Re: IS_BUILTIN vs USE_BUILTIN

On Fri, Aug 14, 2026 at 07:52:09AM +0100, Greg Troxel wrote:
> I'm finding confusion in the builtin scheme, specifically looking at
> expat.
> 
> There is
> 
>   IS_BUILTIN.expat
>   USE_BUILTIN.expat
> 
> As I understand from mk/, IS_BUILTIN is about whether there is some
> implementation of expat in the base system, *regardless of whether it is
> good enough*.
>
> Then, USE_BUILTIN should be set to yes if expat's builtin.mk decides
> that the base version is good enough.

I think that's correct.

> However textproc/expat/builtin.mk seems to check if IS_BUILTIN is not
> set, and then set it conditionally.

It sets it to 'yes' or 'no' to avoid having to look for the relevant
header each time this file is parsed.

> My question is:
> 
>   Is IS_BUILTIN supposed to include "good enough" and USE_BUILTIN
>   supposed to include "PREFER machinery says use it"?
> 
> or is the good enough check supposed to be in IS_BUILTIN?

I think you misunderstood the IS_BUILTIN code in expat/builtin.mk (or
I misunderstood your question).

BUILTIN_FIND_HEADERS_VAR:=      H_EXPAT
BUILTIN_FIND_HEADERS.H_EXPAT=   expat.h

.include "../../mk/buildlink3/bsd.builtin.mk"
...
.if !defined(IS_BUILTIN.expat)
.  if empty(H_EXPAT:M__nonexistent__)
IS_BUILTIN.expat=       yes
.  else
IS_BUILTIN.expat=       no
.  endif
.endif

The H_EXPAT variable contains a path to expat.h when
../../mk/buildlink3/bsd.builtin.mk is done, and IS_BUILTIN.expat is
just a yes-no variable saying if such a header was found or not.
 Thomas