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Question

I am new to socket programming and trying to figure out how to do a simple connection between client and server.

I found a website which explains about it. http://www.tutorialspoint.com/unix_sockets/socket_client_example.htm

I tried executing the codes in my ubuntu terminal ( gcc -o client client.c)

However it prints out the following error messages

client.c In function 'main' :
client.c:32:12: warning: assignment makes pointer from integer without a cast[enabled by defaulted]
client.c:40:25: error : dereferencing pointer to incomplete type
client.c:40:23: error : dereferencing pointer to incomplete type
client.c 46:5: warning: passing argument 2 of 'connect' from incompatible pointer type [enabled by default]

In file included from client c:1:0:
/usr/include/x86_64-linux-gnu/sys/socket.h:138:12: note:expected 'const struct sockaddr *' but argument is of type 'stuct sockaddr_in'

client.c code from the website

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

int main(int argc, char *argv[])
{
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;

    char buffer[256];

    if (argc < 3) {
        fprintf(stderr,"usage %s hostname port\n", argv[0]);
        exit(0);
    }
    portno = atoi(argv[2]);
    /* Create a socket point */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
    {
        perror("ERROR opening socket");
       exit(1);
    }

   //warning: assignment makes pointer from integer without a cast[enabled by defaulted]
   server = gethostbyname(argv[1]);

   if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
   }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;

    //dereferencing pointer to incomplete type
    bcopy((char *)server->h_addr,(char *)&serv_addr.sin_addr.s_addr,server->h_length);
    serv_addr.sin_port = htons(portno);

    /* Now connect to the server */
    if (connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr)) < 0) 
    {
         perror("ERROR connecting");
         exit(1);
    }   
    /* Now ask for a message from the user, this message
    * will be read by server
    */
    printf("Please enter the message: ");
    bzero(buffer,256);
    fgets(buffer,255,stdin);
    /* Send message to the server */
    n = write(sockfd,buffer,strlen(buffer));
    if (n < 0) 
    {
         perror("ERROR writing to socket");
         exit(1);
    }
    /* Now read server response */
    bzero(buffer,256);
    n = read(sockfd,buffer,255);
   if (n < 0) 
    {
         perror("ERROR reading from socket");
         exit(1);
    }
    printf("%s\n",buffer);
    return 0;
}

I am kind of lost and need help. thanks in advance

Answer 1

Not really a full answer, but my advice is to compile with more warnings turned on (eg with -Wall) :

$ gcc -Wall -o client client.c
client.c: In function `main':
client.c:16: warning: implicit declaration of function `exit'
client.c:18: warning: implicit declaration of function `atoi'
client.c:26: warning: implicit declaration of function `gethostbyname'
client.c:26: warning: assignment makes pointer from integer without a cast
client.c:32: warning: implicit declaration of function `bzero'
client.c:34: warning: implicit declaration of function `bcopy'
client.c:34: error: dereferencing pointer to incomplete type
client.c:34: error: dereferencing pointer to incomplete type
client.c:38: warning: passing arg 2 of `connect' from incompatible pointer type
client.c:50: warning: implicit declaration of function `write'
client.c:50: warning: implicit declaration of function `strlen'
client.c:58: warning: implicit declaration of function `read'
$

This makes a lot of the problems much clearer.

For all the implicit declaration of function warnings, just look up the relevant manpage to find what header you need:

$ man -s3 exit | grep '#include'
       #include <stdlib.h>
$ man -s2 write | grep '#include'
       #include <unistd.h>
$ man -s3 gethostbyname | grep '#include'
       #include <netdb.h>
       #include <sys/socket.h>       /* for AF_INET */
$ 

In general, these types of calls will be in section 2 or 3 of the manpages, but you might have to hunt around a bit.

$ man man | grep calls
       2   System calls (functions provided by the kernel)
       3   Library calls (functions within program libraries)
$