C Socket programming on Linux

Question

I am new to socket programming and linux I could find some code about socket programming I want to use this code to connect to a printer , this code is using gethostbyname function which is responsible for getting hostent I think everything is fine except that I have not the host name I just have an IP address (of printer), So what function should I use to connect to the printer by IP ?

this is the code

#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> 

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

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]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) 
    error("ERROR opening socket");
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;
bcopy((char *)server->h_addr, 
     (char *)&serv_addr.sin_addr.s_addr,
     server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
    error("ERROR connecting");
printf("Please enter the message: ");
bzero(buffer,256);
fgets(buffer,255,stdin);
n = write(sockfd,buffer,strlen(buffer));
if (n < 0) 
     error("ERROR writing to socket");
bzero(buffer,256);
n = read(sockfd,buffer,255);
if (n < 0) 
     error("ERROR reading from socket");
printf("%s\n",buffer);
close(sockfd);
return 0;
}

For connecting to a printer, using CUPS is probably more appropriate.... — Basile Starynkevitch
– Basile Starynkevitch, Commented Apr 6, 2013 at 16:27

score 8 · Accepted Answer · 2013-04-06 13:31:31Z

8

Use getaddrinfo instead of gethostbyname. It should accept host names and both IPv4 and IPv6 addresses transparently as long as it uses standard notation.

Do something like this:

int connect_to(char const *host, char const *port)
{
    int sock = -1;
    struct addrinfo hints;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    struct addrinfo *servinfo;
    if (getaddrinfo(host, port, &hints, &servinfo) != 0) {
        return sock;
    }

    for (struct addrinfo *serv = servinfo; NULL != serv; serv = serv->ai_next) {
        int tmp = socket(serv->ai_family, serv->ai_socktype, serv->ai_protocol);
        if (tmp == -1) {
            continue;
        }

        if (connect(tmp, serv->ai_addr, serv->ai_addrlen) == -1) {
            close(tmp);
            continue;
        }

        sock = tmp;
        break;
    }

    freeaddrinfo(servinfo);
    return sock;
}

edited Apr 6, 2013 at 13:31

answered Apr 6, 2013 at 13:07

user858224

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1 Comment

Bahram Over a year ago

I did not test your code but in my code I did wrong about the IP address. I mean the gethostbyname works fine with IP address (I had seted wrong IP and when I use correct IP it worked OK) anyway thanks for your answer

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