feat: use CMake to control symbol visibility (#574)

In contrast to #572 this solution does not need any extra headers. But it is also limited to GCC-like compilers.

Fix By: Gregor Jasny (@gjasny)

CMakeLists.txt

@@ -42,6 +42,7 @@ OPTION (CARES_STATIC_PIC "Build the static library as PIC (position independent)
 OPTION (CARES_BUILD_TESTS "Build and run tests"                                                  OFF)
 OPTION (CARES_BUILD_CONTAINER_TESTS "Build and run container tests (implies CARES_BUILD_TESTS, Linux only)" OFF)
 OPTION (CARES_BUILD_TOOLS "Build tools"                                                          ON)
+OPTION (CARES_SYMBOL_HIDING "Hide private symbols in shared libraries"                           OFF)
 SET    (CARES_RANDOM_FILE "/dev/urandom" CACHE STRING "Suitable File / Device Path for entropy, such as /dev/urandom")
 
 
@@ -74,6 +75,21 @@ IF (MSVC)
 	ENDIF ()
 ENDIF ()
 
+IF (CARES_SYMBOL_HIDING)
+	IF (CMAKE_VERSION VERSION_LESS 3.12)
+		MESSAGE (FATAL_ERROR "Hiding symbols requires CMake 3.12")
+	ENDIF ()
+	CMAKE_POLICY (SET CMP0063 NEW)
+	SET (CARES_SYMBOL_SCOPE_EXTERN [=[__attribute__ ((visibility("default")))]=])
+	CHECK_C_SOURCE_COMPILES ("
+		${CARES_SYMBOL_SCOPE_EXTERN} int somefunc() { return 0; }
+		int main() { return somefunc(); }
+	" HAVE_VISIBILITY_ATTRIBUTE)
+	IF (NOT HAVE_VISIBILITY_ATTRIBUTE)
+		MESSAGE (FATAL_ERROR "C compiler does not accept visibility attribute")
+	ENDIF ()
+ENDIF ()
+
 # Keep build organized.
 SET (CMAKE_RUNTIME_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}/${CMAKE_INSTALL_BINDIR}")
 SET (CMAKE_LIBRARY_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR}")

src/lib/CMakeLists.txt

@@ -37,6 +37,13 @@ IF (CARES_SHARED)
 		C_STANDARD                   90
 	)
 
+	IF (CARES_SYMBOL_HIDING)
+		SET_TARGET_PROPERTIES (${PROJECT_NAME} PROPERTIES
+			C_VISIBILITY_PRESET hidden
+			VISIBILITY_INLINES_HIDDEN YES
+		)
+	ENDIF ()
+
 	TARGET_INCLUDE_DIRECTORIES (${PROJECT_NAME}
 		PUBLIC "$<BUILD_INTERFACE:${PROJECT_BINARY_DIR}>"
 		       "$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>"

src/lib/ares_config.h.cmake

@@ -17,10 +17,10 @@
 #undef CARES_EXPOSE_STATICS
 
 /* Defined for build with symbol hiding. */
-#undef CARES_SYMBOL_HIDING
+#cmakedefine CARES_SYMBOL_HIDING
 
 /* Definition to make a library symbol externally visible. */
-#undef CARES_SYMBOL_SCOPE_EXTERN
+#define CARES_SYMBOL_SCOPE_EXTERN @CARES_SYMBOL_SCOPE_EXTERN@
 
 /* Use resolver library to configure cares */
 #cmakedefine CARES_USE_LIBRESOLV