Network programming in C (data is not being sent from client to server using send())

Question

i have two files a client and a server , i want to send the value of the variable (choice) from the client to the server , but the value is not being sent and i have no idea what the problem is

this is my code for client

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

    int main(int argc , char *argv[])
    {

        char buf;
        int clientSocket, cnnt,fileDiscriptor,sizeInByte=0 , size=0 , serverSocket,choice=2 ;
        struct sockaddr_in serverInfo;

        if(argc != 3)
        {
            printf("Few arguments\nUsage : ./client <ip> <port>\n");
            exit(1);    
        }

        memset(&serverInfo,0,sizeof(serverInfo));   

        serverInfo.sin_family = AF_INET;
        inet_aton(argv[1],&serverInfo.sin_addr.s_addr);
        serverInfo.sin_port = htons(atoi(argv[2]));
        clientSocket = socket(AF_INET,SOCK_STREAM,0);
        if(clientSocket < 0)
        {
            printf("Could not create client socket\n");
            exit(1);
        }

        cnnt = connect(clientSocket,(struct sockaddr *)&serverInfo,sizeof(serverInfo));
        if(cnnt < 0)
        {   
            printf("Could not create connection to the server\n");   
            exit(1);    
        }
        recv(clientSocket , &serverSocket , sizeof(int) , 0);
        send(serverSocket , &choice , sizeof(int),0);
        close(clientSocket);
        return 0;

    }

and this is my code for the server

    #include<stdio.h>
    #include<stdlib.h>
    #include<string.h>
    #include<sys/types.h>
    #include<netinet/in.h>
    #include<sys/socket.h>
    #include<netdb.h>
    #include<arpa/inet.h>
    #include<sys/stat.h>
    #include<fcntl.h>
    #include <sys/wait.h>


    int main(int argc , char *argv[])
    {   
        int choice;
        int serverSocket, bnd, lstn , clientSize, connectionFd;
        struct sockaddr_in serverInfo, clientInfo  ;

        if(argc != 2)
        {   
            printf("Few arguments\nUsage : ./server <port>\n");
            exit(1);    
        }   

        memset(&serverInfo , 0 , sizeof(serverInfo));
        memset(&clientInfo , 0 , sizeof(clientInfo));   

        serverInfo.sin_family = AF_INET;
        serverInfo.sin_port = htons(atoi(argv[1]));
        serverInfo.sin_addr.s_addr = INADDR_ANY;
        serverSocket = socket(AF_INET,SOCK_STREAM,0);
        if(serverSocket < 0)
        {
            printf("Could not create server socket\n");
            exit(1);    
        }

        bnd = bind(serverSocket , (struct sockaddr *)&serverInfo , sizeof(serverInfo));
        if(bnd < 0)
        {
            printf("Could not bind\n");
            exit(1);    
        }

        lstn = listen(serverSocket , 1);
        if(lstn < 0)
        {
            printf("Could not listen on server socket\n");
            exit(1);    
        }
        clientSize = sizeof(clientInfo);
        while(1)
        {
            connectionFd =  accept(serverSocket , (struct sockaddr *)&clientInfo , &clientSize);
            if(connectionFd < 0)
                continue;
                    send(connectionFd , &serverSocket , sizeof(int) , 0);
            recv(serverSocket , &choice , sizeof(int),0);
            close(connectionFd);
            break;
        }
        printf("CHOICE IS :%d\n",choice);
        return 0;
    }

Answer 1

after checking it turns out that there is an error in send and receiving in both the client and the server ..

as in man page

ssize_t send(int sockfd, const void *buf, size_t len, int flags);

ssize_t recv(int sockfd, void *buf, size_t len, int flags);

as in my code

send(clientSocket , &choice , sizeof(int),0);

recv(serverSocket , &choice , sizeof(int),0);

what is causing the error if i am applying each function correctly ?

Answer 2

are you sure it is not send-ing? IMO,

recv(clientSocket , &serverSocket , sizeof(int) , 0);

in client is blocking, thus not reaching the send() call. Can you put the recv() call [which is not intended and required, IMO] after send() in client side?

Hint: Server is listening for incoming connection , and as long as there is no imcoming connection, it doesn't has an address to send the communicate back.

• General flow for server : open socket --> bind --> listen --> accept --> receive --> send.
• General flow for client : open socket --> connect --> send --> receive.

---

EDIT:

As you're sending int type, [more than 1 byte size], I would recommend using htonl() in client side and ntohl() is server side to take care of the endianness issue [if any].

Answer 3

The line in the server code that does:

 send(connectionFd , &serverSocket , sizeof(int) , 0);

is not needed, so remove it. There is no reason to send the value of the server socket to the client, the client cannot make use of that anyway, a file descriptor is a resource connected to a process, and you cannot transfer that resource to another process by writing its value over a TCP connection to the client.

On the client,you need to send the choice variable on the connection that you just opened,so you need to change these 2 lines:

recv(clientSocket , &serverSocket , sizeof(int) , 0);
send(serverSocket , &choice , sizeof(int),0);

To this:

 send(clientSocket , &choice , sizeof(int),0);

In addition to that, you should make a habit of checking all your calls for errors, so you know if something fails, e.g.

int rc;

rc = send(clientSocket , &choice , sizeof(int),0);
if (rc == -1) {
    perror("send()");
    //handle error, e.g. exit the app
}