There is no magic in .h files, you can think them as plaintext. The actual magic happens in .c files. Since C requires objects to be declared before using them,

There is no magic in .h files, you can think them as plaintext. The actual magic happens in .c files. Since C requires objects to be declared before using them, you need to declare objects used in multiple units together. Let's say you have a function `int f(int)`. You want to use this function both in apple.c and pear.c. How would you do it. Well you need to write a f.h such as int f(int); then in apple.c #include "f.h" int apple() { int x = f(10); // rest of the code } You can think this as a copy paste of f.h such that, apple.c is equivalent to: int f(int); int apple() { int x = f(10); // rest of the code } but what "is" f. To "define" f you need another .cpp file that implements f, let's call it f.c: #include "f.h" int f(int x) { return x*x; } now you compile f.c, apple.c, pear.c. These will create f.o, apple.o and pear.o object files. Then you link these together to create your application. e.g. if f.h is in include/ dir: gcc -c apple.c -I include/ -o apple.o gcc main.c -I include/ apple.o pear.o f.o -o main Here, assume that main.c defines int main().