Basic socket help needed

[johngoodman]

I'm trying to complete a basic socket program. I have a class which is the server and I have a client which is in a different class. The master is supposed to pass a given range of values to the client and find all the prime values in that range. But I'm having problems getting the server to pass the "starting" & "ending" variables to the client. Right now when I run the server, type in the values, followed by running the client it just sits there. If you see anything wrong with my code, please point it out because socket code is pretty ugly, thanks!

Here is my code for the server.

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

int main() {
    
    char* names[] = {"localhost"};
    int ports[] = {7000};
    

    int starting, ending;
    printf("Enter starting & ending values: \n");
    scanf("%d %d",&starting, &ending);
    

  // Get a socket of the right type
  int sockfd = socket(AF_INET, SOCK_STREAM, 0);
  if (sockfd < 0) {
    printf("ERROR opening socket");
    exit(1);
  }

  // port number
  int portno = 7000;

  // server address structure
  struct sockaddr_in serv_addr;

  // Set all the values in the server address to 0
  memset(&serv_addr, '0', sizeof(serv_addr)); 

  // Setup the type of socket (internet vs filesystem)
  serv_addr.sin_family = AF_INET;

  // Basically the machine we are on...
  serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);

  // Setup the port number
  // htons - is host to network byte order
  // network byte order is most sig bype first
  //   which might be host or might not be
  serv_addr.sin_port = htons(portno);

  // Bind the socket to the given port
  if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
    printf("ERROR on binding\n");
    exit(1);
  }

  // set it up to listen
  listen(sockfd,5);
  

  int newsockfd;
  struct sockaddr_in cli_addr;
  socklen_t clilen = sizeof(cli_addr);

  // Wait for a call
  newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
  if (newsockfd < 0) {
    printf("ERROR on accept");
    exit(1);
  }

  int buffer = 0;
  memset(&buffer, '\0', sizeof(buffer));
  bzero(&buffer,sizeof(buffer));
  int n = read(newsockfd,&buffer,sizeof(buffer));
  if (n < 0) {
    printf("ERROR reading from socket\n");
    exit(1);
  }
  read(newsockfd,&starting,sizeof(starting));
  read(newsockfd,&ending,sizeof(ending));

  printf("Number of primes found as master is: %d\n",buffer);
  n = write(newsockfd,"I FOUND %D PRIME NUMBERS",sizeof(buffer));
  if (n < 0) {
    printf("ERROR writing to socket\n");
    exit(1);
  }

  close(newsockfd);
  close(sockfd);
  return 0;
}

Here is my code for the client.

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


int isPrime(int num);

int main(){
    
  // Socket pointer
  int sockfd;
  sockfd = socket(AF_INET, SOCK_STREAM, 0);
  if (sockfd < 0) {
    fprintf(stderr,"ERROR opening socket\n");
    exit(0);
  }
    
  // port number
  int portno = 7000;

  // server address structure
  struct sockaddr_in serv_addr;

  // Set all the values in the server address to 0
  memset(&serv_addr, '0', sizeof(serv_addr)); 

  // Setup the type of socket (internet vs filesystem)
  serv_addr.sin_family = AF_INET;

   // Setup the port number
  // htons - is host to network byte order
  // network byte order is most sig bype first
  //   which might be host or might not be
  serv_addr.sin_port = htons(portno);


  // Setup the server host address
  struct hostent *server;
  server = gethostbyname("localhost");
  if (server == NULL) {
    fprintf(stderr,"ERROR, no such host\n");
    exit(0);
  }
  memcpy(&serv_addr.sin_addr.s_addr, server->h_addr, server->h_length);  /// dest, src, size

  // Connect to the server
  if (connect(sockfd,(struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
    printf("ERROR connecting\n");
    exit(0);
  }
  int starting;
  int ending;
  read(sockfd,&starting,sizeof(starting));
  read(sockfd,&ending,sizeof(ending));
  int buffer = 0;
  int primeCount = 0;
  printf("starting: %d ending: %d", starting, ending);
    
      for(int i = starting; i < ending; i++){
        if(isPrime(i)){
            buffer++;
        }
    }
    printf("Number of primes in the given range: %d\n", primeCount);
    

  int n = write(sockfd,&buffer,sizeof(buffer));
  if (n < 0) {
    printf("ERROR writing to socket\n");
    exit(0);
  }

  memset(&buffer, '\0', sizeof(buffer));
  n = read(sockfd,&buffer,sizeof(buffer));
  if (n < 0) {
    printf("ERROR reading from socket\n");
    exit(0);
  }
  
  close(sockfd);
  return 0;
}

    int isPrime(int num){
        for (int i = 2; i < num; i++){
            if (num % i == 0 && i != num) return 0;
        }
        return 1;
    }

[AndiPersti]

Right now when I run the server, type in the values, followed by running the client it just sits there.

In your client code you wait for "starting" and "ending" from the server (lines 58 and 59) but the server itself waits for something to read (read() on line 72; only after two other read()'s on lines 77 and 78 you have a write() on line 81).

// Set all the values in the server address to 0
memset(&serv_addr, '0', sizeof(serv_addr));

The comment is wrong. This call doesn't set all bytes to 0. It's setting all bytes to 48 (the ASCII code of the character '0').

Bye, Andreas

[johngoodman]

In your client code you wait for "starting" and "ending" from the server (lines 58 and 59) but the server itself waits for something to read (read() on line 72; only after two other read()'s on lines 77 and 78 you have a write() on line 81).

// Set all the values in the server address to 0
memset(&serv_addr, '0', sizeof(serv_addr));

The comment is wrong. This call doesn't set all bytes to 0. It's setting all bytes to 48 (the ASCII code of the character '0').

Bye, Andreas

Is there a way of passing all the variables I want from the server to the client in one read? Otherwise, yes, I see what you have pointed out but I don't really know how to properly pass the values over to the client.

[Salem]

Personally, I would suggest you read beej, then write a client/server pair test program to send and receive a single integer.

When that works, you can readily extend it to any sized message you want.

> Is there a way of passing all the variables I want from the server to the client in one read?
You're using SOCK_STREAM, so the only guarantees are that bytes sent are received in the order they are sent.

These are all valid permutations:
S: "hello world"
R: "hell"
R: "o"
R: " world"

S: "he"
S: "llo "
S: "world"
R: "hello world"

While you're talking to localhost, you'll probably never see fragmentation.
But try talking to some remote server on the other side of the planet out in the boondock's, then you're probably going to need to pay attention to it.