[SOLVED] Why doesn't CMake initiate a rebuild when dependent imported library is updated?

Issue

If we create libFoo.a first with add_library(Foo STATIC foo.cpp foo.h) and then do

add_library(Foo STATIC IMPORTED)
set_target_properties(Foo PROPERTIES IMPORTED_LOCATION ${CMAKE_BINARY_DIR}/libFoo.a)
add_library(Bar STATIC bar.cpp bar.h)
target_link_libraries(Bar Foo)

After touch libFoo.a and then make there's no rebuild. Why is that?

What happened in my real situation is that after I update one of my third party dependencies by compiling their source and running make install, the build folder in my project that finds the installed third party dependencies via find_package does not get rebuild. I had to delete the build folder and compile from scratch to get the proper updates. Is this a bug in CMake?

Solution

While you write

target_link_libraries(Bar Foo)

there is no dependency between libBar.a and libFoo.a, so modifying the latter doesn't trigger rebuilding the former.

This is because Bar is a STATIC library, and creation of a static library doesn't involve linking step.

However, modifying libFoo.a would trigger rebuilding a shared library (or executable) which is linked with Bar via target_link_libraries:

add_library(Baz SHARED baz.c)
# That linkage makes libBaz.so to be dependent from libFoo.a
target_link_libraries(Baz Bar)

Despite that Baz is explicitly linked only with Bar, CMake automatically adds libFoo.a to be linked libraries of Baz and creates appropriate dependency.

Answered By - Tsyvarev

Answer Checked By - Senaida (WPSolving Volunteer)