gethostbyname在OSX（Yosemite 10.10.4）上失败

Question

"gethostbyname" returns a pointer to this structure:

 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 from name server */
 };

When I try to use it, h_name points to a valid string: the partial name I supply is expanded to the correct fully qualified host name.

The value of h_addr_list is 4

h_name is valid
h_aliasis is a valid pointer to a null pointer
h_addrtype is 2 (AF_INET, IPV4)
h_length is 0 (should be 4, or perhaps a multiple of 4)
h_addr_list is 4, fails when dereferenced.

I'm running a 32 bit process (MS Office), the h_name pointer is a valid 32 bit pointer. WTF am I doing wrong? Does gethostbyname work for other people, or on other versions of OSX?

Answer 1

I was able to run this small example successfully on 10.10.4 (taken from paulschreiber.com )

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>

int main(int argc, char **argv) {
    if (argc < 2) {
        printf("Usage: %s hostname", argv[0]);
        exit(-1);
    }

    struct hostent *hp = gethostbyname(argv[1]);

    if (hp == NULL) {
       printf("gethostbyname() failed\n");
    } else {
       printf("%s = ", hp->h_name);
       unsigned int i=0;
       while ( hp -> h_addr_list[i] != NULL) {
          printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i])));
          i++;
       }
       printf("\n");
    }
}

However, it did segfault on 64-bit without #include <arpa/inet.h : without that, no prototype for inet_ntoa is found, the return type is assumed to be an int (when it's actually a char * ), and on 64-bit this truncates the pointer and causes a segfault.