libs: fix API export/import and 'inconsistent linkage' on MSVC

For each lib we build export its own API in headers when we're
building it, otherwise import the API from the headers.

This fixes linker warnings on Windows when building with MSVC.

The problem was that we had defined all GST_*_API decorators
unconditionally to GST_EXPORT. This was intentional and only
supposed to be temporary, but caused linker warnings because
we tell the linker that we want to export all symbols even
those from externall DLLs, and when the linker notices that
they were in external DLLS and not present locally it warns.

What we need to do when building each library is: export
the library's own symbols and import all other symbols. To
this end we define e.g. BUILDING_GST_FOO and then we define
the GST_FOO_API decorator either to export or to import
symbols depending on whether BUILDING_GST_FOO is set or not.
That way external users of each library API automatically
get the import.

While we're at it, add new GST_API_EXPORT in config.h and use
that for GST_*_API decorators instead of GST_EXPORT.

The right export define depends on the toolchain and whether
we're using -fvisibility=hidden or not, so it's better to set it
to the right thing directly than hard-coding a compiler whitelist
in the public header.

We put the export define into config.h instead of passing it via the
command line to the compiler because it might contain spaces and brackets
and in the autotools scenario we'd have to pass that through multiple
layers of plumbing and Makefile/shell escaping and we're just not going
to be *that* lucky.

The export define is only used if we're compiling our lib, not by external
users of the lib headers, so it's not a problem to put it into config.h

Also, this means all .c files of libs need to include config.h
to get the export marker defined, so fix up a few that didn't
include config.h.

This commit depends on a common submodule commit that makes gst-glib-gen.mak
add an #include "config.h" to generated enum/marshal .c files for the
autotools build.

https://bugzilla.gnome.org/show_bug.cgi?id=797185

5 files changed, 14 insertions, 2 deletions

diff --git a/gst-libs/gst/basecamerabinsrc/Makefile.am b/gst-libs/gst/basecamerabinsrc/Makefile.am
index 5c68f0fbb..3a669ad62 100644
--- a/gst-libs/gst/basecamerabinsrc/Makefile.am
+++ b/gst-libs/gst/basecamerabinsrc/Makefile.am
@@ -19,6 +19,7 @@ libgstbasecamerabinsrc_@GST_API_VERSION@_la_CFLAGS = \
 	$(GST_PLUGINS_BAD_CFLAGS) \
 	$(GST_PLUGINS_BASE_CFLAGS) \
 	-DGST_USE_UNSTABLE_API \
+	-DBUILDING_GST_BASE_CAMERA_BIN_SRC \
 	$(GST_CFLAGS)
 libgstbasecamerabinsrc_@GST_API_VERSION@_la_LIBADD = \
 	-lgstapp-$(GST_API_VERSION) $(GST_PLUGINS_BASE_LIBS) $(GST_BASE_LIBS) $(GST_LIBS)
diff --git a/gst-libs/gst/basecamerabinsrc/basecamerabinsrc-prelude.h b/gst-libs/gst/basecamerabinsrc/basecamerabinsrc-prelude.h
index 6dc1477e1..ec9a76608 100644
--- a/gst-libs/gst/basecamerabinsrc/basecamerabinsrc-prelude.h
+++ b/gst-libs/gst/basecamerabinsrc/basecamerabinsrc-prelude.h
@@ -25,7 +25,11 @@
 #include <gst/gst.h>
 
 #ifndef GST_BASE_CAMERA_BIN_SRC_API
-#define GST_BASE_CAMERA_BIN_SRC_API GST_EXPORT
+# ifdef BUILDING_GST_BASE_CAMERA_BIN_SRC
+#  define GST_BASE_CAMERA_BIN_SRC_API GST_API_EXPORT         /* from config.h */
+# else
+#  define GST_BASE_CAMERA_BIN_SRC_API GST_API_IMPORT
+# endif
 #endif
 
 #endif /* __GST_BASE_CAMERA_BIN_SRC_PRELUDE_H__ */
diff --git a/gst-libs/gst/basecamerabinsrc/gstcamerabin-enum.c b/gst-libs/gst/basecamerabinsrc/gstcamerabin-enum.c
index bd9088c94..f55680627 100644
--- a/gst-libs/gst/basecamerabinsrc/gstcamerabin-enum.c
+++ b/gst-libs/gst/basecamerabinsrc/gstcamerabin-enum.c
@@ -17,6 +17,9 @@
  * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
 
 #include "gstcamerabin-enum.h"
 
diff --git a/gst-libs/gst/basecamerabinsrc/gstcamerabinpreview.c b/gst-libs/gst/basecamerabinsrc/gstcamerabinpreview.c
index 48ec2fec8..6d8545f12 100644
--- a/gst-libs/gst/basecamerabinsrc/gstcamerabinpreview.c
+++ b/gst-libs/gst/basecamerabinsrc/gstcamerabinpreview.c
@@ -26,6 +26,10 @@
  * #GstCameraBinVideo.
  *
  */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
 #include <gst/app/gstappsrc.h>
 #include <gst/app/gstappsink.h>
 #include <gst/glib-compat-private.h>
diff --git a/gst-libs/gst/basecamerabinsrc/meson.build b/gst-libs/gst/basecamerabinsrc/meson.build
index b1ba10689..830e9157a 100644
--- a/gst-libs/gst/basecamerabinsrc/meson.build
+++ b/gst-libs/gst/basecamerabinsrc/meson.build
@@ -13,7 +13,7 @@ install_headers(camerabin_headers, subdir : 'gstreamer-1.0/gst/basecamerabinsrc'
 
 gstbasecamerabin = library('gstbasecamerabinsrc-' + api_version,
   camerabin_sources,
-  c_args : gst_plugins_bad_args + ['-DGST_USE_UNSTABLE_API'],
+  c_args : gst_plugins_bad_args + ['-DGST_USE_UNSTABLE_API', '-DBUILDING_GST_BASE_CAMERA_BIN_SRC'],
   include_directories : [configinc, libsinc],
   version : libversion,
   soversion : soversion,