focal (3) getifaddrs.3.gz

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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).

       ┌────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├────────────────────────────┼───────────────┼─────────┤
       │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.

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       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, *ifa;
           int family, s, n;
           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 (ifa = ifaddr, n = 0; ifa != NULL; ifa = ifa->ifa_next, n++) {
               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

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