diff --git a/CMakeLists.txt b/CMakeLists.txt
index 36283a7..7807873 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -21,7 +21,7 @@ configure_file (
 #
 if (UNIX)
     set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wall -Wextra -Wno-unused-parameter -fprofile-arcs -ftest-coverage -O0 -pthread -DDEBUG=1")
-    set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} -Wall -Wno-unused-parameter -Os -pthread")
+    set(CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE} -Wall -Wno-unused-parameter -fvisibility=hidden -fvisibility-inlines-hidden -Os -pthread")
     set(CMAKE_EXE_LINKER_FLAGS_DEBUG "-lgcov")
 endif() # UNIX
 
diff --git a/src/mqtt.h.in b/src/mqtt.h.in
index 08d54ad..e58e444 100644
--- a/src/mqtt.h.in
+++ b/src/mqtt.h.in
@@ -21,7 +21,11 @@ extern "C" {
 #if MSVC
 #define API __declspec(dllexport)
 #else
-#define API
+#if DEBUG
+#define API __attribute__((visibility("visible")))
+#else
+#define API __attribute__((visibility("visible")))
+#endif
 #endif
 
 #include <stdint.h>