Basic socket programming (TCP)

Sockets are end points on a machine for communication.

Below code demonstatres connection oriented program (TCP).

Client:

#include following libraries stdio.h, sys/socket.h, sys/types.h, netinet/in.h, netdb.h

int main(int argc,char **argv)

{

  int sockfd,newsockfd;  // socket descriptors 

  struct sockaddr_in serv_addr;

  struct hostent *he;

  char a[50],a1[50];

  sockfd=socket(AF_INET,SOCK_STREAM,0); // creates a socket

  if(sockfd<0)

  {

   printf("Socket failed");

   exit(0);

  }

// Use either green or yellow code, if you want to give server name as command line argument use the green one. 

  if((he=gethostbyname(argv[2]))==NULL)  // gets the ip address

  {

  printf("gethost error");

  exit(1);

  }

  serv_addr.sin_family=AF_INET;

  serv_addr.sin_addr.s_addr=inet_addr("127.127.127.1");

  memcpy(&(serv_addr.sin_addr),he->h_addr,he->h_length);

  serv_addr.sin_port=htons(atoi(argv[1])); 

  if(connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr))<0)

  {

   printf("\nConnection failed");

   exit(0);

  }

  printf("\nEnter a Message");

  scanf("%s",&a);

  write(sockfd,a,50);

  read(sockfd,a1,50);

  printf("\nClient received Message %s",a1);

  close(sockfd); // closing the socket descriptor

}

Server: 

#include following libraries stdio.h, sys/socket.h, sys/types.h, netinet/in.h

int main(int argc,char **argv)

{

  int sockfd,newsockfd,clilen;

  struct sockaddr_in serv_addr,cli_addr;

  char a[50];

  sockfd=socket(AF_INET,SOCK_STREAM,0);

  if(sockfd<0)

  {

   printf("\nSocket failed!");

   exit(0);

  }

  serv_addr.sin_family=AF_INET;

  serv_addr.sin_addr.s_addr=htonl(INADDR_ANY);

  serv_addr.sin_port=htons(atoi(argv[1]));

  if(bind(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr))<0)

  {

   printf("\nBind failed");

   exit(1);

  }

  if(listen(sockfd,5)<0)

  {

   printf("\nListen fails");

   exit(0);

  }

  clilen=sizeof(cli_addr);

  newsockfd=accept(sockfd,(struct sockaddr *)&cli_addr,&clilen);

  read(newsockfd,a,80);

  printf("Server received %s",a);

  write(newsockfd,"Server received message",26);

  close(newsockfd);

  close(sockfd);

}

compilation:

client :   gcc -o client client.c

server :   gcc -o server server.c

Execution:

server : ./server (portno)

client : ./client (portno) (servername)

Communication between client and server.

Client side:

client creates a socket and sends a request to server

Server side:

1. Server creates a socket and waiting for a request.
   
2. when request arrives to server socket then it creates a new socket and forward the request to that new socket. 
   
3. Further communication is goes on in between client's socket and server's new socket.