GETIFADDRS(3)             (2020-11-01)              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 struc-
          tures 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 con-
          sulted 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

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          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).  allbox; lbw27 lb lb l l l.
          Interface Attribute Value T{ getifaddrs(), freeifaddrs()
          T}   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 actu-
          ally 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

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          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];

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              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);

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          }

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

     COLOPHON
          This page is part of release 5.10 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/.

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