Trying to make a program with header files in C

Question

I'm trying to make a program in C that uses header files and I'm getting an error. The program is supposed to add two numbers, add two numbers with a macro, and print a component in a typedef. There are three separate files:

functions.c

int add(int n1, int n2) {
    return n1 + n2;
}

functions.h

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

// function declaration
int add(int, int);

// macro declaration
#define ADD(n1, n2) n1 + n2

// typedef declaration
typedef struct {
    int age;
    char *name;
} person;

#endif

main.c (This is the main program that uses everything from functions.c and functions.h and prints the result.)

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

int main(void) {
    printf("%d\n", add(2, 6));
    printf("%d\n", ADD(3, 7));

    person engineer_man;
    engineer_man.age = 32;
    printf("engineer man's age is: %d\n", engineer_man.age);
}

I tried compiling by doing gcc -g -std=c99 -Wall -Wvla -Werror -fsanitize=address,undefined -o main main.c, but then it gave this error:

/usr/bin/ld: /tmp/ccbVAioe.o: in function `main':
/common/home/user/folder/main.c:5: undefined reference to `add'
collect2: error: ld returned 1 exit status

How do I fix this error?

Answer 1

You never compile functions.c.

You need this:

gcc -g -std=c99 -Wall -Wvla -Werror -fsanitize=address,undefined -o main main.c functions.c
                                                                 you forgot this ^

BTW, you also need to include "functions.h" in "functions.c":

#include "functions.h"

int add(int n1, int n2) {
    return n1 + n2;
}

Without including "functions.h" your program will build, but if for some reason the function declaration in functions.h doesn't match the function definiton in "functions.c", your program will have undefined behaviour.

Answer 2

gcc -c functions.c
gcc -o main main.c functions.o

should also work well. I omitted the compiler flags that need to be added in each case, depending on your need.