Thread: UDP Client/Server in C

Hello, I'm currently working on an assignment that requires C but I only have experience with C++ so I'm having some trouble.

The assignment is asking me to implement a client/server that communicates with each other over UDP. Basically the client is supposed to send over a number as a command line input(up to 128 characters) and the server needs to check if it is indeed a number. If it is, then it adds up the individual digits until there is only one digit and then sends the result to the client.

Example
Enter string: 123456789101234567891012345678910
From server: 138
From server: 12
From server: 3

Here is what I have so far:


Server

Code:

/* Creates a datagram server.  The port   number is passed as an argument.  This
   server runs forever */


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


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


int main(int argc, char *argv[])
{
   int sock, length, n;
   socklen_t fromlen;
   struct sockaddr_in server;
   struct sockaddr_in from;
   char buf[1024];
   int sum = 0;




   if (argc < 2) {
      fprintf(stderr, "ERROR, no port provided\n");
      exit(0);
   }


   sock=socket(AF_INET, SOCK_DGRAM, 0);
   if (sock < 0) error("Opening socket");
   length = sizeof(server);
   bzero(&server,length);
   server.sin_family=AF_INET;
   server.sin_addr.s_addr=INADDR_ANY;
   server.sin_port=htons(atoi(argv[1]));
   if (bind(sock,(struct sockaddr *)&server,length)<0)
       error("binding");
   fromlen = sizeof(struct sockaddr_in);
   while (1) {
       n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
       if (n < 0) error("recvfrom");
       write(1,"Received a datagram: ",21);
       write(1,buf,n);



       for(int i = 0; i < strlen(buf); i++){
                if(!isdigit(buf[i])){
                        n = sendto(sock,"We can't compute!\n",19,0,(struct sockaddr *)&from,fromlen);
                }
        }
       n = sendto(sock,"We can compute!\n",17,0,(struct sockaddr *)&from,fromlen);


       snprintf(buf, sizeof(buf), "%c", sum);
       n = sendto(sock,buf,128,0,(struct sockaddr *)&from,fromlen);




       if (n  < 0) error("sendto");
   }
   return 0;
 }

Client

Code:

/* UDP client in the internet domain */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>


void error(const char *);
int main(int argc, char *argv[])
{
   int sock, n;
   unsigned int length;
   struct sockaddr_in server, from;
   struct hostent *hp;
   char buffer[256];


   if (argc != 3) { printf("Usage: server port\n");
                    exit(1);
   }
   sock= socket(AF_INET, SOCK_DGRAM, 0);
   if (sock < 0) error("socket");


   server.sin_family = AF_INET;
   hp = gethostbyname(argv[1]);
   if (hp==0) error("Unknown host");


   bcopy((char *)hp->h_addr,
        (char *)&server.sin_addr,
         hp->h_length);
   server.sin_port = htons(atoi(argv[2]));
   length=sizeof(struct sockaddr_in);
   printf("Please enter the message: ");
   bzero(buffer,256);
   fgets(buffer,255,stdin);
   n=sendto(sock,buffer,
            strlen(buffer),0,(const struct sockaddr *)&server,length);
   if (n < 0) error("Sendto");


   n = recvfrom(sock,buffer,256,0,(struct sockaddr *)&from, &length);
   if (n < 0) error("recvfrom");
   write(1,"From server: ",13);
   write(1,buffer,n);


   close(sock);
   return 0;
}


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

The part I'm stuck on is this part from the server code

Code:

 while (1) {
       n = recvfrom(sock,buf,1024,0,(struct sockaddr *)&from,&fromlen);
       if (n < 0) error("recvfrom");
       write(1,"Received a datagram: ",21);
       write(1,buf,n);



       for(int i = 0; i < strlen(buf); i++){                
                if(isdigit(buf[i])){
                        sum += buf[i] - '0';
                        printf("%d", sum);
                }
                else{
                        n = sendto(sock,"We can't compute!\n",19,0,(struct sockaddr *)&from,fromlen);
                        break;
                }


        }




        //snprintf(buf, sizeof(buf), "%c", sum);
        //n = sendto(sock,buf,128,0,(struct sockaddr *)&from,fromlen);




       if (n  < 0) error("sendto");
   }

I can't seem to even get the server to recognize a number even when I send a simple number, like 10. This part was working before but I must've changed something and I can't figure it out. I always get the "We can't compute!" response from the server.

Also I am completely stumped on how to convert an integer to a char[]. I'm able to get the buffer from the client and convert it into an integer but I don't know how to convert it back to a char[] to send the sum back to the client. This is my attempt so far using a hard-coded buffer.

Code:

int main(void) {
  char buf[128] = "123456789";
  
  int sum;


  for(int i = 0; i < 128; i++){
    if(isdigit(buf[i])){
      sum += buf[i] - '0';
    
    }
  }  
  printf("%d\n", sum); 
    
 return 0; 
}

Any help would be appreciated.