man3/getifaddrs.3

   1 '\" t
   2 .\" Copyright (c) 2008 Petr Baudis <pasky@suse.cz>
   3 .\" and copyright (c) 2009, Linux Foundation, written by Michael Kerrisk
   4 .\"     <mtk.manpages@gmail.com>
   5 .\"
   6 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   7 .\"
   8 .\" Redistribution and use in source and binary forms, with or without
   9 .\" modification, are permitted provided that the following conditions
  10 .\" are met:
  11 .\"
  12 .\" 2008-12-08 Petr Baudis <pasky@suse.cz>
  13 .\"    Rewrite the BSD manpage in the Linux man pages style and account
  14 .\"    for glibc specificities, provide an example.
  15 .\" 2009-01-14 mtk, many edits and changes, rewrote example program.
  16 .\"
  17 .TH getifaddrs 3 (date) "Linux man-pages (unreleased)"
  18 .SH NAME
  19 getifaddrs, freeifaddrs \- get interface addresses
  20 .SH LIBRARY
  21 Standard C library
  22 .RI ( libc ", " \-lc )
  23 .SH SYNOPSIS
  24 .nf
  25 .B #include <sys/types.h>
  26 .B #include <ifaddrs.h>
  27 .PP
  28 .BI "int getifaddrs(struct ifaddrs **" "ifap" );
  29 .BI "void freeifaddrs(struct ifaddrs *" "ifa" );
  30 .fi
  31 .SH DESCRIPTION
  32 The
  33 .BR getifaddrs ()
  34 function creates a linked list of structures describing
  35 the network interfaces of the local system,
  36 and stores the address of the first item of the list in
  37 .IR *ifap .
  38 The list consists of
  39 .I ifaddrs
  40 structures, defined as follows:
  41 .PP
  42 .in +4n
  43 .EX
  44 struct ifaddrs {
  45     struct ifaddrs  *ifa_next;    /* Next item in list */
  46     char            *ifa_name;    /* Name of interface */
  47     unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
  48     struct sockaddr *ifa_addr;    /* Address of interface */
  49     struct sockaddr *ifa_netmask; /* Netmask of interface */
  50     union {
  51         struct sockaddr *ifu_broadaddr;
  52                          /* Broadcast address of interface */
  53         struct sockaddr *ifu_dstaddr;
  54                          /* Point\-to\-point destination address */
  55     } ifa_ifu;
  56 #define              ifa_broadaddr ifa_ifu.ifu_broadaddr
  57 #define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
  58     void            *ifa_data;    /* Address\-specific data */
  59 };
  60 .EE
  61 .in
  62 .PP
  63 The
  64 .I ifa_next
  65 field contains a pointer to the next structure on the list,
  66 or NULL if this is the last item of the list.
  67 .PP
  68 The
  69 .I ifa_name
  70 points to the null-terminated interface name.
  71 .\" The constant
  72 .\" .B IF NAMESIZE
  73 .\" indicates the maximum length of this field.
  74 .PP
  75 The
  76 .I ifa_flags
  77 field contains the interface flags, as returned by the
  78 .B SIOCGIFFLAGS
  79 .BR ioctl (2)
  80 operation (see
  81 .BR netdevice (7)
  82 for a list of these flags).
  83 .PP
  84 The
  85 .I ifa_addr
  86 field points to a structure containing the interface address.
  87 (The
  88 .I sa_family
  89 subfield should be consulted to determine the format of the
  90 address structure.)
  91 This field may contain a null pointer.
  92 .PP
  93 The
  94 .I ifa_netmask
  95 field points to a structure containing the netmask associated with
  96 .IR ifa_addr ,
  97 if applicable for the address family.
  98 This field may contain a null pointer.
  99 .PP
 100 Depending on whether the bit
 101 .B IFF_BROADCAST
 102 or
 103 .B IFF_POINTOPOINT
 104 is set in
 105 .I ifa_flags
 106 (only one can be set at a time),
 107 either
 108 .I ifa_broadaddr
 109 will contain the broadcast address associated with
 110 .I ifa_addr
 111 (if applicable for the address family) or
 112 .I ifa_dstaddr
 113 will contain the destination address of the point-to-point interface.
 114 .PP
 115 The
 116 .I ifa_data
 117 field points to a buffer containing address-family-specific data;
 118 this field may be NULL if there is no such data for this interface.
 119 .PP
 120 The data returned by
 121 .BR getifaddrs ()
 122 is dynamically allocated and should be freed using
 123 .BR freeifaddrs ()
 124 when no longer needed.
 125 .SH RETURN VALUE
 126 On success,
 127 .BR getifaddrs ()
 128 returns zero;
 129 on error, \-1 is returned, and
 130 .I errno
 131 is set to indicate the error.
 132 .SH ERRORS
 133 .BR getifaddrs ()
 134 may fail and set
 135 .I errno
 136 for any of the errors specified for
 137 .BR socket (2),
 138 .BR bind (2),
 139 .BR getsockname (2),
 140 .BR recvmsg (2),
 141 .BR sendto (2),
 142 .BR malloc (3),
 143 or
 144 .BR realloc (3).
 145 .SH ATTRIBUTES
 146 For an explanation of the terms used in this section, see
 147 .BR attributes (7).
 148 .TS
 149 allbox;
 150 lbx lb lb
 151 l l l.
 152 Interface       Attribute       Value
 153 T{
 154 .na
 155 .nh
 156 .BR getifaddrs (),
 157 .BR freeifaddrs ()
 158 T}      Thread safety   MT-Safe
 159 .TE
 160 .sp 1
 161 .SH STANDARDS
 162 None.
 163 .SH HISTORY
 164 This function first appeared in BSDi and is
 165 present on the BSD systems, but with slightly different
 166 semantics documented\[em]returning one entry per interface,
 167 not per address.
 168 This means
 169 .I ifa_addr
 170 and other fields can actually be NULL if the interface has no address,
 171 and no link-level address is returned if the interface has an IP address
 172 assigned.
 173 Also, the way of choosing either
 174 .I ifa_broadaddr
 175 or
 176 .I ifa_dstaddr
 177 differs on various systems.
 178 .\" , but the BSD-derived documentation generally
 179 .\" appears to be confused and obsolete on this point.
 180 .\" i.e., commonly it still says one of them will be NULL, even if
 181 .\" the ifa_ifu union is already present
 182 .PP
 183 .BR getifaddrs ()
 184 first appeared in glibc 2.3, but before glibc 2.3.3,
 185 the implementation supported only IPv4 addresses;
 186 IPv6 support was added in glibc 2.3.3.
 187 Support of address families other than IPv4 is available only
 188 on kernels that support netlink.
 189 .SH NOTES
 190 The addresses returned on Linux will usually be the IPv4 and IPv6 addresses
 191 assigned to the interface, but also one
 192 .B AF_PACKET
 193 address per interface containing lower-level details about the interface
 194 and its physical layer.
 195 In this case, the
 196 .I ifa_data
 197 field may contain a pointer to a
 198 .IR "struct rtnl_link_stats" ,
 199 defined in
 200 .I <linux/if_link.h>
 201 (in Linux 2.4 and earlier,
 202 .IR "struct net_device_stats" ,
 203 defined in
 204 .IR <linux/netdevice.h> ),
 205 which contains various interface attributes and statistics.
 206 .SH EXAMPLES
 207 The program below demonstrates the use of
 208 .BR getifaddrs (),
 209 .BR freeifaddrs (),
 210 and
 211 .BR getnameinfo (3).
 212 Here is what we see when running this program on one system:
 213 .PP
 214 .in +4n
 215 .EX
 216 $ \fB./a.out\fP
 217 lo       AF_PACKET (17)
 218                 tx_packets =        524; rx_packets =        524
 219                 tx_bytes   =      38788; rx_bytes   =      38788
 220 wlp3s0   AF_PACKET (17)
 221                 tx_packets =     108391; rx_packets =     130245
 222                 tx_bytes   =   30420659; rx_bytes   =   94230014
 223 em1      AF_PACKET (17)
 224                 tx_packets =          0; rx_packets =          0
 225                 tx_bytes   =          0; rx_bytes   =          0
 226 lo       AF_INET (2)
 227                 address: <127.0.0.1>
 228 wlp3s0   AF_INET (2)
 229                 address: <192.168.235.137>
 230 lo       AF_INET6 (10)
 231                 address: <::1>
 232 wlp3s0   AF_INET6 (10)
 233                 address: <fe80::7ee9:d3ff:fef5:1a91%wlp3s0>
 234 .EE
 235 .in
 236 .SS Program source
 237 \&
 238 .EX
 239 #define _GNU_SOURCE     /* To get defns of NI_MAXSERV and NI_MAXHOST */
 240 #include <arpa/inet.h>
 241 #include <sys/socket.h>
 242 #include <netdb.h>
 243 #include <ifaddrs.h>
 244 #include <stdio.h>
 245 #include <stdlib.h>
 246 #include <unistd.h>
 247 #include <linux/if_link.h>
 248 \&
 249 int main(int argc, char *argv[])
 250 {
 251     struct ifaddrs *ifaddr;
 252     int family, s;
 253     char host[NI_MAXHOST];
 254 \&
 255     if (getifaddrs(&ifaddr) == \-1) {
 256         perror("getifaddrs");
 257         exit(EXIT_FAILURE);
 258     }
 259 \&
 260     /* Walk through linked list, maintaining head pointer so we
 261        can free list later. */
 262 \&
 263     for (struct ifaddrs *ifa = ifaddr; ifa != NULL;
 264              ifa = ifa\->ifa_next) {
 265         if (ifa\->ifa_addr == NULL)
 266             continue;
 267 \&
 268         family = ifa\->ifa_addr\->sa_family;
 269 \&
 270         /* Display interface name and family (including symbolic
 271            form of the latter for the common families). */
 272 \&
 273         printf("%\-8s %s (%d)\en",
 274                ifa\->ifa_name,
 275                (family == AF_PACKET) ? "AF_PACKET" :
 276                (family == AF_INET) ? "AF_INET" :
 277                (family == AF_INET6) ? "AF_INET6" : "???",
 278                family);
 279 \&
 280         /* For an AF_INET* interface address, display the address. */
 281 \&
 282         if (family == AF_INET || family == AF_INET6) {
 283             s = getnameinfo(ifa\->ifa_addr,
 284                     (family == AF_INET) ? sizeof(struct sockaddr_in) :
 285                                           sizeof(struct sockaddr_in6),
 286                     host, NI_MAXHOST,
 287                     NULL, 0, NI_NUMERICHOST);
 288             if (s != 0) {
 289                 printf("getnameinfo() failed: %s\en", gai_strerror(s));
 290                 exit(EXIT_FAILURE);
 291             }
 292 \&
 293             printf("\et\etaddress: <%s>\en", host);
 294 \&
 295         } else if (family == AF_PACKET && ifa\->ifa_data != NULL) {
 296             struct rtnl_link_stats *stats = ifa\->ifa_data;
 297 \&
 298             printf("\et\ettx_packets = %10u; rx_packets = %10u\en"
 299                    "\et\ettx_bytes   = %10u; rx_bytes   = %10u\en",
 300                    stats\->tx_packets, stats\->rx_packets,
 301                    stats\->tx_bytes, stats\->rx_bytes);
 302         }
 303     }
 304 \&
 305     freeifaddrs(ifaddr);
 306     exit(EXIT_SUCCESS);
 307 }
 308 .EE
 309 .SH SEE ALSO
 310 .BR bind (2),
 311 .BR getsockname (2),
 312 .BR socket (2),
 313 .BR packet (7),
 314 .BR ifconfig (8)