You need to create a project in DevC++ and add both cpp files to the project. Including the .h file makes the compiler happy, i.e., it can generate code to call the function per the function prototype in the header file. But if the function is implemented in another .cpp and not included in main. cpp, the linker need to see the object code to put the executable. That's why a non trivial program in C almost always needs a project file in IDEs like VC++ and DevC++, or a makefile with the traditio