CIS241

System-Level Programming and Utilities

C - Header Files

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Time to make our code reusable

Let's say we want to reuse a function, or our incredible struct from last time...

typedef struct Player {
  int roomID;
  int health;
  char* name;
} Player;

How do we reuse this across projects?

Header files!

Header files contain C code, filenames, end in .h

Where have we seen these before?

• The files we’ve been including, example:
  • #include <stdio.h>

For system headers, we use

#include <file.h>

For local headers that we write:

#include "file.h"

Traditionally...

Traditionally, header files contain structs, variables, and function prototypes

• (You can put function definitions in headers, there are tradeoffs)

Example header file

This extra stuff is a header guard

• #ifndef checks to see if macro (PRINT_H here) is defined, if so, it cuts out all code between it and #endif

Because we define the macro inside this if, we ensure we have, at most, one copy of this code!

• #pragma once is a modern alternative

Function definition (i.e., the code?)

The code goes in a separate .c file with the same name

print.c:

#include "print.h"

void PrintArray(int* arr, int len) {
    printf("[");
    for (int i = 0; i < len; i++) {
        if (i != 0) printf(" ");
        printf("%d", *(arr + i));
        if (i != len - 1) printf(",");
    }
    printf("]\n");
}

Using the header file

#include <stdio.h>
#include "print.h"

int main() {
    int arr_1[] = {1, 2, 3, 4, 5, 6, 7};
    PrintArray(arr_1, 7);
    return 0;
}

However...

However!

How do we tell main.c about print.c?

• Compile it in!
• gcc main.c print.c -o printer.o

Saving intermediate builds

What if we don’t want to compile all files each time?

• E.g., maybe print.c rarely changes, no reason to recompile.

We can compile them separately using object files:

• gcc -c print.c
  • (creates print.o)
• gcc program.c print.o -o printer.o

This is called linking files

• Ever have a linker error?

Where are includes located?

A local include like: #include "file.h" will first look in current directory

• System also has some go-to directories that can be checked.

We can also specify paths to check for files in our gcc call using -I

Example:

• gcc -Iother_proj/headers
  • Note the lack of space after I

Preprocessor directives (i.e., more-of)

Remember: these operate on text, they do not consider code context

List (part 1):

• #include <header.h> -> paste contents of header.h

• #define NAME X -> Replace instances of NAME with X

• #define NAME(a) printf(“a”); -> Can also take arguments

• #undef NAME -> Undefines NAME

Preprocessor directives (i.e., more-of)

Remember: these operate on text, they do not consider code context

List (part 2):

• #if X -> Includes following code if X is not 0

• #elsif and #else -> Go with if

• #endif -> Ends if / elsif / else blocks

• #ifdef X -> Like if; includes code if X is defined

• #ifndef X -> Includes if X is NOT defined