target_link_libraries and add_dependencies

Question

Is there any use case in which

target_link_libraries(my-lib x y z)

add_dependencies(my-lib x) # this is not just a waste of bytes?

If so, can someone explain what it would be?

Answer 1

I don't know in what particularly you're interesting...

From conceptual point of view -- I think you're right it is a waste of bytes

From cmake documentation point of view -- You should prefer make so to guarantee correct build order

According to the documentation target_link_libraries, add_dependencies concepts was ideologically splitted. Such idea of split depencies, and linker options is also persist in Makefile format in GNU make tool

target_link_libraries

..Specify libraries or flags to use when linking a given target..

add_dependencies

...Make a top-level depend on other top-level targets to ensure that they build before does...

In modern cmake from 3.* you can omit add_dependencies if you will perform linking with aliased target

add_library(fooLib 1.cpp 2.cpp)
add_library(my::fooLib ALIAS fooLib)
...
target_link_libraries(fooBin my::fooLib)

Answer 2

In current CMake releases:

After some error checking add_dependencies results in a call to Target->AddUtility(). x is added to the list of utilities for my-lib.

target_link_libraries does not result in a call to AddUtility, but it does add the arguments to the LINK_LIBRARIES target property.

Later, both the content of the LINK_LIBRARIES target property and the list of utilities are used to compute the dependencies of the target in cmComputeTargetDepends.

The list of utilities in a target can not be queried at configure time, and is only used at generate time, so the use of add_dependencies with arguments which are libraries already added with target_link_libraries is redundant.

Answer 3

It's used in cases where top-level targets depend on each other. That is, if x is a something that you add to your project (at top-level) with

add_library( x x.c y.c z.c )

you shall tell CMake about it. And you do that with add_dependencies.