implicit header includes in C

Question

program.c :

int main () {

    hello();
    return 0;
}

tools.c :

void hello (void) {

    printf("hello world\n");
}

Makefile :

program : program.o tools.o

In the set of files of this program I have no tools.h file, even though it compiles fine with no errors, could someone explain the purpose of header files in C programs ?

For now I only have one idea : variables such as structures are required at the compilation layer...

But in my case, if the header file only contains function prototypes, is it longer required to build it ? (the makefile linker syntax is a bit easier to catch).

Answer 1

Header files usually would contain declarations of the functions which are defined in source c files.

What purpose does it serve?

• It gives you additional safety, the compiler checks the parameters passed to a function against the declaration and reports errors if it finds an discrepancy.
• They allow seperation of interface from the implementation.Basically, this allows you to provide your code(implementation) as an library, which clients need to link against while just including the interface header file in their applications.