CMake 2.6 Notes

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This page documents some of the changes and new features available in CMake 2.6.

Exporting and Importing Targets

Please see our tutorial on Exporting and Importing Targets.

Packages

Please see our tutorial on Packaging.

Preprocessor Definitions

Preprocessor definitions may now be added to builds with much finer granularity than in previous versions of CMake. There is a new property called COMPILE_DEFINITIONS that is defined directories, targets, and source files. For example, the code

 add_library(mylib src1.c src2.c)
 add_executable(myexe main1.c)
 set_property(
   DIRECTORY
   PROPERTY COMPILE_DEFINITIONS A AV=1
   )
 set_property(
   TARGET mylib
   PROPERTY COMPILE_DEFINITIONS B BV=2
   )
 set_property(
   SOURCE src1.c
   PROPERTY COMPILE_DEFINITIONS C CV=3
   )

will build the source files with these definitions:

 src1.c:   -DA -DAV=1 -DB -DBV=2 -DC -DCV=3
 src2.c:   -DA -DAV=1 -DB -DBV=2
 main2.c:  -DA -DAV=1

When the add_definitions command is called with flags like "-DX" the definitions are extracted and added to the current directory's COMPILE_DEFINITIONS property. When a new subdirectory is created with add_subdirectory the current state of the directory-level property is used to initialize the same property in the subdirectory.

Note in the above example that the set_property command will actually set the property and replace any existing value. The command provides the APPEND option to help add more definitions without removing existing ones. For example, the code

 set_property(
   SOURCE src1.c
   APPEND PROPERTY COMPILE_DEFINITIONS D DV=4
   )

will add the definitions "-DD -DDV=4" when building src1.c.

Definitions may also be added on a per-configuration basis using the COMPILE_DEFINITIONS_<CONFIG> property. For example, the code

 set_property(
   TARGET mylib
   PROPERTY COMPILE_DEFINITIONS_DEBUG MYLIB_DEBUG_MODE
   )

will build sources in mylib with -DMYLIB_DEBUG_MODE only when compiling in a Debug configuration.

Link Line Generation

CMake 2.6 implements a new approach to generating link lines for targets.

Consider these libraries:

  /path/to/libfoo.a
  /path/to/libfoo.so

Previously if someone wrote

  target_link_libraries(myexe /path/to/libfoo.a)

CMake would generate this code to link it:

  ... -L/path/to -Wl,-Bstatic -lfoo -Wl,-Bdynamic ...

This worked most of the time, but some platforms (such as OS X) do not support the -Bstatic or equivalent flag. This made it impossible to link to the static version of a library without creating a symlink in another directory and using that one instead.

Now CMake will generate this code:

  ... /path/to/libfoo.a ...

This guarantees that the correct library is chosen. However there are some side-effects that affect compatibility with existing projects (documented in the next two subsections).

Missing Linker Search Directories

Projects used to be able to write this (wrong) code and it would work by accident:

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

where "B" is meant to link "/path/to/libB.so". This code is incorrect because it asks CMake to link to B but does not provide the proper linker search path for it. It used to work by accident because the -L/path/to would get added as part of the implementation of linking to A. The correct code would be

  link_directories(/path/to)
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

or even better

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)

In order to support projects that have this bug, we've added a compatibility feature that adds the "-L/path/to" paths for all libraries linked with full paths even though the linker will not need those paths to find the main libraries. See policy CMP0003 for details.

Linking to System Libraries

System libraries on UNIX-like systems are typically provided in /usr/lib or /lib. These directories are considered implicit linker search paths because linkers automatically search these locations even without a flag like -L/usr/lib. Consider the code

 find_library(M_LIB m)
 target_link_libraries(myexe ${M_LIB})

Typically the find_library command would find the math library

 /usr/lib/libm.so

Some platforms provide multiple versions of libraries corresponding to different architectures. For example, on an IRIX machine one might find the libraries

 /usr/lib/libm.so         (ELF o32)
 /usr/lib32/libm.so       (ELF n32)
 /usr/lib64/libm.so       (ELF 64)

On a Solaris machine one might find

 /usr/lib/libm.so          (sparcv8 architecture)
 /usr/lib/sparcv9/libm.so  (sparcv9 architecture)

Unfortunately find_library may not know about all the architecture-specific system search paths used by the linker. In fact when it finds /usr/lib/libm.so it may be finding a library of incorrect architecture. If the link computation were to produce the line

 ... /usr/lib/libm.so ...

the linker might complain if /usr/lib/libm.so does not match the architecture it wants.

One solution to this problem is for the link computation to recognize that the library is in a system directory and ask the linker to search for the library. It could produce the link line

 ... -lm ...

and the linker would search through its architecture-specific implicit link directories to find the correct library. Unfortunately this solution suffers from the original problem of distinguishing between static and shared versions:

 /usr/lib/libm.a
 /usr/lib/libm.so

In order to ask the linker to find the static system library of the correct architecture it must produce the link line

 ... -Wl,-Bstatic -lm ... -Wl,-Bshared ...

This solution directly contradicts the original motivation to give the linker paths to libraries instead of -l options: not all platforms have an option like -Bstatic. Fortunately the platforms that do not provide such flags also tend to not have architecture-specific implicit link directories.

The solution used by CMake is:

  • Libraries not in implicit system locations are linked by passing the file path to the linker
  • Libraries in implicit system locations are linked by
    • passing the -l option if a flag like -Bstatic is available
    • passing the file path to the linker otherwise

Users can override this behavior by using the IMPORTED targets feature:

 add_library(math STATIC IMPORTED)
 set_property(TARGET math PROPERTY IMPORTED_LOCATION /usr/lib/libm.a)
 add_executable(foo foo.c)
 target_link_libraries(foo math) # will link using full path

CMake Policy Mechanism

CMake 2.6 introduces a new mechanism for backwards compatibility support. See CMake/Policies for more information.