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Re: per-deplib static/dynamic flags

From: Ralf Wildenhues
Subject: Re: per-deplib static/dynamic flags
Date: Thu, 2 Feb 2006 07:13:18 +0100
User-agent: Mutt/1.5.9i

Hi Bob, Albert,

* Bob Friesenhahn wrote on Wed, Feb 01, 2006 at 07:47:51AM CET:
> On Tue, 31 Jan 2006, Albert Chin wrote:
> >On Mon, Jan 30, 2006 at 10:28:52PM +0100, Ralf Wildenhues wrote:
> >>- Should the corresponding libtool flags be named `-Bstatic' resp.
> >>  `-Bdynamic'?  Those were the most common names I could find, but IMHO
> >>  they are not very self-explanatory for users not used to them.
> >
> >-prefer-static, -prefer-dynamic/-prefer-shared? The -Bxxx doesn't seem
> >similar with current libtool options.
> At least for the static case, I would prefer the link to entirely fail 
> if a static library is requested but one can not be found.  Usually 
> there is a very important reason to use a static library if it has 
> specifically been requested.  So the wording should not be 'prefer' 
> for the static case.  I agree that the -B syntax does not fit the 
> style of other libtool options (but does match many linkers).

Several different issues here:
- the option naming, which I will not delve into in this post, and
- whether what I'll call -Bstatic for now should fail if it does not
  find a static library to link against.

The latter point is intricate, and requires much more elaboration to be
completely specified.  That's the main reason I kept the semantics so
vague in my proposal.  The issues are: library search algorithm,
difference between libtool and non-libtool libraries, linker capability.

1) The linker may not allow to specify per-deplib linkage at all, or may
not have a flag to force static linkage (as opposed to only _prefer_ it).
Examples for the former are Darwin; I haven't found an example for the
latter yet (good!).  But this means, that for non-libtool libraries we
cannot guarantee a certain linking mode, unless we were to change our
search algorithm quite drastically (which I don't see as an option ATM).

2) Difference between libtool libraries (*.la) and non-libtool ones, and
libtool search algorithm and native linker search algorithm.

The current libtool library search algorithm looks a bit like this
(before my patch):

- given `path/to/', that is taken and nothing else.
- given `path/to/libfoo.a' ($libext), that is taken and nothing else.
- given `-lfoo'
    for all searchdirs that we look at,
      for the extensions `.la', `$std_shrext', `.so', `.a'   [1]
        check whether there exists a file libfoo.$extension
          if yes, exit search algorithm

My patch skips `$std_shrext' and `.so' when in Bstatic mode, but it
still happily picks the first `.la' file it can find, even iff that one
was shared-only, for example.  Currently it then links shared or fails
to link, depending on hardcoding features of the linker, and on whether
the linker fails to link shared in Bstatic mode.  It may be more useful
to either
- fail outright if the first .la library wasn't good, or
- go back and scan the rest of the searchdirs for a better candidate
But this would open new questions: would we then accept .la libraries as
better candidates only, or also $libext archives?

The same question already goes for `-static' and `-static-libtool-libs':
Should they fail upon encounter of a disable-static libtool library?
Obviously that is more of an issue for `-static-libtool-libs' because it
also involves libraries not in the current package tree.

Now about non-libtool libaries: while I believe all linkers (that have
Bstatic flags) will fail if they find a shared but no static library
anywhere in the search path, this needs testing.  I will extend to do this as well, but even then there's no guarantee another
system may have this limitation.

Hope that helps a bit.


[1] we need to exchange `.a' with $libext, or add $libext if != `.a' for
    w32 systems, I believe.

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