This is process 1: He is annoying and will only stop once it receives a signal from his neighbour.

neighbour1.c

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

/*If he causes any problem, do a `kill -9 <Neighbour ID> on a separate terminal.*/

int shmid;
pid_t *shmptr;
void __gcov_flush(void);//check in gcc sources gcc/gcov-io.h for the prototype, in case it creates problems.

void sig1_handler(volatile sig_atomic_t usr_sig)
{
        signal(usr_sig, SIG_IGN);
        printf("Received instructions %u\nDumping neighbour's fight logs.\n I am happy you erred\n",usr_sig);
        __gcov_flush();
        shmdt(shmptr);
        shmctl(shmid, IPC_RMID, NULL);
        exit(1);
}

void sig2_handler(volatile sig_atomic_t usr_sig)
{
        signal(usr_sig, SIG_IGN);
        printf("Received instructions %u\nDumping neighbour's complaints logs.\n I am kidnapped.\n",usr_sig);
        __gcov_flush();
        shmdt(shmptr);
        shmctl(shmid, IPC_RMID, NULL);
        exit(2);
}

void sig3_handler(volatile sig_atomic_t usr_sig)
{
        signal(usr_sig, SIG_IGN);
        printf("Received instructions %u\nDumping neighbour's quarrel logs.\n I am killed\n",usr_sig);
        __gcov_flush();
        shmdt(shmptr);
        shmctl(shmid, IPC_RMID, NULL);
        exit(3);
}

void sig4_handler(volatile sig_atomic_t usr_sig)
{
        signal(usr_sig, SIG_IGN);
        printf("Received instruction %u\ndumping error logs.\n I am spared\n",usr_sig);
        __gcov_flush();
        shmdt(shmptr);
        shmctl(shmid, IPC_RMID, NULL);
        exit(4);
}

void main(int argc, char **argv[])
{
        int i;
        pid_t pid=getpid();
        key_t keygen;

        if (signal(SIGUSR1, sig1_handler)==SIG_ERR){
                printf("Error1. interrupt and try again\n");
                //signal(SIGINT, sigdf_handler);
                exit(0);
        }
        if (signal(SIGINT, sig2_handler)==SIG_ERR){
                printf("Error2. interrupt and try again\n");
                
                exit(0);
        }
        if (signal(SIGQUIT, sig3_handler)==SIG_ERR){
                printf("Error3. interrupt and try again\n");
                
                exit(0);
        }
        if (signal(SIGUSR2, sig4_handler)==SIG_ERR){
                printf("Error4. interrupt and try again\n");
                
                exit(0);
        }
        
        /*This is the shared memory created to be given access to the neighbbour 2.
        The signal arrives here through the following route*/
        
        keygen=ftok(".",'s');
        shmid=shmget(keygen, sizeof(pid_t), IPC_CREAT | 0666);
        shmptr=(pid_t *) shmat(shmid, NULL, 0);
        *shmptr=pid;
        for(i=0; ;i++){
                printf("I am Neighbour ID:%d:%dI like to annoy you\n",pid,i);
                sleep(2);}
        
}