Socket Programming.md

Points

• On a single system, Two process can communicate through
  1. Pipes
  2. Message queues
  3. Shared memory
• To communicate between two process over network, you need SOCKET
• Socket = End point of communication between two systems on a network OR Combination of IP & Port Number

Server Example

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

void ErrorAndExit(const char *str);

#define IP       "127.0.0.1"    /* Local Host OR Should be your local IP for test */
#define PORT     5000

int main(int argc, char *argv[])
{
        int listenfd = 0, connfd = 0;
        struct sockaddr_in serv_addr = {0};

        if( (listenfd = socket(AF_INET, SOCK_STREAM, 0) ) < 0 )
                ErrorAndExit("Could not create socket");

        serv_addr.sin_family = AF_INET;
        serv_addr.sin_addr.s_addr = inet_addr(IP);
        serv_addr.sin_port = htons(PORT);

        if( (bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) ) < 0 )
                ErrorAndExit("Could not bind on Given IP");

        if( (listen(listenfd, 10)  ) < 0 )
                ErrorAndExit("Could not listen");

        if( ( connfd = accept(listenfd, (struct sockaddr*)NULL, NULL) ) < 0 )
                ErrorAndExit("Could not accept");

        write(connfd, "Server Message", strlen("Server Message"));

        close(connfd);
        
        return 0;
}

void ErrorAndExit(const char *str)
{
        printf("Error : %s\n", str);
        exit(EXIT_FAILURE);
}

• socket() creates socket inside the kernel & returns socket descriptor.
  1. AF_INET represents IPv4 addresses.
  2. SOCK_STREAM specifies communication semantics means how communcations would carried out.
  3. The 3rd argument is zero to let the kernel decide the default protocol to use for this connection. The default protocol used is TCP.
• bind(), wait for client requests on particular IP-Port specified in the structure serv_addr.
• listen() with second argument as 10 specifies maximum number of client connections that server will queue for this listening socket.
• accept(), put the server into sleep & when client requests, the three way TCP handshake* is complete, the function wakes up & returns the socket descriptor representing the client socket.
• As soon as server gets a request from client, it prepares its message & writes on the client socket through the descriptor returned by accept().

Client Example

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

void ErrorAndExit(const char *str);

#define IP       "127.0.0.1"    /* Local Host OR Should be your local IP for test */
#define PORT     5000

int main()
{
        int sockfd = 0, n = 0;
        char recvBuff[1024];
        struct sockaddr_in serv_addr = {0};

        memset(recvBuff, `0`,sizeof(recvBuff));

        if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
                ErrorAndExit("Could not create socket");

        serv_addr.sin_family = AF_INET;
        serv_addr.sin_port = htons(PORT);
        serv_addr.sin_addr.s_addr = inet_addr(IP);

        if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
                ErrorAndExit("Connect Failed");

        while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0)
        {
                recvBuff[n] = 0;
                if(fputs(recvBuff, stdout) == EOF)
                        ErrorAndExit("fputs error");
        }

        if(n < 0)
                ErrorAndExit("Read error");

        return 0;
}

void ErrorAndExit(const char *str)
{
        printf("Error : %s\n", str);
        exit(EXIT_FAILURE);
}

• socket() do work same as mentioned in server
• connect() will connect this socket with remote host whose IP-Port & other info bundled up in a structure sockaddr_in.
• Once the sockets are connected, the server sends the data on clients socket through clients socket descriptor and client can read it through normal read call on the its socket descriptor.