#include <stdio.h>
#include <stdlib.h>
int main(){

  int * int_ptr;
  int_ptr = (int *)malloc(2*sizeof(int));
  
  if(!int_ptr) {
    printf("Something went wrong while allocating memory. Exiting...");
    exit(1);
  }
  
  printf("Enter first integer: ");
  scanf("%i", &int_ptr[0]);
  
  printf("Enter second integer: ");
  scanf("%i", &int_ptr[1]);
  
  printf("Original values: 1st = %i 2nd = %i\n",int_ptr[0], int_ptr[1]);
  
  int_ptr[0] = int_ptr[0] ^ int_ptr[1];
  //printf("%i\n", int_ptr[0]);
  int_ptr[1] = int_ptr[0] ^ int_ptr[1];
  //printf("%i\n", int_ptr[1]);
  int_ptr[0] = int_ptr[0] ^ int_ptr[1];
  //printf("%i\n", int_ptr[0]);
  
  printf("Swapped values: 1st = %i 2nd = %i\n", int_ptr[0], int_ptr[1]);

  free(int_ptr);
  exit(0);
}

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

int glbvar = 6;
int main() {
  int locvar = 88;
  pid_t pid;
  printf("Before fork()\n");
  if ((pid = fork()) == 0) {
    /* child */
    glbvar++;
    locvar++;
  } else if (pid > 0) {
    /* parent */
    sleep(2);
  } else
    perror("fork error");
  printf("pid=%d, glbvar=%d, locvar=%d\n", getpid(), glbvar, locvar);
  return 0;
} /* end main */

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

int main() {
  char * cmd[] = {
    "who",
    "ls",
    "date"
  };
  int i;
  printf("0=who 1=ls 2=date : ");
  scanf("%d", & i);
  execlp(cmd[i], cmd[i], (char * ) 0);
  printf("execlp failed\n");
  return 0;
}

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

int main() {
  char * cmd[] = {
    "who",
    "ls",
    "date"
  };
  int i;
  while (1) {
    printf("0=who 1=ls 2=date : ");
    scanf("%d", & i);
    if (fork() == 0) {
      /* child */
      execlp(cmd[i], cmd[i], (char * ) 0);
      printf("execlp failed\n");
      exit(1);
    } else {
      /* parent */
      wait((int * ) 0);
      printf("child finished\n");
    }
  } /* while */
} /* main */

// Server side implementation of UDP client-server model
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>

#define PORT 8008
#define RECIEVE_BUFFER_SIZE 1024

// Driver code
int main() {

        //Create a UDP socket
        //int socket(int domain, int type, int protocol);
        //Domain is the place where the data goes. AF_BLUETOOTH, AF_INET6, AF_UNIX is local communication between different programs
        //Type is the type of connection, so connection based (TCP), connectionless (UDP) or somewhere in between or beyond
        //Protocol is an enum or flags that give some more options specific to that type. For SOCK_STREAM, you can force it to use SCTP instead of TCP. For SOCK_RAW, it can give you control over the ip frame, and even the ethernet frame.
        //So this function makes a file descriptor to an IPV4 UDP 'connection' with no options.
        int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
        // Creating socket file descriptor
        if ( sockfd < 0) {
                perror("socket creation failed");
                exit(EXIT_FAILURE);
        }


        struct sockaddr_in serverAddress;
        memset(&serverAddress, 0, sizeof(serverAddress));

        // Filling server information
        serverAddress.sin_family = AF_INET; // IPv4
        serverAddress.sin_addr.s_addr = INADDR_ANY;
        serverAddress.sin_port = htons(PORT);

        // Bind the socket with the server address
        if(bind(sockfd, (const struct sockaddr*)&serverAddress, sizeof(serverAddress))){        //Bind returns 0 if succeeded, or -1 if failed. -1 is still true.
                perror("bind failed");
                exit(EXIT_FAILURE);
        }

        struct sockaddr_in clientAddress; unsigned int clientAddressSize = sizeof(clientAddress);
        memset(&clientAddress, 0, clientAddressSize);

        char buffer[RECIEVE_BUFFER_SIZE];
        while(1){
                int bytesRecieved = recvfrom(sockfd, (char*)buffer, RECIEVE_BUFFER_SIZE, MSG_WAITALL, (struct sockaddr*) &clientAddress, &clientAddressSize); //sets clientAddress to the client's address and the new size
                buffer[bytesRecieved] = '\0';
                //printf("Client : %s\n", buffer);

                //The client might be like PINGUIN or something, but this is to ensure PING\n\r also gets accepted
                if(bytesRecieved >= 4 && (strncmp("PING", buffer, 4) == 0)){
                        const char* responseMessage = "PONG";
                        sendto(sockfd, (const char*)responseMessage, strlen(responseMessage), MSG_CONFIRM, (const struct sockaddr*) &clientAddress, clientAddressSize);
                        //printf("Hello message sent.\n");
                }else{
                        printf("Server bytes recieved: %d\nServer message recieved: %s", bytesRecieved, buffer);
                }
        }

        return 0;
}

// Client side implementation of UDP client-server model
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>

#define PORT 8008
#define MAXLINE 1024

// Driver code
int main() {
        char buffer[MAXLINE];
        struct sockaddr_in servaddr;

        // Creating socket file descriptor

        int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
        if ( sockfd < 0 ) {
                perror("socket creation failed");
                exit(EXIT_FAILURE);
        }

        struct timeval timeout;
        timeout.tv_sec = 1; //Wait 1 second
        timeout.tv_usec = 0;

        if (setsockopt (sockfd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof timeout) < 0) perror("setsockopt failed\n");

        if (setsockopt (sockfd, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof timeout) < 0) perror("setsockopt failed\n");

        memset(&servaddr, 0, sizeof(servaddr));

        // Filling server information
        servaddr.sin_family = AF_INET;
        servaddr.sin_port = htons(PORT);
        servaddr.sin_addr.s_addr = inet_addr("192.168.4.65");

        for(int i = 0; i < 10; ++i){
                char* message = "PING";
                int sendStatus = sendto(sockfd, (const char *)message, strlen(message), MSG_CONFIRM, (const struct sockaddr *) &servaddr, sizeof(servaddr));
                if(sendStatus >= 0){
                        printf("Sent PING\n");
                }else{
                        printf("Send failed\n");
                        continue;
                }

                int len = sizeof(struct sockaddr_in);
                int bytesRecieved = recvfrom(sockfd, (char *)buffer, MAXLINE, MSG_WAITALL, (struct sockaddr *) &servaddr, &len); //We can reuse servaddr because the port the server sends messages from is the same one we send to
                if(bytesRecieved >= 0){
                        buffer[bytesRecieved] = '\0';
                        printf("Recieved %s\n", buffer);
                }else{
                        printf("Recieved nothing: Packet Dropped\n", buffer);
                }
        }

        close(sockfd);

        return 0;
}