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| __TOC__
| | {{CMake/Template/Moved}} |
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| =Exporting and Importing Targets=
| | This page has moved [https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/notes/2.6 here]. |
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| =Linking=
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| CMake 2.6 implements a new approach to generating link lines for targets.
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| Consider these libraries:
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| /path/to/libfoo.a
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| /path/to/libfoo.so
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| Previously if someone wrote
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| target_link_libraries(myexe /path/to/libfoo.a)
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| CMake would generate this code to link it:
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| ... -L/path/to -Wl,-Bstatic -lfoo -Wl,-Bdynamic ...
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| This worked most of the time, but some platforms (such as OS X) do not
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| support the <code>-Bstatic</code> or equivalent flag. This made it impossible to
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| link to the static version of a library without creating a symlink in
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| another directory and using that one instead.
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| Now CMake will generate this code:
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| ... /path/to/libfoo.a ...
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| This guarantees that the correct library is chosen. However there are some side-effects.
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| ==Missing Linker Search Directories==
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| Projects used to be able to write this (wrong) code and it would work by accident:
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| where "<code>B</code>" is meant to link "<code>/path/to/libB.so</code>". This code is incorrect
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| because it asks CMake to link to <code>B</code> but does not provide the proper
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| linker search path for it. It used to work by accident because the
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| <code>-L/path/to</code> would get added as part of the implementation of linking to
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| A. The correct code would be
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| link_directories(/path/to)
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so B)
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| or even better
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| add_executable(myexe myexe.c)
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| target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)
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| In order to support projects that have this bug, we've added a
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| compatibility feature that adds the "<code>-L/path/to</code>" paths for all libraries
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| linked with full paths even though the linker will not need those paths
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| to find the main libraries. The compatibility mode is enabled when a
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| link line contains a non-full-path library (like <code>B</code>) and either
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| <code>CMAKE_BACKWARDS_COMPATIBILITY</code> is set to 2.4 or lower or
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| <code>CMAKE_LINK_OLD_PATHS</code> is set to true.
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| If you are trying to build a project and run into this problem, a quick-fix is to run
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| cmake -DCMAKE_LINK_OLD_PATHS:BOOL=ON .
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| in the top of the build tree.
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| ==Linking to System Libraries==
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| System libraries on UNIX-like systems are typically provided in <code>/usr/lib</code> or <code>/lib</code>. These directories are considered implicit linker search paths because linkers automatically search these locations even without a flag like <code>-L/usr/lib</code>. Consider the code
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| find_library(M_LIB m)
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| target_link_libraries(myexe ${M_LIB})
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| Typically the <code>find_library</code> command would find the math library
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| /usr/lib/libm.so
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| In CMake 2.4 and lower this value would be assigned directly to <code>M_LIB</code>. Then the link line generation would split off the link directory <code>/usr/lib</code> and the library <code>libm.so</code> and produce the
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| ... -lm ...
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| Note that the <code>-L/usr/lib</code> option is left out because it is an implicit linker search path. The linker would see <code>-lm</code> and search for the math library. Typically the linker would find /usr/lib/libm.so too. However some platforms provide multiple versions of libraries correesponding to different architectures. For example, on an IRIX machine one might find the libraries
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| /usr/lib/libm.so (ELF o32)
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| /usr/lib32/libm.so (ELF n32)
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| /usr/lib64/libm.so (ELF 64)
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| On a Solaris machine one might find
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| /usr/lib/libm.so (sparcv8 architecture)
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| /usr/lib/sparcv9/libm.so (sparcv9 architecture)
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| When the linker sees <code>-lm</code> it in fact searches the system path corresponding to the current architecture. Internally it might use <code>-L/usr/lib/sparcv9</code> instead of <code>-L/usr/lib</code>.
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| In CMake 2.6 the code
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| target_link_libraries(myexe /usr/lib/libm.so)
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| would generate the link line
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| ... /usr/lib/libm.so ...
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| no matter what architecture is getting linked. This might cause the linker to complain if <code>/usr/lib/libm.so</code> does not match the architecture it wants. This is not a problem with the link line computation. CMake is linking <code>myexe</code> to the library to which it was told to link.
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| The problem is created because <code>find_library</code> may not know about all the architecture-specific system search paths used by the linker. In fact when it finds <code>/usr/lib/libm.so</code> it may be finding a library of incorrect architecture. The solution is for the command to recognize that when it finds a library in a system search path that it should ask the linker to find the correct version of the library at link time. Consider the original example:
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| find_library(M_LIB m)
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| target_link_libraries(myexe ${M_LIB})
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| In CMake 2.6 the <code>find_library</code> command will set <code>M_LIB</code> to contain just "<code>libm.so</code>" when it finds <code>/usr/lib/libm.so</code>. The link command will then be
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| target_link_libraries(myexe libm.so)
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| which will be converted to the link command line
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| ... -lm ...
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| and the linker will locate the correct version of the library. If <code>find_library</code> does not find the library in an implicit link directory it will report the full path as usual. The user might also edit the cache to set <code>M_LIB</code> to a full path. In both cases the full path given to target_link_libraries will be preserved on the final link line.
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