Problem: when linking to libzmq in my project, I want zmq symbols to remain invisible to users of my library. There is no way to control this, since visibility is set automatically through ZMQ_EXPORT based on OS and compiler.

Solution: add a preprocessor variable ZMQ_NO_EXPORT that, when set, bypasses the automatic ZMQ_EXPORT determination block and just sets ZMQ_EXPORT to empty.

By combining this solution at configuration time with manually passing -fvisibility=hidden to CXXFLAGS, I solved my visibility problem. Just passing -fvisibility=hidden is not enough, because __attribute__ ((visibility ("default"))) has higher priority.

CMakeLists.txt

@@ -1828,3 +1828,9 @@ if(MSVC
    AND BUILD_STATIC)
   add_dependencies(libzmq-static libzmq)
 endif()
+
+option(ENABLE_NO_EXPORT "Build with empty ZMQ_EXPORT macro, bypassing platform-based automated detection" OFF)
+if(ENABLE_NO_EXPORT)
+  message(STATUS "Building with empty ZMQ_EXPORT macro")
+  add_definitions(-DZMQ_NO_EXPORT)
+endif()

configure.ac

@@ -1150,6 +1150,11 @@ AC_ARG_WITH([pkgconfigdir],
     [pkgconfigdir='${libdir}/pkgconfig'])
 AC_SUBST([pkgconfigdir])
 
+AC_ARG_ENABLE([no-export],
+    [AS_HELP_STRING([--enable-no-export],
+        [Build libzmq with empty ZMQ_EXPORT macro, bypassing platform-based automated detection [default=no]])],
+    [CPPFLAGS="-DZMQ_NO_EXPORT $CPPFLAGS"])
+
 AC_CONFIG_FILES([ \
     Makefile \
     src/libzmq.pc \

include/zmq.h

@@ -59,6 +59,9 @@ extern "C" {
 #include <stdio.h>
 
 /*  Handle DSO symbol visibility                                             */
+#if defined ZMQ_NO_EXPORT
+#define ZMQ_EXPORT
+#else
 #if defined _WIN32
 #if defined ZMQ_STATIC
 #define ZMQ_EXPORT
@@ -76,6 +79,7 @@ extern "C" {
 #define ZMQ_EXPORT
 #endif
 #endif
+#endif
 
 /*  Define integer types needed for event interface                          */
 #define ZMQ_DEFINED_STDINT 1