linker test

apple.c

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

// int global = -42;

void apple(void)
{
  printf("apple\n");
  global = 42;
}

banana.c

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

// int global = -99;

void banana(void)
{
  printf("banana\n");
  global = 99;
}

global.c

#include "global.h"

// int global = -666;
int global;

global.h

extern int global;

main.c

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

void apple(void);
void banana(void);

int main()
{
  printf("global = %d\n", global);
  apple();
  printf("global = %d\n", global);
  banana();
  printf("global = %d\n", global);
  return 0;
}

Makefile

all:    main

main:   main.o libfruit.a
        $(CC) -o main main.o -L. -lfruit

libfruit.a: apple.o banana.o global.o
        ar cr libfruit.a apple.o banana.o global.o
  
clean:
        rm -f *.o *.a main