisites, including order-only prerequisites, are brought up
> to date following the standard algorithm and we're determining whether
> the target is outdated, we skip order-only prerequisites.
> 
> In the case of your example, make considers "all", then considers "b",
> then considers "t", then considers "a.o", then considers "a.h", then
> considers "foo", then considers "b".  It doesn't matter what the result
> of considering "foo" was: make still needs to see if "b" needs to be
> brought up to date.  In order to see if "b" is out of date, it needs to
> consider "t", etc. and you have a cycle.

That's too bad, because it doesn't allow to have targets which can
only be updated by one of their dependencies.

In Emacs, we have a header file that is generated from an certain data
file, but its generation requires an Emacs binary, which runs a Lisp
command to massage the data file into a C header.  And the binary
depends on that header because one of the source files #include's the
header.  The header is also kept in the VCS, so some, perhaps outdated
version of it is always available.  But I cannot find a way of having
a target that remakes the header when the data file changes, because
this creates a circular dependency which Make throws away.

I thought order-only dependencies could be a way out of this
conundrum, but they aren't :-(

Thanks.