[Gdal-dev] Reducing number of exported symbols

[Frank Warmerdam]

Even,

I am quite interested in this change.  My suggestions would be that for now
(GDAL/OGR 1.5.0) we drive it by an optional configure switch defaulting to
the old behavior, and for 1.6 we switch to defaulting to use the new behavior
if we have an appropriate gcc.

Even Rouault wrote:
> I give it a try on my system. This required only another change in shapefil.h 
> that doesn't use CPL_DLL but SHPAPI_CALL that is equivalent.
> 
> #ifndef SHPAPI_CALL
> #  if defined(__GNUC__) && __GNUC__ >= 4
> #    define SHPAPI_CALL     __attribute__ ((visibility("default")))
> #    define SHPAPI_CALL1(x) __attribute__ ((visibility("default")))     x
> #  else
> #    define SHPAPI_CALL
> #  endif
> #endif

Presumably we could pre-define SHPAPI_CALL in the makefile.

> Result : only 3 442 symbols ! Roughly like on Windows :-)
> 
> Another great benefit is that it should prevent subtle and annoying problems 
> when linking an executable with a GDAL library build with internal 
> libtiff/libpng/libjpeg/etc... and one of these libraries.Now most of them 
> have disappeared form libgdal.so. This may be becoming necessary with new 
> libtiff 4.0 that has become the internal GDAL libtiff, whereas most *nix 
> systems/distributios still uses 3.8.2. People will probably want to use 
> BigTIFF capabilities without messing up their system libtiff. Mixing symbols 
> from both versions is definitely not a good idea as there has been ABI 
> changes.
> The case for internal libgeotiff is a bit different since it uses CPL_DLL too 
> in its header. So, symbols for internal libgeotiff are still exported. We 
> should fine a way to distinguish the case when it's built in GDAL tree from 
> when it's build standalone.

We have a CPL_ODLL macro we use for APIs that would not normally be
public, but that we sometimes want to export.  On windows this was
introduced so that stuff like the libtiff and libgeotiff API could
be exported optionally. I would suggest we aim to use it in a similar
fashion with gcc.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org