Hopefully the explanation that follows helps clarify what PRIVATE, PUBLIC and INTERFACE mean and do. From my understanding of things, I think there may have been some subtle inaccuracies in some of the discussions so far, so hopefully the following is helpful and if I've got something wrong, then by all means please point out the inaccuracies.
- When A links in 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 in 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 in 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 in 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 in B 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 in B as PRIVATE and another target C links in 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 in 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. As a final note, if you call target_link_libraries() and do not specify any of PRIVATE, PUBLIC or INTERFACE, you may be tempted to believe that it will be treated as PUBLIC. The situation is actually more complicated than that though. It may be treated as PUBLIC or PRIVATE, depending on what other target_link_library() calls and/or target property manipulations have been performed. The documentation for target_link_libraries() talks a bit about this, but you have to go digging into the documentation for the target properties it mentions to get an understanding of what circumstances lead to PRIVATE or PUBLIC behaviour. Hope that helps clarify some things. Sorry if this has gone off on a tangent from the original enquiry, I'm coming in late to this thread. On Wed, May 11, 2016 at 8:33 PM, iosif neitzke < iosif.neitzke+cm...@gmail.com> wrote: > >> I *think* that these public/private rules behave a bit differently > >> for static libraries than they do for shared ones. > > They do. Assuming main calls a() and b() defined in A_lib and B_lib > respectively, for: > add_library(A_lib STATIC a.c) > add_library(B_lib STATIC b.c) > target_link_libraries(A_lib PRIVATE B_lib) > add_executable(main main.c) > target_link_libraries(main A_lib) > > The PRIVATE in "target_link_libraries(A_lib PRIVATE B_lib)" is > useless. It is the same as writing "target_link_libraries(A_lib > PUBLIC B_lib)", only more confusing to the reader. Static libraries > always link to their dependencies publically. > > > https://cmake.org/cmake/help/v3.5/command/target_link_libraries.html#libraries-for-a-target-and-or-its-dependents > > However, if you change A_lib to be a shared library with > "add_library(A_lib SHARED a.c)" and left the rest of the code the > same, you would now get link errors for main not able to find b(), > because A_lib now does not pass on its dependency on B, it hides it > from main. Change the last line to "target_link_libraries(main A_lib > B_lib)" and main builds again. > -- > > Powered by www.kitware.com > > Please keep messages on-topic and check the CMake FAQ at: > http://www.cmake.org/Wiki/CMake_FAQ > > Kitware offers various services to support the CMake community. For more > information on each offering, please visit: > > CMake Support: http://cmake.org/cmake/help/support.html > CMake Consulting: http://cmake.org/cmake/help/consulting.html > CMake Training Courses: http://cmake.org/cmake/help/training.html > > Visit other Kitware open-source projects at > http://www.kitware.com/opensource/opensource.html > > Follow this link to subscribe/unsubscribe: > http://public.kitware.com/mailman/listinfo/cmake > -- Craig Scott Melbourne, Australia http://crascit.com
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