How to include headers from other packages

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I'm using ROS Groovy with catkin.

I created a package1 with a file1.cpp in the src directory and a header file1.h in its include/package directory.

I need to #include this header in another file2.cpp of another package2 in the same workspace and then call those declared function.

file1.cpp doesn't have any main() function. It's only a list of functions and their body definitions.

I'm not secure with this tool because everything I tried using tutorials is not working. Please, give me full informations about what to write in every CMakeLists.txt or package.xml and how to include that header in both packages.

Thanks in advance.