C code, url=inet_ntoa(*(struct in_addr *)ip->h_addr); I don’t understand what’s going on

Asked

Viewed 108 times

1

I was studying sockets for this stuff https://www.exploit-db.com/papers/13634/ and I came across a code that I don’t understand what’s going on. That’s the part of the code: 

struct hostent *ip;
...
// função da lib netdb.h para converter ai nosso host para IP
  ip = gethostbyname(host);
  url=inet_ntoa(*(struct in_addr *)ip->h_addr);
  strcat(getpage,page);
  printf("%s",getpage);

The line that starts with 'url' is that I don’t understand what’s going on. From what I’ve researched so far I’ve discovered that: struct hostent has the fields 

char  *h_name;            /* official name of host */
char **h_aliases;         /* alias list */
int    h_addrtype;        /* host address type */
int    h_length;          /* length of address */
char **h_addr_list; /* list of addresses */

and the countryside h_addr that equals char **h_addr_list[0]; //primeira posiçao do vetor h_addr_list. gethostbyname() returns a hostent(struct! ) for the variable 'ip', then accessing ip->h_addr I am accessing a pointer that is of the type char, but is being cast for pointer struct in_addr:

url=inet_ntoa(*(struct in_addr *)ip->h_addr); but h_addr is not an in_addr struct (which has an unsigned long) and to complete, this * asterisk that comes before (struct in_addr *) What are you doing? I don’t understand this syntax... I researched a lot but I still don’t understand.

c c++ socket syntax

1 answer

1


And look, man! All right?

but h_addr is not an in_addr struct (which has an unsigned long)

Exactly, how gethostbyname() can return a hostent.h_addr_list containing addrs of the type in_addr or in6_addr and for this reason hostent.h_addr_list is a **char. this way being necessary the cast.

Obs. to check which type of addr, you can cast to sockaddr and check the field sa_family and cast the corresponding cast.


sockaddr *addr = (struct sockaddr *)ip->h_addr;

if(addr.sa_family == INET_ADDR)
{
  in_addr* ipv4 = (struct in_addr*)addr;
}
else if(addr.sa_family == INET6_ADDR)
{
  in6_addr* ipv6 = (struct in_addr*)addr;
}
else
{
  // erro
}

to complete, this asterisk * that comes before (struct in_addr *) what you are doing?

according to the declaration char *inet_ntoa(struct in_addr in); the parameter in is not a pointer and that is why the * on the pointer is used, to take the instance of the Object and the Object is copied and not referenced.

I hope I was clear.

  • Very clear, congratulations and thank you very much man.

  • Noting that you have a good knowledge on the subject I would just like to ask if you know about any material that covers well sockets and typecasting, because although you understand I haven’t completely mastered how I want that part(typecasting) And I’m learning sockets but some materials I picked up aren’t very good. The only one I consider worthy of those is the Hacking: the art of exploitation. That has a content on C Very good, and is also a tip in thanks.

  • socket and Typecast no, but for socket I have a great material that is this link. If you want to understand better about Typecast, try to understand about memory padding and align. This will give you a good understanding of the subject. link.

  • if my answer was correct, please mark the post as resolved to help other people with the same question (and help with dots! kkk...).

  • user with no experience in stackoverflow here, then forgot to mark kkk but is there marked, and thanks again.

Browser other questions tagged c c++ socket syntax

You are not signed in. Login or sign up in order to post.