c - getaddrinfo and IPv6 -

i'm trying understand getaddrinfo function returns :

#include <stdlib.h> #include <sys/types.h> #include <unistd.h> #include <sys/socket.h> #include <netdb.h>  int main (int argc, char *argv[]) {   struct addrinfo *res = 0 ;    getaddrinfo("localhost", null ,null,&res);   printf("ai_flags -> %i\n", res->ai_flags) ;   printf("ai_family -> %i\n", res->ai_family) ;   printf("ai_socktype -> %i\n", res->ai_socktype) ;   printf("ai_protocol -> %i\n", res->ai_protocol) ;   printf("ai_addrlen -> %i\n", res->ai_addrlen) ;   struct sockaddr_in* saddr = (struct sockaddr_in*)res->ai_addr;   printf("ai_addr hostname ->  %s\n", inet_ntoa(saddr->sin_addr));    freeaddrinfo(res);    return 0 ; } 

results :

ai_flags -> 40 ai_family -> 2 ai_socktype -> 1 ai_protocol -> 6 ai_addrlen -> 16 ai_addr hostname ->  127.0.0.1 

in /etc/hosts, 've got :

127.0.0.1 localhost     ::1     localhost 

getaddrinfo returns 127.0.0.1 , not ::1 ? don't understand why ?

the second question can find meaning of ints (40,2,1,6 etc) ? i've read man there nothing that.

i wanted know if it's possible give ipv6 adress (for example ::1) , function returns name : localhost ?

thanks lot !!

@jwodder , @onteria_ covered ipv6 portion well, i'll tackle numbers portion:

ai_flags -> 40 

probably going sum of following 2 in /usr/include/netdb.h:

# define ai_v4mapped    0x0008  /* ipv4 mapped addresses acceptable.  */ # define ai_addrconfig  0x0020  /* use configuration of host choose 

this protocol family, inet, inet6, apx, unix, etc.:

ai_family -> 2  bits/socket.h:78:#define    pf_inet     2   /* ip protocol family.  */ bits/socket.h:119:#define   af_inet     pf_inet 

this socket type, stream, dgram, packet, rdm, seqpacket:

ai_socktype -> 1  bits/socket.h:42:  sock_stream = 1,     /* sequenced, reliable, connection-based 

the higher-level protocol, tcp, udp, tcp6, udp6, udplite, ospf, icmp, etc:

ai_protocol -> 6 

funny enough, in /etc/protocols:

tcp 6   tcp     # transmission control protocol 

the size of struct sockaddr. (differs based on address family! ugh.)

ai_addrlen -> 16 

this because you're getting struct sockaddr_in, see linux/in.h:

#define __sock_size__   16      /* sizeof(struct sockaddr)  */ struct sockaddr_in {   sa_family_t       sin_family; /* address family       */   __be16        sin_port;   /* port number          */   struct in_addr    sin_addr;   /* internet address     */    /* pad size of `struct sockaddr'. */   unsigned char     __pad[__sock_size__ - sizeof(short int) -             sizeof(unsigned short int) - sizeof(struct in_addr)]; }; 

and last one, /etc/hosts :)

ai_addr hostname ->  127.0.0.1