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libs/lib3/CMakeLists.txt

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+# Defines an INTERFACE library target named 'lib3'.
+# - INTERFACE: This type of library does not compile any source files itself.
+#              It is used purely to propagate usage requirements (like include directories)
+#              to targets that link to it.
+add_library(lib3 INTERFACE)
+
+# Specifies include directories that are part of lib3's interface.
+# - INTERFACE: Means that targets linking to lib3 (like cmake_tutorialApp or lib1) will
+#              inherit this include path. This allows them to find lib3.h.
+target_include_directories(lib3 INTERFACE
+    ${CMAKE_CURRENT_SOURCE_DIR}/inc
+)
+

libs/lib3/inc/lib3.h

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+#pragma once
+#include <stdio.h> // Standard I/O header is needed here because printf is used within this header-only function.
+
+// Declared as static to prevent multiple definition errors.
+// Since this is a header-only function, it will be included and compiled
+// into every translation unit that includes lib3.h. 'static' limits its
+// linkage to the current translation unit, preventing linker conflicts.
+static void lib3_function()
+{
+  printf("This is a function from Header-Only Library 3.\n");
+}