getifaddrs

GETIFADDRS(3)               Linux Programmer's Manual              GETIFADDRS(3)



NAME
       getifaddrs, freeifaddrs - get interface addresses

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 describing
       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.)  This field may contain a null pointer.

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

       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.

       The ifa_data field points to a buffer containing address-family-specific
       data; this field may be NULL if there is no such data for this interface.

       The data returned by getifaddrs() is dynamically allocated and should be
       freed using freeifaddrs() when no longer needed.

RETURN VALUE
       On success, getifaddrs() returns zero; on error, -1 is returned, and
       errno is set appropriately.

ERRORS
       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 supported only IPv4 addresses; IPv6 support was
       added in glibc 2.3.3.  Support of address families other than IPv4 is
       available only on kernels that support netlink.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌────────────────────────────┬───────────────┬─────────┐
       │Interface                   Attribute     Value   │
       ├────────────────────────────┼───────────────┼─────────┤
       │getifaddrs(), freeifaddrs() │ Thread safety │ MT-Safe │
       └────────────────────────────┴───────────────┴─────────┘

CONFORMING TO
       Not in POSIX.1.  This function first appeared in BSDi and is present on
       the BSD systems, but with slightly different semantics documented—
       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 rtnl_link_stats, defined in <linux/if_link.h> (in Linux 2.4
       and earlier, struct net_device_stats, defined in <linux/netdevice.h>),
       which contains various interface attributes and statistics.

EXAMPLES
       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       AF_PACKET (17)
                           tx_packets =        524; rx_packets =        524
                           tx_bytes   =      38788; rx_bytes   =      38788
           wlp3s0   AF_PACKET (17)
                           tx_packets =     108391; rx_packets =     130245
                           tx_bytes   =   30420659; rx_bytes   =   94230014
           em1      AF_PACKET (17)
                           tx_packets =          0; rx_packets =          0
                           tx_bytes   =          0; rx_bytes   =          0
           lo       AF_INET (2)
                           address: <127.0.0.1>
           wlp3s0   AF_INET (2)
                           address: <192.168.235.137>
           lo       AF_INET6 (10)
                           address: <::1>
           wlp3s0   AF_INET6 (10)
                           address: <fe80::7ee9:d3ff:fef5:1a91%wlp3s0>

   Program source

       #define _GNU_SOURCE     /* To get defns of NI_MAXSERV and NI_MAXHOST */
       #include <arpa/inet.h>
       #include <sys/socket.h>
       #include <netdb.h>
       #include <ifaddrs.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <linux/if_link.h>

       int main(int argc, char *argv[])
       {
           struct ifaddrs *ifaddr;
           int family, s;
           char host[NI_MAXHOST];

           if (getifaddrs(&ifaddr) == -1) {
               perror("getifaddrs");
               exit(EXIT_FAILURE);
           }

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

           for (struct ifaddrs *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("%-8s %s (%d)\n",
                      ifa->ifa_name,
                      (family == AF_PACKET) ? "AF_PACKET" :
                      (family == AF_INET) ? "AF_INET" :
                      (family == AF_INET6) ? "AF_INET6" : "???",
                      family);

               /* 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("\t\taddress: <%s>\n", host);

               } else if (family == AF_PACKET && ifa->ifa_data != NULL) {
                   struct rtnl_link_stats *stats = ifa->ifa_data;

                   printf("\t\ttx_packets = %10u; rx_packets = %10u\n"
                          "\t\ttx_bytes   = %10u; rx_bytes   = %10u\n",
                          stats->tx_packets, stats->rx_packets,
                          stats->tx_bytes, stats->rx_bytes);
               }
           }

           freeifaddrs(ifaddr);
           exit(EXIT_SUCCESS);
       }

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

COLOPHON
       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                                2020-11-01                      GETIFADDRS(3)