Normally, you’ll create a header file with the prototype ... For things like C++ classes — or anything, really — you can enclose a block of code inside a special preprocessor directive ...

Don't put variable declarations in header files, IMO. Make a globals.c file and put them in there ... as long as the text inside is syntactically correct, right?) Obviously not a good practice ...

In C programming ... Also called "include files," because the #include statement is used to incorporate them with the program, header files use a .h extension. See precompiled header.