Add support for building shared libraries with MSVC

This requires the makedef perl script by Derek, from the
c89-to-c99 repo. That scripts produces a .def file, listing
the symbols to be exported, based on the gcc version scripts
and the built object files.

To properly load non-function symbols from DLL files, the
data symbol declarations need to have the attribute
__declspec(dllimport) when building the calling code. (On mingw,
the linker can fix this up automatically, which is why it has not
been an issue so far. If this attribute is omitted, linking
actually succeeds, but reads from the table will not produce the
desired results at runtime.)

MSVC seems to manage to link DLLs (and run properly) even if
this attribute is present while building the library itself
(which normally isn't recommended) - other object files in the
same library manage to link to the symbol (with a small warning
at link time, like "warning LNK4049: locally defined symbol
_avpriv_mpa_bitrate_tab imported" - it doesn't seem to be possible
to squelch this warning), and the definition of the tables
themselves produce a warning that can be squelched ("warning C4273:
'avpriv_mpa_bitrate_tab' : inconsistent dll linkage, see previous
definition of 'avpriv_mpa_bitrate_tab').

In this setup, mingw isn't able to link object files that refer to
data symbols with __declspec(dllimport) without those symbols
actually being linked via a DLL (linking avcodec.dll ends up with
errors like "undefined reference to `__imp__avpriv_mpa_freq_tab'").
The dllimport declspec isn't needed at all in mingw, so we simply
choose not to declare it for other compilers than MSVC that requires
it. (If ICL support later requires it, the condition can be extended
later to include both of them.)

This also implies that code that is built to link to a certain
library as a DLL can't link to the same library as a static library.
Therefore, we only allow building either static or shared but not
both at the same time. (That is, static libraries as such can be,
and actually are, built - this is used for linking the test tools to
internal symbols in the libraries - but e.g. libavformat built to
link to libavcodec as a DLL cannot link statically to libavcodec.)

Also, linking to DLLs is slightly different from linking to shared
libraries on other platforms. DLLs use a thing called import
libraries, which is basically a stub library allowing the linker
to know which symbols exist in the DLL and what name the DLL will
have at runtime.

In mingw/gcc, the import library is usually named libfoo.dll.a,
which goes next to a static library named libfoo.a. This allows
gcc to pick the dynamic one, if available, from the normal -lfoo
switches, just as it does for libfoo.a vs libfoo.so on Unix. On
MSVC however, you need to literally specify the name of the import
library instead of the static library.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -2095,7 +2095,7 @@ msvc_flags(){
             -Wall)                echo -W4 -wd4244 -wd4127 -wd4018 -wd4389 \
                                        -wd4146 -wd4057 -wd4204 -wd4706 -wd4305 \
                                        -wd4152 -wd4324 -we4013 -wd4100 -wd4214 \
-                                       -wd4996 ;;
+                                       -wd4996 -wd4273 ;;
             -std=c99)             ;;
             -fno-math-errno)      ;;
             -fno-common)          ;;
@@ -2761,6 +2761,25 @@ case $target_os in
         add_cppflags -U__STRICT_ANSI__
         ;;
     win32|win64)
+        if enabled shared; then
+            # Link to the import library instead of the normal static library
+            # for shared libs.
+            LD_LIB='%.lib'
+            # Cannot build shared and static libraries at the same time with
+            # MSVC.
+            disable static
+        fi
+        shlibdir_default="$bindir_default"
+        SLIBPREF=""
+        SLIBSUF=".dll"
+        SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
+        SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
+        SLIB_CREATE_DEF_CMD='makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
+        SLIB_INSTALL_NAME='$(SLIBNAME_WITH_MAJOR)'
+        SLIB_INSTALL_LINKS=
+        SLIB_INSTALL_EXTRA_SHLIB='$(SLIBNAME:$(SLIBSUF)=.lib)'
+        SLIB_INSTALL_EXTRA_LIB='$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.def)'
+        SHFLAGS='-dll -def:$$(@:$(SLIBSUF)=.def) -implib:$(SUBDIR)$(SLIBNAME:$(SLIBSUF)=.lib)'
         objformat="win32"
         ranlib=:
         enable dos_paths

libavcodec/ac3tab.h

@@ -33,7 +33,7 @@
 
 extern const uint16_t ff_ac3_frame_size_tab[38][3];
 extern const uint8_t  ff_ac3_channels_tab[8];
-extern const uint16_t avpriv_ac3_channel_layout_tab[8];
+extern av_export const uint16_t avpriv_ac3_channel_layout_tab[8];
 extern const uint8_t  ff_ac3_enc_channel_map[8][2][6];
 extern const uint8_t  ff_ac3_dec_channel_map[8][2][6];
 extern const uint16_t ff_ac3_sample_rate_tab[3];

libavcodec/dca.h

@@ -26,6 +26,7 @@
 #define AVCODEC_DCA_H
 
 #include <stdint.h>
+#include "libavutil/internal.h"
 
 /** DCA syncwords, also used for bitstream type detection */
 #define DCA_MARKER_RAW_BE 0x7FFE8001
@@ -36,6 +37,6 @@
 /** DCA-HD specific block starts with this marker. */
 #define DCA_HD_MARKER     0x64582025
 
-extern const uint32_t avpriv_dca_sample_rates[16];
+extern av_export const uint32_t avpriv_dca_sample_rates[16];
 
 #endif /* AVCODEC_DCA_H */

libavcodec/mjpeg.h

@@ -137,16 +137,16 @@ static inline void put_marker(PutBitContext *p, int code)
         case 7: ret= (left + top)>>1; break;\
     }
 
-extern const uint8_t avpriv_mjpeg_bits_dc_luminance[];
-extern const uint8_t avpriv_mjpeg_val_dc[];
+extern av_export const uint8_t avpriv_mjpeg_bits_dc_luminance[];
+extern av_export const uint8_t avpriv_mjpeg_val_dc[];
 
-extern const uint8_t avpriv_mjpeg_bits_dc_chrominance[];
+extern av_export const uint8_t avpriv_mjpeg_bits_dc_chrominance[];
 
-extern const uint8_t avpriv_mjpeg_bits_ac_luminance[];
-extern const uint8_t avpriv_mjpeg_val_ac_luminance[];
+extern av_export const uint8_t avpriv_mjpeg_bits_ac_luminance[];
+extern av_export const uint8_t avpriv_mjpeg_val_ac_luminance[];
 
-extern const uint8_t avpriv_mjpeg_bits_ac_chrominance[];
-extern const uint8_t avpriv_mjpeg_val_ac_chrominance[];
+extern av_export const uint8_t avpriv_mjpeg_bits_ac_chrominance[];
+extern av_export const uint8_t avpriv_mjpeg_val_ac_chrominance[];
 
 void ff_mjpeg_build_huffman_codes(uint8_t *huff_size, uint16_t *huff_code,
                                   const uint8_t *bits_table,

libavcodec/mpeg4audio.h

@@ -40,7 +40,7 @@ typedef struct MPEG4AudioConfig {
     int ps;  ///< -1 implicit, 1 presence
 } MPEG4AudioConfig;
 
-extern const int avpriv_mpeg4audio_sample_rates[16];
+extern av_export const int avpriv_mpeg4audio_sample_rates[16];
 extern const uint8_t ff_mpeg4audio_channels[8];
 
 /**

libavcodec/mpegaudiodata.h

@@ -29,11 +29,13 @@
 
 #include <stdint.h>
 
+#include "libavutil/internal.h"
+
 #define MODE_EXT_MS_STEREO 2
 #define MODE_EXT_I_STEREO  1
 
-extern const uint16_t avpriv_mpa_bitrate_tab[2][3][15];
-extern const uint16_t avpriv_mpa_freq_tab[3];
+extern av_export const uint16_t avpriv_mpa_bitrate_tab[2][3][15];
+extern av_export const uint16_t avpriv_mpa_freq_tab[3];
 extern const int ff_mpa_sblimit_table[5];
 extern const int ff_mpa_quant_steps[17];
 extern const int ff_mpa_quant_bits[17];

libavutil/internal.h

@@ -47,6 +47,12 @@
 #endif
 #endif
 
+#if defined(_MSC_VER) && CONFIG_SHARED
+#    define av_export __declspec(dllimport)
+#else
+#    define av_export
+#endif
+
 #ifndef INT_BIT
 #    define INT_BIT (CHAR_BIT * sizeof(int))
 #endif