//All in One C file #include void Hello(); int main(){ Hello(); return 0; } void Hello(){ printf("hello\n"); } -------------------------------------------------------------------- //main.c #include #include "hello.h" int main(){ print_hello(); return 0; } //hello.h void print_hello(); //hello.c #include void print_hello(){ printf("hello\n"); } gcc -c main.c hello.c gcc -o a main.o hello.o ./a or..