Does struct hostent have a field "h_addr"?

Question

I encountered the following code snapshot:

struct hostent *hp;
 hp = my_gethostbyname(localhost);
    if (hp == NULL) {
       ls_syslog(LOG_ERR, I18N_FUNC_FAIL, fname, "my_gethostbyname()");
       return -1;
    }
    strcpy(localhost, hp->h_name);

    memcpy(&addr, hp->h_addr, hp->h_length);

I am rather confused by the last statement, the declaration of struct hostent is like this:

struct hostent {
   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 */
};

It doesn't have a field named "h_addr", but the code did can compile, can anyone tell me why? thanks.

I'm getting an error on VSCode that says, "struct "hostent" has no field "h_addr" but it compiles and runs fine. The code was copy pasted from here: https://www.cs.cmu.edu/afs/cs/academic/class/15213-f99/www/class26/tcpclient.c — mLstudent33, Dec 31 '19 at 04:42

score 29 · Accepted Answer · answered Jul 10 '12 at 02:36

29

You missed this bit right under it:

#define h_addr h_addr_list[0] /* for backward compatibility */

So no, there is no problem.

answered Jul 10 '12 at 02:36

Ignacio Vazquez-Abrams

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score 4 · Answer 2 · edited Mar 29 '22 at 05:00

4

In the GNU libc manual (or see here for the entire libc manual all on one page) they say:

Recall that the host might be connected to multiple networks and have different addresses on each one

They also provide the h_addr variable which is just the first element of the vector h_addr_list.

edited Mar 29 '22 at 05:00

Gabriel Staples

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answered Jan 22 '15 at 04:37

Ratul Sharker

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One might wonder: "how long is the `h_addr_list` array?" Answer: Iterate down it until you find a `NULL` ptr. An address string inside the `h_addr_list` which consists of nothing but a `NULL` ptr indicates the end of the list. The [GNU libc documentation link (see here for it all one 1 pg)](https://www.gnu.org/software/libc/manual/html_mono/libc.html#index-struct-hostent) states that too: `The vector is terminated by a null pointer.` – Gabriel Staples Mar 29 '22 at 02:08

score 2 · Answer 3 · answered Apr 20 '20 at 01:38

2

h_addr is not POSIX. See POSIX netdb.h. Using h_addr could result in error: ‘struct hostent’ has no member named ‘h_addr’. Portable code should use h_addr_list instead.

answered Apr 20 '20 at 01:38

mortmann

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kralyk · Answer 4 · 2018-05-16T12:55:01.297

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Note that the h_addr macro is on some systems only visible if you define _BSD_SOURCE and/or _DEFAULT_SOURCE before including header files.

edited May 16 '18 at 12:55

answered May 16 '18 at 09:49

kralyk

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Does struct hostent have a field "h_addr"?

4 Answers4