[CMake] Best practices/preferred methods for linking external libraries

[Alan W. Irwin]

On 2011-12-01 21:56-0000 Matthew LeRoy wrote:

> We began using CMake a few months ago for a couple of small cross-platform projects, and we're still
> learning all the ins and outs and capabilities CMake has to offer, as well as how to get the most
> out of CMake by using it "The Right Way". Right now, we're trying to figure out how to link
> to external libraries that don't have "find" modules or CMake config-mode files. After lots of
> reading in the wiki, mailing list, etc, it seems like there are several different ways to do
> this, like writing "find" modules and/or config-mode files ourselves, using the LINK_DIRECTORIES()
> command, importing/exporting targets, and others. What we're unsure of is, what is the "preferred"
> or "officially supported" method?
>
> To be a little more specific, we have two different library projects (call them ProjectA and
> ProjectB) that both use CMake, but are developed independently. Further, ProjectB depends (or will
> depend) on ProjectA; we've just recently gotten to the point on ProjectB where we want to use some
> of the functionality in ProjectA, so we need to link to the ProjectA library(ies). At first we
> thought we needed to write a "find" module for ProjectA -- but we really have very little idea how
> to go about doing that. Other than the wiki page for finding libraries that talks a little about
> writing "find" modules (http://www.vtk.org/Wiki/CMake:How_To_Find_Libraries), is there any other
> documentation for writing a proper "find" module? Is there a particular module that ships with CMake
> that is well-written that we should look at as a guide?
>
> Then we came across the wiki page for module maintainers
> (http://www.itk.org/Wiki/CMake:Module_Maintainers), which states in the Introduction that we
> should *not* write find modules for packages that themselves build with CMake, and instead to create
> a CMake package configuration file. We followed the link to the tutorials on CMake Packages, but
> don't fully understand everything that's going on there. Regardless, is this the preferred method?
>
> Another general question we have is that we may not want to have to build and install ProjectA in
> order to build ProjectB; we may instead just want to provide pre-built binaries and headers for
> ProjectA in ProjectB's source tree in version control -- part of the reason for this is to control
> which version of ProjectA is used by ProjectB. In that case, how would a find module or
> package configuration file find the binaries and headers, since they're not installed in the
> standard install location on the system? Further, regarding "standard install locations": is there a
> particular standard location to install packages on Windows systems, similar to /usr/local (or
> whatever install prefix is specified) on a Unix/Linux system?
>
> Thanks in advance for whatever direction/guidance you can give!

If both ProjectA and ProjectB are built with cmake and ProjectB's
libraries depend on ProjectA's libraries, then use the EXPORT
signature for install(TARGETS ... for each library in A, and use the
install(EXPORT ... command _once_ for A.  For example, PLplot has the
following cmake logic to do this:

install(TARGETS plplot${LIB_TAG}
   EXPORT export_plplot
   ARCHIVE DESTINATION ${LIB_DIR}
   LIBRARY DESTINATION ${LIB_DIR}
   RUNTIME DESTINATION ${BIN_DIR}
   )

(and similarly for all our installed libraries) and there is also the
following line:

install(EXPORT export_plplot DESTINATION ${DATA_DIR}/examples/cmake/modules)

which installs the required information for all of PLplot's libraries in
that location.

Then all B has to do is set CMAKE_MODULE_PATH appropriately then use
the include command to gain access to the exported information for all
installed libraries from A.

For example, the installed examples for PLplot (which have their own
independent CMake-based build system) we use

set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules)
include(export_plplot)

to get access to the required build information for the installed
libraries for the core build.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
__________________________

Linux-powered Science
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