Description of problem:
In the getifaddrs(3) manual page, there is an 'EXAMPLE' section which contains code demonstrating how to use the getifaddrs() API. This code is not robust and will crash depending on the host OS network configuration

# gcc -g -o if if.c 
# gdb ./if
(gdb) run
Starting program: /home/berrange/if 
lo  address family: 17 (AF_PACKET)
wlan0  address family: 17 (AF_PACKET)
virbr0  address family: 17 (AF_PACKET)
virbr0-nic  address family: 17 (AF_PACKET)

Program received signal SIGSEGV, Segmentation fault.
0x00000000004006f7 in main (argc=1, argv=0x7fffffffe1e8) at if.c:25
25	               family = ifa->ifa_addr->sa_family;
(gdb) print ifa->ifa_addr
$1 = (struct sockaddr *) 0x0
(gdb) print *ifa
$2 = {ifa_next = 0x602088, ifa_name = 0x602074 "tun0", ifa_flags = 69841, ifa_addr = 0x0, 
  ifa_netmask = 0x0, ifa_ifu = {ifu_broadaddr = 0x0, ifu_dstaddr = 0x0}, ifa_data = 0x6027b8}

This bit of the example code:


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

           for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
               family = ifa->ifa_addr->sa_family;

Should be changed to

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

           for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
                family = ifa->ifa_addr ? ifa->ifa_addr->sa_family : AF_UNSPEC;


ie, allow for  ifa->ifa_addr being possibly NULL


Version-Release number of selected component (if applicable):
man-pages-3.25-1.fc14.noarch
glibc-2.13-1.x86_64

How reproducible:
Only if 'ifconfig -a' shows an interface with "Link encap:UNSPEC", eg tun0 from a VPNC connection


Steps to Reproduce:
1. Start VPNC client
2. Run getifaddrs() example program
3.
  
Actual results:
$ ./if 
lo  address family: 17 (AF_PACKET)
wlan0  address family: 17 (AF_PACKET)
virbr0  address family: 17 (AF_PACKET)
virbr0-nic  address family: 17 (AF_PACKET)
Segmentation fault

Expected results:
$ ./if 
lo  address family: 17 (AF_PACKET)
wlan0  address family: 17 (AF_PACKET)
virbr0  address family: 17 (AF_PACKET)
virbr0-nic  address family: 17 (AF_PACKET)
tun0  address family: 0
eth0  address family: 17 (AF_PACKET)
lo  address family: 2 (AF_INET)
	address: <127.0.0.1>
wlan0  address family: 2 (AF_INET)
	address: <192....>
virbr0  address family: 2 (AF_INET)
	address: <192.....>
tun0  address family: 2 (AF_INET)
	address: <10.....>
lo  address family: 10 (AF_INET6)
	address: <::1>
wlan0  address family: 10 (AF_INET6)
	address: <2a01:....>
wlan0  address family: 10 (AF_INET6)
	address: <fe80:....>


Additional info: