freeifaddrs


SYNOPSIS
       #include <sys/types.h>
       #include <ifaddrs.h>

       int getifaddrs(struct ifaddrs **ifap);

       void freeifaddrs(struct ifaddrs *ifa);

DESCRIPTION
       The  getifaddrs() function creates a linked list of structures describ-
       ing the network interfaces of the local system, and stores the  address
       of  the  first item of the list in *ifap.  The list consists of ifaddrs
       structures, defined as follows:

           struct ifaddrs {
               struct ifaddrs  *ifa_next;    /* Next item in list */
               char            *ifa_name;    /* Name of interface */
               unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
               struct sockaddr *ifa_addr;    /* Address of interface */
               struct sockaddr *ifa_netmask; /* Netmask of interface */
               union {
                   struct sockaddr *ifu_broadaddr;
                                    /* Broadcast address of interface */
                   struct sockaddr *ifu_dstaddr;
                                    /* Point-to-point destination address */
               } ifa_ifu;
           #define              ifa_broadaddr ifa_ifu.ifu_broadaddr
           #define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
               void            *ifa_data;    /* Address-specific data */
           };

       The ifa_next field contains a pointer to  the  next  structure  on  the
       list, or NULL if this is the last item of the list.

       The ifa_name points to the null-terminated interface name.

       The  ifa_flags  field  contains the interface flags, as returned by the
       SIOCGIFFLAGS ioctl(2) operation (see netdevice(7) for a list  of  these
       flags).

       The  ifa_addr  field  points  to  a  structure containing the interface
       address.  (The sa_family subfield should be consulted to determine  the
       format of the address structure.)

       The  ifa_netmask  field  points  to  a structure containing the netmask
       associated with ifa_addr, if applicable for the address family.

       Depending on whether the bit IFF_BROADCAST or IFF_POINTOPOINT is set in
       ifa_flags  (only  one  can be set at a time), either ifa_broadaddr will
       contain the broadcast address associated with ifa_addr  (if  applicable
       for  the  address  family)  or ifa_dstaddr will contain the destination
       address of the point-to-point interface.

       getifaddrs() may fail and set errno for any of the errors specified for
       socket(2),  bind(2),  getsockname(2), recvmsg(2), sendto(2), malloc(3),
       or realloc(3).

VERSIONS
       The getifaddrs() function first appeared in glibc 2.3, but before glibc
       2.3.3,  the  implementation only supported IPv4 addresses; IPv6 support
       was added in glibc 2.3.3.  Support of address families other than  IPv4
       is only available on kernels that support netlink.

CONFORMING TO
       Not  in  POSIX.1-2001.   This  function  first  appeared in BSDi and is
       present on the BSD systems, but with slightly different semantics docu-
       mented--returning one entry per interface, not per address.  This means
       ifa_addr and other fields can actually be NULL if the interface has  no
       address,  and no link-level address is returned if the interface has an
       IP address assigned.  Also, the way of choosing either ifa_broadaddr or
       ifa_dstaddr differs on various systems.

NOTES
       The  addresses  returned  on  Linux  will  usually be the IPv4 and IPv6
       addresses assigned to the interface, but also one AF_PACKET address per
       interface  containing  lower-level  details about the interface and its
       physical layer.  In this case, the ifa_data field may contain a pointer
       to  a  struct  net_device_stats,  defined in <linux/netdevice.h>, which
       contains various interface attributes and statistics.

EXAMPLE
       The program below demonstrates the use of getifaddrs(),  freeifaddrs(),
       and  getnameinfo(3).   Here is what we see when running this program on
       one system:

           $ ./a.out
           lo      address family: 17 (AF_PACKET)
           eth0    address family: 17 (AF_PACKET)
           lo      address family: 2 (AF_INET)
                   address: <127.0.0.1>
           eth0    address family: 2 (AF_INET)
                   address: <10.1.1.4>
           lo      address family: 10 (AF_INET6)
                   address: <::1>
           eth0    address family: 10 (AF_INET6)
                   address: <fe80::2d0:59ff:feda:eb51%eth0>

   Program source

       #include <arpa/inet.h>
       #include <sys/socket.h>
       #include <netdb.h>
       #include <ifaddrs.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

           /* Walk through linked list, maintaining head pointer so we
              can free list later */

           for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
               if (ifa->ifa_addr == NULL)
                   continue;

               family = ifa->ifa_addr->sa_family;

               /* Display interface name and family (including symbolic
                  form of the latter for the common families) */

               printf("%s  address family: %d%s\n",
                       ifa->ifa_name, family,
                       (family == AF_PACKET) ? " (AF_PACKET)" :
                       (family == AF_INET) ?   " (AF_INET)" :
                       (family == AF_INET6) ?  " (AF_INET6)" : "");

               /* For an AF_INET* interface address, display the address */

               if (family == AF_INET || family == AF_INET6) {
                   s = getnameinfo(ifa->ifa_addr,
                           (family == AF_INET) ? sizeof(struct sockaddr_in) :
                                                 sizeof(struct sockaddr_in6),
                           host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
                   if (s != 0) {
                       printf("getnameinfo() failed: %s\n", gai_strerror(s));
                       exit(EXIT_FAILURE);
                   }
                   printf("\taddress: <%s>\n", host);
               }
           }

           freeifaddrs(ifaddr);
           exit(EXIT_SUCCESS);
       }

SEE ALSO
       bind(2), getsockname(2), socket(2), packet(7), ifconfig(8)

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



GNU                               2010-10-06                     GETIFADDRS(3)
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