code in C, url = inet_ntoa (* (struct in_addr *) ip-h_addr); I do not understand what is happening.

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code in C, url = inet_ntoa (* (struct in_addr *) ip-h_addr); I do not understand what is happening.

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I was studying sockets for this material link and I came across a code that I do not understand what is going on. The code part is this:

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 do not understand what's going on. From what I've researched so far I've found that: the 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 h_addr field equals char **h_addr_list[0]; //primeira posiçao do vetor h_addr_list . gethostbyname() returns a hostent (struct!) for the 'ip' variable, then accessing ip-> h_addr I'm accessing a pointer that is of type char , but is being cast for pointer of struct in_addr :

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

c c++ sintaxe socket

asked by anonymous 20.07.2018 / 12:31

1 answer

___ ___ erkimt Javascript function Onclick In conflict with the form? ______ qstntxt ___

I have a form that sends data to another page, I would like to reset some inputs after submitting the form, so I tried this:

%pre%

But I did not succeed, I tried to reset the input several ways, putting in a function js (which would be right in the case), tried jquery among others but I did not succeed, so I believe there may be some conflict within my form that might be causing this function problem not to generate the expected result (which in case is reset the text area description).

______ azszpr316756 ___

%code% is in the wrong place. You should put it in the %code% tag. Another problem is that %code% is only applicable to the %code% object, not to an individual form element.

As you can not clear the field in the submit (otherwise it will be sent empty), call a function after a small delay using %code% on %code% :

%pre%

And put the %code% function in JavaScript by clearing the field with %code% :

%pre%

Or you could also do everything in %code% without calling function:

%pre%

___ C # - Remove ASPAS - Replace - Remove ASP Double

score 1 · Accepted Answer

Hey, man! Is that okay?

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

Exactly, as gethostbyname() can return a hostent.h_addr_list containing addrs of type in_addr or in6_addr and for that reason hostent.h_addr_list is **char . so casting is required.

obs. to check which type of addr, you can cast% to sockaddr and check the sa_family field 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 are you doing?

According to the statement char *inet_ntoa(struct in_addr in); the in parameter is not a pointer and that is why * is used in the pointer, to get the object instance and the object to be copied and not referenced.

I hope I have been clear.