# Locating packages with cmake

When building programs with cmake on non-UNIX systems, it can be a pain to specify the location of external libraries. I’ve been upgrading mkg3a to support using libpng to load icons in addition to the old bmp loader, but that means I need to link against libpng, and also zlib (since libpng depends on zlib to handle the image compression). Compiling it all on Windows, however, is not an easy task, since there’s no standard search path for libraries like there is on UNIX systems (eg /usr/include for libraries, /usr/lib for libraries..). I didn’t find any good resources on how to make it work in my own searches, so here’s a quick write-up of the process in the hopes that it’ll be useful to somebody else.

I grabbed the zlib and libpng static libraries from gnuwin32 and extracted them near my mkg3a source tree, in the same directory. Setting up to build, then, my directory tree looks something like the following (some files omitted for brevity):

build\
- libs\
- include\
+ libpng12\
| png.h
| pngconf.h
| zconf.h
| zlib.h
- lib\
| libpng.lib
| zlib.lib
+ manifest\
- mkg3a\
| CMakeLists.txt
| config.h.in

In order to tell cmake where to find zlib and libpng now, we can use the CMAKE_PREFIX_PATH variable, which is a path relative to the source directory. In this case, the following command will pick up the libraries in libs and generate project files for Visual Studio 2010 (note we’re executing from within the build tree):
H:\Desktop\build> cmake -G "Visual Studio 10" -D CMAKE_PREFIX_PATH=../libs ../mkg3a

If the build tree were instead under the source tree (mkg3a/build/ instead of just build/), the value for CMAKE_PREFIX_PATH would not need to change, since it is specified relative to the source directory.
In short: set CMAKE_PREFIX_PATH to help it find packages when they’re not in the usual system locations. It’s much easier to combine all your external libraries into one directory (libs in my example), but you could also specify a list of paths and keep them separate.