undefined reference to function fork and getppid

Question

The error I am getting on compiling the following code is undefined reference to 'fork' undefined reference to 'getppid' error: ld returned 1 exit status

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>

int main()
{
    int pid;
    pid=fork();    //creates child process and stores the return value 
    if(pid>0)
        printf("parent process id:%d",getpid());    //prints the process id
    else if(pid==0)
        printf("Child process id:%d parent id:%d Parent process id:%d",getpid(),getppid());    //prints the process and parent id
    else
        printf("Error");
    return 0;
}