By convention header files use the extension '.h' rather than the '.c' extension for C source files. While header files may contain any legal C code, it is good practice to limit this code to variable and function declarations and preprocessor directives. If an external variable is defined (allocated) or a complete function definition is given,
then errors may occur when the overall program is linked (i.e., when the compiled source files and the standard function libraries are combined into an executable application). The reason is that the header may need to be #included into several source files, leading to multiple definitions for the same variable or function.
