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Linux 2.6 - man page for getifaddrs (linux section 3)

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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 associ-
       ated 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 appro-
       priately.

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 imple-
       mentation 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.

CONFORMING TO
       Not  in POSIX.1-2001.  This function first appeared in BSDi and is present on the BSD sys-
       tems, 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 inter-
       face 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 ear-
       lier, 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>

       int
       main(int argc, char *argv[])
       {
	   struct ifaddrs *ifaddr, *ifa;
	   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 (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 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU					    2012-11-11				    GETIFADDRS(3)
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