Tutorial: Client/Server on the Raspberry Pi

--D. Thiebaut (talk) 16:52, 8 July 2013 (EDT)

---

A simple example to get started. The Raspberry Pi runs a server that waits for connection from a laptop, and expects integers from it. It multiplies each integer by 2 and sends it back. The laptop runs a client that initiates a connection, sends a bunch of positive integers that it gets back multiplied by two, and closes the connection by sending a -1. Sending a -2 causes the server to stop.

Hardware Setup

• The setup is that proposed in the excellent blog page at http://pihw.wordpress.com/guides/direct-network-connection/ from which the image above is taken.
• No need to try to copy what they already did very well, so simply follow their recommendations and connect your laptop to the Raspberry Pi using an ethernet cable.

Server/Client setup

• Here we take another good example from User Moorthy at RPI and adapt it slightly for our use.

Server code

/* A simple server in the internet domain using TCP.
myServer.c
D. Thiebaut
Adapted from http://www.cs.rpi.edu/~moorthy/Courses/os98/Pgms/socket.html
The port number used in 51717.
This code is compiled and run on the Raspberry as follows:
   
    g++ -o myServer myServer.c 
    ./myServer

The server waits for a connection request from a client.
The server assumes the client will send positive integers, which it sends back multiplied by 2.
If the server receives -1 it closes the socket with the client.
If the server receives -2, it exits.
*/

#include <stdio.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>


void error( char *msg ) {
  perror(  msg );
  exit(1);
}

int func( int a ) {
   return 2 * a;
}

void sendData( int sockfd, int x ) {
  int n;

  char buffer[32];
  sprintf( buffer, "%d\n", x );
  if ( (n = write( sockfd, buffer, strlen(buffer) ) ) < 0 )
    error( const_cast<char *>( "ERROR writing to socket") );
  buffer[n] = '\0';
}

int getData( int sockfd ) {
  char buffer[32];
  int n;

  if ( (n = read(sockfd,buffer,31) ) < 0 )
    error( const_cast<char *>( "ERROR reading from socket") );
  buffer[n] = '\0';
  return atoi( buffer );
}

int main(int argc, char *argv[]) {
     int sockfd, newsockfd, portno = 51717, clilen;
     char buffer[256];
     struct sockaddr_in serv_addr, cli_addr;
     int n;
     int data;

     printf( "using port #%d\n", portno );
    
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0) 
         error( const_cast<char *>("ERROR opening socket") );
     bzero((char *) &serv_addr, sizeof(serv_addr));

     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons( portno );
     if (bind(sockfd, (struct sockaddr *) &serv_addr,
              sizeof(serv_addr)) < 0) 
       error( const_cast<char *>( "ERROR on binding" ) );
     listen(sockfd,5);
     clilen = sizeof(cli_addr);
  
     //--- infinite wait on a connection ---
     while ( 1 ) {
        printf( "waiting for new client...\n" );
        if ( ( newsockfd = accept( sockfd, (struct sockaddr *) &cli_addr, (socklen_t*) &clilen) ) < 0 )
            error( const_cast<char *>("ERROR on accept") );
        printf( "opened new communication with client\n" );
        while ( 1 ) {
	     //---- wait for a number from client ---
             data = getData( newsockfd );
             printf( "got %d\n", data );
             if ( data < 0 ) 
                break;
                
             data = func( data );

             //--- send new data back --- 
	     printf( "sending back %d\n", data );
             sendData( newsockfd, data );
	}
        close( newsockfd );

        //--- if -2 sent by client, we can quit ---
        if ( data == -2 )
          break;
     }
     return 0; 
}

Client code

/* A simple client program to interact with the myServer.c program on the Raspberry.
myClient.c
D. Thiebaut
Adapted from http://www.cs.rpi.edu/~moorthy/Courses/os98/Pgms/socket.html
The port number used in 51717.
This code is compiled and run on the Macbook laptop is below:
   
    g++ -o myClient myClient.c 
    ./myClient


*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 
#include <netdb.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> 

void error(char *msg) {
    perror(msg);
    exit(0);
}

void sendData( int sockfd, int x ) {
  int n;

  char buffer[32];
  sprintf( buffer, "%d\n", x );
  if ( (n = write( sockfd, buffer, strlen(buffer) ) ) < 0 )
      error( const_cast<char *>( "ERROR writing to socket") );
  buffer[n] = '\0';
}

int getData( int sockfd ) {
  char buffer[32];
  int n;

  if ( (n = read(sockfd,buffer,31) ) < 0 )
       error( const_cast<char *>( "ERROR reading from socket") );
  buffer[n] = '\0';
  return atoi( buffer );
}

int main(int argc, char *argv[])
{
    int sockfd, portno = 51717, n;
    char serverIp[] = "169.254.0.2";
    struct sockaddr_in serv_addr;
    struct hostent *server;
    char buffer[256];
    int data;

    if (argc < 3) {
      // error( const_cast<char *>( "usage myClient2 hostname port\n" ) );
      printf( "contacting %s on port %d\n", serverIp, portno );
      // exit(0);
    }
    if ( ( sockfd = socket(AF_INET, SOCK_STREAM, 0) ) < 0 )
        error( const_cast<char *>( "ERROR opening socket") );

    if ( ( server = gethostbyname( serverIp ) ) == NULL ) 
        error( const_cast<char *>("ERROR, no such host\n") );
    
    bzero( (char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy( (char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
    serv_addr.sin_port = htons(portno);
    if ( connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr)) < 0) 
        error( const_cast<char *>( "ERROR connecting") );

    for ( n = 0; n < 10; n++ ) {
      sendData( sockfd, n );
      data = getData( sockfd );
      printf("%d ->  %d\n",n, data );
    }
    sendData( sockfd, -2 );

    close( sockfd );
    return 0;
}