How do I catkin-depend on the SDFormat library?

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SDFormat is not a catkin package; it is a plain cmake package. Gazebo uses pkg-config to find sdformat and explicitly links against the sdformat libraries where necessary.

If you also want to build sdformat from source in a catkin workspace, you will need to add a package.xml file (see the package_xml branch in sdformat for an example).

Normally, you would add a system dependency on sdformat, which is released with Hydro. Unfortunately, it does not seem to have been defined via rosdep yet. You should probably contribute the missing definition to fix that problem.

Then, in your package.xml, add:

<build_depend>sdformat</build_depend>
<run_depend>sdformat</run_depend>

With sdformat installed, in your CMakeLists.txt, find the SDF system library:

find_package(PkgConfig REQUIRED)
pkg_check_modules(SDF sdformat REQUIRED)
include_directories(include ${SDF_INCLUDE_DIRS})

Then your own code will need something like this to build with it:

add_executable(your_node ${YOUR_NODE_SOURCES})
target_link_libraries(your_node
                      ${catkin_LIBRARIES}
                      ${SDF_LIBRARIES})

as addition to scpeters answer:

You can add sdformat as system dependency. You can do so by calling the catkin_create_pkg script with the parameter -s sdformat. This should solve the problem without adding a package.xml.

If you already have a package you can just add sdformat like you would do with gazebo. If you are already linking gazebo as sysdep this should not be necessary...