在不使用getaddrinfo（）函数的情况下调用socket :: connect，socket :: bind，socket :: listen

Question

In all the example including Beej's Guide , the IP address is provided in dot notation and then it's fed to ::getaddrinfo() . This post doesn't answer my question.

After which the addrinfo struct is used for socket related functions (eg connect() , bind() , listen() ). For example:

struct addrinfo hints, *res;
// ... create socket etc.
connect(sockfd, res->ai_addr, res->ai_addrlen);

Example
The variable ai_addr is of type sockaddr which can be safely typecasted to sockaddr_storage , sockaddr_in and sockaddr_in6 .

Question :

If I typecast sockaddr to sockaddr_in (or sockaddr_in6 )

sockaddr_in& ipv4 = (sockaddr_in&)(sockaddr_variable);

and feed below info:

• ipv4.sin_family = AF_INET
• ipv4.sin_addr = [IP Address in net byte order]
• ipv4.sin_port = [Port number in net byte order]

Can I call the connect() method directly using above info?

connect(sockfd, &ipv4, sizeof(ipv4));

With my program it doesn't appear to work. Am I missing something, or is there a better way?

The motivation behind is that, if we have the information of IPAddress, Port etc. in socket readable format then why to go through the cycle of getaddrinfo()

Answer 1

Be sure you're placing your values in network order, here's a small example:

#include<stdio.h>
#include<sys/socket.h>
#include<arpa/inet.h>

int main(int argc, char *argv[])
{
    int sock;
    struct sockaddr_in server;

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock == -1)
    {
        printf("Could not create socket\n");
    }
    printf("Socket created\n");

    server.sin_family = AF_INET;
    // 173.194.32.207 is a google address
    server.sin_addr.s_addr = 173 | 194 << 8 | 32 << 16 | 207 << 24;
    server.sin_port = 0x5000; // port 80

    if (connect(sock, (struct sockaddr *)&server, sizeof(server)) < 0)
    {
        perror("connect failed. Error");
        return 1;
    }

    printf("Connected\n");

    close(sock);
    return 0;
}

Answer 2

First check whether the machine is reachable & the server application is running on the machine using "netstat" utility. Use inet_aton method to convert dotted address to network byte order. Finally, log the error value returned by the connect to get the exact reason of failure.

Answer 3

It's worth noting that calling socket::{connect, bind, ...} is wrong: these are C API s and C doesn't have namespaces, classes and so on.

You should use getaddrinfo as it's much easier and safer to use. But nothing prevents you from using struct sockaddr and all its variants. Indeed, getaddrinfo is a sort of wrapper as stated in man(3) getaddrinfo :

The getaddrinfo() function combines the functionality provided by the gethostbyname(3) and getservbyname(3) functions into a single interface, but unlike the latter functions, getaddrinfo() is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependen‐ cies.

An example:

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

int main()
{
   struct sockaddr_in addr = {0};
   addr.sin_family = AF_INET;
   addr.sin_port   = htons(80);

  inet_pton(addr.sin_family, "198.252.206.16", &addr.sin_addr);

  int fd = socket(addr.sin_family, SOCK_STREAM, 0);

  if (fd == -1)
       ; /* could not create socket */

  if (connect(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
        ;  /* could not connect */

   close(fd);

}