libexample.c

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

void helloworld() {
  printf("Hello World\n");
}

libexample.h

void helloworld();

Makefile

INSTALL = /usr/bin/install -c
prefix = /usr/local
libdir = $(prefix)/lib

default:
        $(CC) -Wall -fPIC -c libexample.c
        $(CC) -Wall -shared -Wl,-soname=libexample.so -o libexample.so libexample.o 

clean:
        @rm -f libexample.o libexample.so

install:
        $(INSTALL) libexample.so $(libdir)


.PHONY: clean

test-fork.c

/* $ gcc -I. -L. -lexample test-fork.c -o test-fork */

#include <libexample.h>
#include <stdio.h>
#include <stdio.h>
#include <unistd.h>

int main() {
  int status;

  printf("[Parent] PID is %d\n", getpid());
  printf("[Parent] ");
  helloworld();

  while(1) {
    pid_t pid = fork();
    if (pid == 0) {
      printf("[Child] ");
      helloworld();
      printf("[Child] PID is %d\n", getpid());
      sleep(1);
    } else if (pid < 0) {
      _exit(-1);
    } else {
      waitpid(pid, &status, 0);
    }
  }
}

test.c

/* $ gcc -I. -L. -lexample test.c -o test */
#include <libexample.h>
#include <stdio.h>
#include <stdio.h>

int main() {
  helloworld();
  printf("PID is %d\n", getpid());
  printf("Press Any Key To Exit...");
  fgetc(stdin);
  return 0;
}