Re: [PATCH] Do not decorate symbols as dllexport on Cygwin

[Corinna Vinschen <vinschen@redhat.com>]

On Feb  5 21:41, Bruno Haible wrote:
> Hi Corinna,
> 
> Thanks for the heads-up and patch.
> 
> > Note that dllimport/dllexport decorations are not at all required on
> > Cygwin for quite some time.
> 
> Is this true in all circumstances? I thought that when --disable-auto-import
> is in use AND the shared library wants to export variables, dllimport
> and dllexport decorations are necessary.
> 
> > On Cygwin --export-all-symbols is default.  However, if just a single
> > symbol is decorated with dllexport, ld switches to exporting only these
> > symbols.
> > ...
> > An example of that is current recode.  Building this package with
> > --enable-shared is broken, unless one adds -Wl,--export-all-symbols to
> > the link stage command line explicitely, because of the decorated
> > gl_get_setlocale_null_lock variable.
> 
> So, there are three types of packages:
>   (A) Those that use --disable-auto-import (e.g. [1][2]) and provide an
>       explicit list of exports for their shared libraries.
>   (B) Those that don't use --disable-auto-import but want to limit
>       the exposed symbols for other reasons (e.g. namespace cleanliness).
>       Such as those that use the libtool options '-export-symbols' or
>       '-export-symbols-regex'.
>   (C) Those like 'recode', which want all symbols to be exported (e.g.
>       if namespace cleanliness is not an issue for them).
> 
> For packages of type (A) or (B), the symbol gl_get_setlocale_null_lock
> *MUST* be exported, otherwise there may be different get_setlocale_null_lock
> functions accessed by different code, which will destroy the purpose of this
> function, i.e. open the door to potential crashes due to use of setlocale().
> 
> For packages of type (C), the symbol gl_get_setlocale_null_lock *MUST NOT*
> be exported, otherwise all other symbols will not be exported.
> 
> AFAIU, your patch fixes packages of type (C), while at the same time breaking
> packages of type (A) or (B).
> 
> Is there an easy way to distinguish packages of type (C) from those of type
> (A), (B)?
> 
> Another option — since we are talking about a single symbol and a single
> platform — would be if the locking for setlocale_null were not necessary
> on Cygwin in the first place. I determined that it is necessary by running
> the unit test gnulib/tests/test-setlocale_null-mt-all.c [3] on Cygwin:
> without the lock, it crashed within less than 1 second. Could the
> implementation of setlocale() in Cygwin be changed in such a way that this
> test does not crash? Then the lock would be necessary.

Well, we could do that by adding Cygwin-internal locking to setlocale
calls.  But that would only be available in the next Cygwin version
of course.

May I ask what's the idea to provide a thread-safe setlocale?  It was
never defined as thread-safe and POSIX explicitely mentions that.  Any
application expecting to call setlocale thread-safe is broken by design.
It should use the newlocale/duplocale/uselocale/freelocale API instead,
isn't it?


Corinna



> 
> Bruno
> 
> [1] https://git.savannah.gnu.org/gitweb/?p=gettext.git;a=blob;f=gettext-tools/woe32dll/export.h;hb=HEAD
> [2] https://haible.de/bruno/woe32dll.html
> [3] https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=tests/test-setlocale_null-mt-all.c;hb=HEAD
> 
>