CMake: Try to (un-)hide private library symbols

Detect support for compiler symbol visibility flags and apply those
according to CURL_HIDDEN_SYMBOLS option.
It should work true to the autotools build except it tries to unhide
symbols on Windows when requested and prints warning if it fails.

Ref: https://github.com/curl/curl/issues/981#issuecomment-242665951
Reported-by: Daniel Stenberg

CMake/CurlSymbolHiding.cmake

@@ -0,0 +1,61 @@
+include(CheckCSourceCompiles)
+
+option(CURL_HIDDEN_SYMBOLS "Set to ON to hide libcurl internal symbols (=hide all symbols that aren't officially external)." ON)
+mark_as_advanced(CURL_HIDDEN_SYMBOLS)
+
+if(CURL_HIDDEN_SYMBOLS)
+    set(SUPPORTS_SYMBOL_HIDING FALSE)
+
+    if(CMAKE_C_COMPILER_ID MATCHES "Clang")
+        set(SUPPORTS_SYMBOL_HIDING TRUE)
+        set(_SYMBOL_EXTERN "__attribute__ ((__visibility__ (\"default\")))")
+        set(_CFLAG_SYMBOLS_HIDE "-fvisibility=hidden")
+    elseif(CMAKE_COMPILER_IS_GNUCC)
+        if(NOT CMAKE_VERSION VERSION_LESS 2.8.10)
+            set(GCC_VERSION ${CMAKE_C_COMPILER_VERSION})
+        else()
+            execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion
+                            OUTPUT_VARIABLE GCC_VERSION)
+        endif()
+        if(NOT GCC_VERSION VERSION_LESS 3.4)
+            # note: this is considered buggy prior to 4.0 but the autotools don't care, so let's ignore that fact
+            set(SUPPORTS_SYMBOL_HIDING TRUE)
+            set(_SYMBOL_EXTERN "__attribute__ ((__visibility__ (\"default\")))")
+            set(_CFLAG_SYMBOLS_HIDE "-fvisibility=hidden")
+        endif()
+    elseif(CMAKE_C_COMPILER_ID MATCHES "SunPro" AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 8.0)
+        set(SUPPORTS_SYMBOL_HIDING TRUE)
+        set(_SYMBOL_EXTERN "__global")
+        set(_CFLAG_SYMBOLS_HIDE "-xldscope=hidden")
+    elseif(CMAKE_C_COMPILER_ID MATCHES "Intel" AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 9.0)
+        # note: this should probably just check for version 9.1.045 but I'm not 100% sure
+        #       so let's to it the same way autotools do.
+        set(SUPPORTS_SYMBOL_HIDING TRUE)
+        set(_SYMBOL_EXTERN "__attribute__ ((__visibility__ (\"default\")))")
+        set(_CFLAG_SYMBOLS_HIDE "-fvisibility=hidden")
+        check_c_source_compiles("#include <stdio.h>
+            int main (void) { printf(\"icc fvisibility bug test\"); return 0; }" _no_bug)
+        if(NOT _no_bug)
+            set(SUPPORTS_SYMBOL_HIDING FALSE)
+            set(_SYMBOL_EXTERN "")
+            set(_CFLAG_SYMBOLS_HIDE "")
+        endif()
+    elseif(MSVC)
+        set(SUPPORTS_SYMBOL_HIDING TRUE)
+    endif()
+
+    set(HIDES_CURL_PRIVATE_SYMBOLS ${SUPPORTS_SYMBOL_HIDING})
+elseif(MSVC)
+    if(NOT CMAKE_VERSION VERSION_LESS 3.7)
+        set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS TRUE) #present since 3.4.3 but broken
+        set(HIDES_CURL_PRIVATE_SYMBOLS FALSE)
+    else()
+        message(WARNING "Hiding private symbols regardless CURL_HIDDEN_SYMBOLS being disabled.")
+        set(HIDES_CURL_PRIVATE_SYMBOLS TRUE)
+    endif()
+elseif()
+    set(HIDES_CURL_PRIVATE_SYMBOLS FALSE)
+endif()
+
+set(CURL_CFLAG_SYMBOLS_HIDE ${_CFLAG_SYMBOLS_HIDE})
+set(CURL_EXTERN_SYMBOL ${_SYMBOL_EXTERN})

CMakeLists.txt

@@ -119,8 +119,7 @@ if(MSVC)
   mark_as_advanced(BUILD_RELEASE_DEBUG_DIRS)
 endif()
 
-option(CURL_HIDDEN_SYMBOLS "Set to ON to hide libcurl internal symbols (=hide all symbols that aren't officially external)." ON)
-mark_as_advanced(CURL_HIDDEN_SYMBOLS)
+include(CurlSymbolHiding)
 
 option(HTTP_ONLY "disables all protocols except HTTP (This overrides all CURL_DISABLE_* options)" OFF)
 mark_as_advanced(HTTP_ONLY)

lib/CMakeLists.txt

@@ -87,6 +87,11 @@ endif()
 
 set_target_properties(${LIB_NAME} PROPERTIES COMPILE_DEFINITIONS BUILDING_LIBCURL)
 
+if(HIDES_CURL_PRIVATE_SYMBOLS)
+  set_property(TARGET ${LIB_NAME} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_HIDDEN_SYMBOLS")
+  set_property(TARGET ${LIB_NAME} APPEND PROPERTY COMPILE_FLAGS ${CURL_CFLAG_SYMBOLS_HIDE})
+endif()
+
 # Remove the "lib" prefix since the library is already named "libcurl".
 set_target_properties(${LIB_NAME} PROPERTIES PREFIX "")
 set_target_properties(${LIB_NAME} PROPERTIES IMPORT_PREFIX "")

lib/curl_config.h.cmake

@@ -67,7 +67,7 @@
 #cmakedefine CURL_DISABLE_VERBOSE_STRINGS 1
 
 /* to make a symbol visible */
-#cmakedefine CURL_EXTERN_SYMBOL 1
+#cmakedefine CURL_EXTERN_SYMBOL ${CURL_EXTERN_SYMBOL}
 /* Ensure using CURL_EXTERN_SYMBOL is possible */
 #ifndef CURL_EXTERN_SYMBOL
 #define CURL_EXTERN_SYMBOL

tests/libtest/CMakeLists.txt

@@ -43,6 +43,10 @@ if(NOT WIN32)
   # library at (tests)/libtest/.libs/libhostname.so
   set_target_properties(hostname PROPERTIES
       LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/.libs)
+  if(HIDES_CURL_PRIVATE_SYMBOLS)
+    set_property(TARGET hostname APPEND PROPERTY COMPILE_DEFINITIONS "CURL_HIDDEN_SYMBOLS")
+    set_property(TARGET hostname APPEND PROPERTY COMPILE_FLAGS ${CURL_CFLAG_SYMBOLS_HIDE})
+  endif()
 endif()
 
 # # files used only in some libcurl test programs