Better handling for export headers (#447)

* Better handling for export headers

* More refinement on export headers.

* Add comment to explain export macros.

* Minor change on comment

CMakeLists.txt

@@ -406,14 +406,16 @@ target_include_directories(avif
                            PUBLIC $<BUILD_INTERFACE:${libavif_SOURCE_DIR}/include>
                                   $<INSTALL_INTERFACE:include>
                            PRIVATE ${AVIF_PLATFORM_INCLUDES} ${AVIF_CODEC_INCLUDES})
+set(AVIF_PKG_CONFIG_EXTRA_CFLAGS "")
 if(BUILD_SHARED_LIBS)
-    target_compile_definitions(avif PRIVATE -DAVIF_BUILDING_SHARED_LIBS)
+    target_compile_definitions(avif PUBLIC AVIF_DLL)
+    target_compile_definitions(avif PRIVATE AVIF_BUILDING_SHARED_LIBS)
+    set(AVIF_PKG_CONFIG_EXTRA_CFLAGS " -DAVIF_DLL")
     if(AVIF_LOCAL_LIBGAV1)
         set_target_properties(avif PROPERTIES LINKER_LANGUAGE "CXX")
     endif()
 endif()
 
-
 option(AVIF_BUILD_EXAMPLES "Build avif Examples." OFF)
 if(AVIF_BUILD_EXAMPLES)
     set(AVIF_EXAMPLES

include/avif/avif.h

@@ -7,23 +7,46 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#ifndef AVIF_API
-#if defined(AVIF_BUILDING_SHARED_LIBS)
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// ---------------------------------------------------------------------------
+// Export macros
+
+// AVIF_BUILDING_SHARED_LIBS should only be defined when libavif is being built
+// as a shared library.
+// AVIF_DLL should be defined if libavif is a shared library. If you are using
+// libavif as CMake dependency, through CMake package config file or
+// through pkg-config, this is defined automatically.
+//
+// Here's what AVIF_API will be defined as in shared build:
+// |       |        Windows        |                  Unix                  |
+// | Build | __declspec(dllexport) | __attribute__((visibility("default"))) |
+// |  Use  | __declspec(dllimport) |                                        |
+//
+// For static build, AVIF_API is always defined as nothing.
+
 #if defined(_WIN32)
-#define AVIF_API __declspec(dllexport)
+#define AVIF_HELPER_EXPORT __declspec(dllexport)
+#define AVIF_HELPER_IMPORT __declspec(dllimport)
 #elif defined(__GNUC__) && __GNUC__ >= 4
-#define AVIF_API __attribute__((visibility("default")))
+#define AVIF_HELPER_EXPORT __attribute__((visibility("default")))
+#define AVIF_HELPER_IMPORT
 #else
-#define AVIF_API
-#endif // if defined(_WIN32)
+#define AVIF_HELPER_EXPORT
+#define AVIF_HELPER_IMPORT
+#endif
+
+#if defined(AVIF_DLL)
+#if defined(AVIF_BUILDING_SHARED_LIBS)
+#define AVIF_API AVIF_HELPER_EXPORT
 #else
-#define AVIF_API
+#define AVIF_API AVIF_HELPER_IMPORT
 #endif // if defined(AVIF_BUILDING_SHARED_LIBS)
-#endif // ifndef AVIF_API
-
-#ifdef __cplusplus
-extern "C" {
-#endif
+#else
+#define AVIF_API
+#endif //if defined(AVIF_DLL)
 
 // ---------------------------------------------------------------------------
 // Constants

libavif.pc.cmake

@@ -7,4 +7,4 @@ Name: @PROJECT_NAME@
 Description: Library for encoding and decoding .avif files
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lavif
-Cflags: -I${includedir}
+Cflags: -I${includedir}@AVIF_PKG_CONFIG_EXTRA_CFLAGS@