C Program to implement CRC (Cyclic Redundancy Code) in Client-Server.

 Client :

#include<stdio.h>
#include<string.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>
#define SERV_TCP_PORT 5035
int main(int argc,char * * argv)
{
       int sockfd;
       struct sockaddr_in serv_addr;
       struct hostent *server;

       int i,j,keylen,msglen,k;
       char input[100],key[30],temp[30],quot[100],key1[30],
       afterror[100], sermessage[200];
     
       sockfd=socket(AF_INET,SOCK_STREAM,0);
       serv_addr.sin_family=AF_INET;
       serv_addr.sin_addr.s_addr=inet_addr("127.0.0.1");
       serv_addr.sin_port=htons(SERV_TCP_PORT);
       connect(sockfd,(struct sockaddr*)&serv_addr,sizeof(serv_addr));
     
       printf("Enter Message Data : ");
        gets(input);
        strcpy(afterror,input);
        printf("Enter Divisor : ");
        gets(key);
        keylen=strlen(key);
        msglen=strlen(input);
        char key2[keylen];
        strcpy(key2,key);
        char rem[keylen-1];
   
        strcpy(key1,key);
        for (i=0;i<keylen-1;i++) {
            input[msglen+i]='0';
        }
        for (i=0;i<keylen;i++)
         temp[i]=input[i];
        for (i=0;i<msglen;i++) {
            quot[i]=temp[0];
            if(quot[i]=='0')
            for (j=0;j<keylen;j++)
            key[j]='0'; else
            for (j=0;j<keylen;j++)
            key[j]=key1[j];
            for (j=keylen-1;j>0;j--)
            {
                if(temp[j]==key[j])
                 rem[j-1]='0'; else
                 rem[j-1]='1';
            }
            rem[keylen-1]=input[i+keylen];
            strcpy(temp,rem);
        }
       strcpy(rem,temp);
       printf("\nQuotient is : ");
       for (i=0;i<msglen;i++)
       printf("%c",quot[i]);
       printf("\nRemainder or Crc is : ");
       for (i=0;i<keylen-1;i++)
       printf("%c",rem[i]);
       rem[i]='\0';
      
      
       printf("\nDo You want introduce an Error In original Message(Press 0) : ");
       scanf("%d",&k);
       if(k==0)
       {
        printf("Enter position where do you want introduce an error : ");
        scanf("%d",&k);
        if(afterror[k-1]=='0')
        afterror[k-1]='1';
        else
        afterror[k-1]='0';
       }
       printf("\nAfter Introducing Error Original Data Will Be : ");
       printf("%s",afterror);
        
      write(sockfd,afterror,100);
      write(sockfd,rem,30);
      write(sockfd,key2,30);
    
     
      printf("\n");
      printf("\nServer Respond : \n");
      read(sockfd,sermessage,200);
      printf("%s\n\n",sermessage);
       close(sockfd);
       return 0;
}



Server :

#include<stdio.h>
#include<string.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>
#define SERV_TCP_PORT 5035
int main(int argc, char **argv)
{
int sockfd,newsockfd,clength;
struct sockaddr_in serv_addr,cli_addr;

int i,j,keylen,msglen,remlen,flag=0;
char input[100], key[30],temp[30],quot[100],rem[30],key1[30],crc[30],retmessage[100];

sockfd=socket(AF_INET,SOCK_STREAM,0);
serv_addr.sin_family=AF_INET;
serv_addr.sin_addr.s_addr=INADDR_ANY;
serv_addr.sin_port=htons(SERV_TCP_PORT);

printf("\n Binding Done...");
bind(sockfd,(struct sockaddr*)&serv_addr,sizeof(serv_addr));
listen(sockfd,5);
clength=sizeof(cli_addr);
newsockfd=accept(sockfd,(struct sockaddr*)&cli_addr,&clength);
read(newsockfd,input,100);
read(newsockfd,rem,30);
read(newsockfd,key,30);



// Message Data After Error Introducing
// CRC Calculated from Original data message

printf("\nMessage Data Received From Client : %s",input);
printf("\nCrc : %s",rem);
printf("\nDivisor : %s",key);


// Crc Implementing

keylen=strlen(key);
        msglen=strlen(input);
        strcpy(key1,key);
        strcat(input,rem);
        for (i=0;i<keylen;i++)
        temp[i]=input[i];
        for (i=0;i<msglen;i++) {
            quot[i]=temp[0];
            if(quot[i]=='0')
            for (j=0;j<keylen;j++)
            key[j]='0'; else
            for (j=0;j<keylen;j++)
            key[j]=key1[j];
            for (j=keylen-1;j>0;j--)
           {
                if(temp[j]==key[j])
                 rem[j-1]='0'; else
                 rem[j-1]='1';
            }
            rem[keylen-1]=input[i+keylen];
            strcpy(temp,rem);
        }

        strcpy(rem,temp);
       printf("\nQuotient is ");
       for (i=0;i<msglen;i++)
       printf("%c",quot[i]);
       printf("\nRemainder or Crc is ");
       for (i=0;i<keylen-1;i++)
       printf("%c",rem[i]);
       printf("\n\n");
       remlen=strlen(rem);
       for(i=0;i<remlen;i++)
       {
          if(rem[i]!='0')
          flag++;
       }
       if(flag>0)
       // If Remainder is Not Zero
       strcpy(retmessage,"Message Received by Server , Message In Error Please Send Again");
       else
       // If Remainder is Zero
       strcpy(retmessage,"Message Received by Server Without Error");

      write(newsockfd,retmessage,200);
close(sockfd);
return 0;
}

  Client Output & Server Output :