CMake target_link_libraries Interface Dependencies

I am new to CMake and a bit confused with the PUBLIC, PRIVATE and INTERFACE keywords related to target_link_libraries(). Documentation mentions that they can be used to specify both the link dependencies and the link interface in one command.

What does link dependencies and link interface actually mean?

If you are creating a shared library and your source cpp files #include the headers of another library (Say, QtNetwork for example), but your header files don't include QtNetwork headers, then QtNetwork is a PRIVATE dependency.

If your source files and your headers include the headers of another library, then it is a PUBLIC dependency.

If your header files other than your source files include the headers of another library, then it is an INTERFACE dependency.

Other build properties of PUBLIC and INTERFACE dependencies are propagated to consuming libraries. http://www.cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#transitive-usage-requirements

@steveire accepted answer is great. I just wanted to add a table to quickly see the difference:

.-----------.------------------.----------------.
|           | Linked by target | Link interface |
:-----------+------------------+----------------:
| PUBLIC    |        X         |        X       |
:-----------+------------------+----------------:
| PRIVATE   |        X         |                |
:-----------+------------------+----------------:
| INTERFACE |                  |        X       |
'-----------'------------------'----------------'

• Linked by target: libraries included in target sources (not a dependency for projects linking the library).
• Link interface: libraries included in target public headers (dependencies for projects linking the library).

Not my brainchild but this extremely useful explanation helped me understand the situation. The most important part is quoted below for reference:

> - When A links B as PRIVATE, it is saying that A uses B in its implementation, but B is not used in any part of A's public API. Any > code that makes calls into A would not need to refer directly to > anything from B. An example of this could be a networking library A > which can be built to use one of a number of different SSL > libraries internally (which B represents). A presents a unified > interface for client code which does not reference any of the > internal SSL data structures or functions. Client code would have > no idea what SSL implementation (B) is being used by A, nor does
> that client code need to care. > - When A links B as INTERFACE, it is saying that A does not use B in its implementation, but B is used in A's public API. Code > that calls into A may need to refer to things from B in order to > make such calls. One example of this is an interface library which > simply forwards calls along to another library but doesn't actually > reference the objects on the way through other than by a pointer or > reference. Another example is where A is defined in CMake as an > interface library, meaning it has no actual implementation itself, > it is effectively just a collection of other libraries (I'm > probably over-simplifying here, but you get the picture). > - When A links B as PUBLIC, it is essentially a combination of PRIVATE and INTERFACE. It says that A uses B in its implementation and > B is also used in A's public API. > > > Consider first what this means for include search paths. If something > links against A, it will also need any include search paths from B if > B is in A's public API. Thus, if A links B either as PUBLIC or > INTERFACE, then any header search paths defined for target B will also > apply to anything that links to A. Any PRIVATE header search path for > B will NOT be carried through to anything that links only to A. The > target_include_directories() command handles this. The situation with > compile flags is analogously handled with target_compile_definitions() > and target_compile_options(). > > Now consider the situation for the actual libraries involved. If A is > a shared library, then A will have encoded into it a dependency on B. > This information can be inspected with tools like ldd on Linux, otool > on Mac and something like Dependency Walker (a.k.a. depends.exe) on > Windows. If other code links directly to A, then it also will have > encoded into it a dependency on A. It will not, however, have a > dependency on B unless A links B either as PUBLIC or INTERFACE. So far, so > good. If, however, A is a static library, the situation changes. > Static libraries do not carry information about other libraries they > depend on. For this reason, when A links B as PRIVATE and another > target C links A, CMake will still add B to the list of libraries > to be linked for C because parts of B are needed by A, but A itself > doesn't have that dependency encoded into it. So even though B is an > internal implementation detail of A, C still needs B added to the > linker command, which CMake conveniently handles for you. > > If you were paying careful attention, you would have noticed that when > A links B as PRIVATE, the include directories of B never propagate > to something linking to A, but if A is a static library, then the > linking of B behaves as though the relationship was PUBLIC. This PRIVATE-becomes-PUBLIC behaviour for static libraries only applies to > the > linking, not to the other dependencies (compiler options/flags and include search paths). The upshot of all this is that if you select > PRIVATE, PUBLIC or INTERFACE based on the explanations in the dot > points above, then CMake will ensure dependencies propagate through to > where they are required, regardless of whether libraries are static or > shared. This does, of course, rely on you the developer not missing > any dependencies or specifying the wrong PRIVATE/PUBLIC/INTERFACE > relationship.

Some answers only said when to use PRIVATE/PUBLIC/INTERFACE, but the affects are ignored. Refer:CMake-Public-Private-Interface

PUBLIC
All the objects following PUBLIC will be used for linking to the current target and providing the interface to the other targets that have dependencies on the current target.

PRIVATE All the objects following PRIVATE will only be used for linking to the current target.

INTERFACE All the objects following INTERFACE will only be used for providing the interface to the other targets that have dependencies on the current target.

Other posts already answered what the meaning of PUBLIC/PRIVATE/INTERFACE keywords. I want to add one more to clarify the terms "link dependencies" and "link interface."

• Link dependencies : the list of libraries to be linked by the target. The target property LINK_LIBRARIES holds this information.
• Link interface : the list of libraries to be linked by the target's dependents. The target property INTERFACE_LINK_LIBRARIES holds this information.

Probably the term "link interface" came from the old CMake wording used around LINK_INTERFACE_LIBRARIES properties, which is deprecated in favor of INTERFACE_LINK_LIBRARIES. See the description of CMP0022, which also uses the term "link interface." https://cmake.org/cmake/help/latest/policy/CMP0022.html > INTERFACE_LINK_LIBRARIES defines the link interface.