unable display or recv() the data at the client buffer

Question

I am trying to read the data from the file /tmp/k on the server side and copying the data to the message and sending to the client side. Here i am able to send the data to the client but my client is not able to recv() or display the data. Can anyone help me with the problem or can propose a better solution to display the data at the client side.

Thanks

client.c

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

#define MAX_SIZE 100000
#define NUM_CLIENT 5
void *connection_handler(void *);

int main(int argc , char *argv[])
{
   int sockfd;
   long i;
   pthread_t sniffer_thread;
   printf("memory allocated for the creation of socket \n");
   for (i=1; i<=NUM_CLIENT; i++) {
        if( pthread_create( &sniffer_thread , NULL ,  connection_handler , (void *) i) < 0)
    {
        perror("could not create thread");
        return 1;
    } 
    printf("Thread created \n");
    sleep(2); 
} 
pthread_exit(NULL);
return 0;
}
void *connection_handler(void *threadid)
{   
    long etid; /*each thread id */
    etid = (long)threadid;
    pthread_t tid;
    tid = pthread_self();
    int sockfd;
    struct sockaddr_in serv_addr;
    char sbuff[MAX_SIZE] , rbuff[MAX_SIZE];                             
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0){
    printf("Failed creating socket\n");
    perror("could not create thread");
    return 0;
    }
    memset(&serv_addr, 0 , sizeof (serv_addr));
    printf("the bytes of memory area pointed by serv_addr is filled with constant '0' \n");
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    serv_addr.sin_port = htons(8888);

    if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof (serv_addr)) < 0) {
         perror("Failed to connect to server. Error");
         return 0;
         }
    printf("Connected successfully client:%ld \n", tid);
    printf("before calling pthread_create getpid: %d getpthread_self: %lu tid:%lu\n",getpid(), pthread_self(), syscall(SYS_gettid)); 
    while(1)
    {
       printf("For thread : %ld\n", etid);
       printf("thread id given by kernel: %ld\n", tid);
       fgets(sbuff, MAX_SIZE , stdin);
       send(sockfd,sbuff,strlen(sbuff),0);

        if(recv(sockfd,rbuff,MAX_SIZE,0)==0)
           printf("Error");
       else
          fputs(rbuff,stdout);
       bzero(rbuff,MAX_SIZE);
       sleep(2);
       } 
       close(sockfd);
       return 0;
}

server.c

 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <pthread.h>
 #include <sys/syscall.h>
 #include<pthread.h> //for threading , link with lpthread
 #include <fcntl.h>

 #define PROCESSLIST "/tmp/k"
 void *connection_handler(void *);
 int main(int argc , char *argv[])
 {
     int sockfd , new_socket , c , *new_sock;
     struct sockaddr_in server , client;
     char *message;

     //Create socket
     sockfd = socket(AF_INET , SOCK_STREAM , 0);
     if (sockfd == -1)
     {
         printf("Could not create socket");
     }

     //Prepare the sockaddr_in structure
     server.sin_family = AF_INET;
     server.sin_addr.s_addr = INADDR_ANY;
     server.sin_port = htons( 8888 );

     //Bind
     if( bind(sockfd,(struct sockaddr *)&server , sizeof(server)) < 0)
    {
       puts("bind failed");
       return 1;
     }
     puts("bind done");

     //Listen
     listen(sockfd , 3);

    //Accept and incoming connection
    puts("Waiting for incoming connections...");
    c = sizeof(struct sockaddr_in);
    while( (new_socket = accept(sockfd, (struct sockaddr *)&client, (socklen_t*)&c)) )
    {
        puts("Connection accepted");

        pthread_t sniffer_thread;
        new_sock = malloc(1);
       *new_sock = new_socket;

      if( pthread_create( &sniffer_thread , NULL ,  connection_handler , (void*) new_sock) < 0)
      {
        perror("could not create thread");
        return 1;
       }
    puts("Handler assigned");
   }

   if (new_socket<0)
   {
    perror("accept failed");
    return 1;
    }

    return 0;
}

  /* This will handle connection for each client */
void *connection_handler(void *sockfd)
{
       //Get the socket descriptor
      int sock = *(int*)sockfd;
      int read_size;
      char message[100000], client_message[2000];
      int fd;
      void *buff;
      //Receive a message from client
     while( (read_size = recv(sock , client_message , 2000 , 0)) > 0 )
     {
        system ("ps -ef > /tmp/k");
        puts("temp file getting created");
        fd = open (PROCESSLIST, O_RDONLY);
        puts("file open has taken place");
        read(fd, buff, sizeof(*buff));
        puts("reading the file");
        write(sock, buff, 1);
        puts("writing to the buffer");
        puts("copied data from buffer to message");
        //Send the message back to client
        send(sock,message,strlen(message),0);
        puts("data has been sent");
        }

       if(read_size == 0)
       {
       puts("Client disconnected");
       fflush(stdout);
       }
       else if(read_size == -1)
       {
          perror("recv failed");
        }

//Free the socket pointer
free(sockfd);

return 0;
}

Answer 1

A couple of issues:

In your server, you do this:

new_sock = malloc(1);

You're only allocating 1 byte to hold an int That's not enough space. Since newsock points to a int, you should allocate enough space for one:

new_sock = malloc(sizeof(int));

Also in the server, you're doing this after you read the message from the client:

    fd = open (PROCESSLIST, O_RDONLY);
    puts("file open has taken place");
    read(fd, buff, sizeof(*buff));
    puts("reading the file");
    write(sock, buff, 1);
    puts("writing to the buffer");
    puts("copied data from buffer to message");
    //Send the message back to client
    send(sock,message,strlen(message),0);

buff is an uninitialized void *, so attempting to write to it results in undefined behavior. The write is sending a single byte over the socket from the uninitialized buff, which is probably not what you want. Then you send data from message over the socket without putting anything into it.

All you really need to do here is read from the file directly into message, then send. You'll first want to clear the memory in message so that after you read from the file, it will be NULL terminated.

So you can replace the above with this:

    fd = open (PROCESSLIST, O_RDONLY);
    puts("file open has taken place");
    memset(message, 0, sizeof(message));
    read(fd, message, sizeof(message));
    puts("reading the file");
    //Send the message back to client
    send(sock,message,strlen(message),0);

In the client, you have 5 threads trying to read a message from the user at the same time. Only one of them will be able to. Since the server is ignoring the data send by the client, it might be better to have each client thread send some dummy data without prompting the user, then exiting once it gets the message from the server.