On Fri, 17 Nov 2017, James Almer wrote:
On 11/16/2017 5:22 AM, Martin Storsjö wrote:
The only purpose of dllexport (which is set while building the library
that exports the symbols) is to have the linker automatically
export such symbols into a DLL without using a def file - it doesn't
affect the generated code.
For MSVC builds, this isn't essential since we override what symbols
to export via an autogenerated def file instead.
Update a comment in configure to refer to the right concept.
With lld, this avoids warnings about duplicate export directives,
when some symbols are requested to be exported both via dllexport
attributes and via the autogenerated def file.
This also reduces the number of lines of code marginally.
---
configure | 2 +-
libavcodec/internal.h | 6 +-----
2 files changed, 2 insertions(+), 6 deletions(-)
diff --git a/configure b/configure
index 3bad7fb..62dffd4 100755
--- a/configure
+++ b/configure
@@ -3883,7 +3883,7 @@ case $target_os in
mingw32*|mingw64*)
target_os=mingw32
if enabled shared; then
- # Cannot build both shared and static libs when using dllexport.
+ # Cannot build both shared and static libs when using dllimport.
disable static
fi
check_ldflags -Wl,--nxcompat
diff --git a/libavcodec/internal.h b/libavcodec/internal.h
index da1b2fa..868e3df 100644
--- a/libavcodec/internal.h
+++ b/libavcodec/internal.h
@@ -285,12 +285,8 @@ int ff_decode_frame_props(AVCodecContext *avctx, AVFrame
*frame);
*/
AVCPBProperties *ff_add_cpb_side_data(AVCodecContext *avctx);
-#if defined(_WIN32) && CONFIG_SHARED
-#ifdef BUILDING_avcodec
-# define av_export_avcodec __declspec(dllexport)
-#else
+#if defined(_WIN32) && CONFIG_SHARED && !defined(BUILDING_avcodec)
Sorry if it's obvious, but when is BUILDING_avcodec not defined, for
that matter? library.mak seems to add -DBUILDING_avcodec to the
preprocessor flags of every libavcodec object.
When object files in libavformat includes this header, these variables
will get the __declspec(dllimport) attribute - which is the whole reason
why this is an issue at all.
Technically, this attribute means that instead of referring directly to
the variables, there's an implicit variable __imp_<name> which is a
pointer to the real one. So this means that libavformat object files will
refer to __imp_avpriv_foo (which is provided by the avcodec DLL import
library), while object files within avcodec themselves refer directly to
the variable itself without this indirection.
// Martin
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