The bug is that the ioctl() must be called twice as the first time it
returns garbage. The other SSIOCGIF* commands (such as NETMASK and BROADCAST)
do not suffer from this even though their code paths in the kernel are in
common. The ifconfig program seems to not suffer from this problem because
it invokes the ioctl() with other SIOCGIF* commands prior to invoking the
SIOCGIFADDR command.
I have observed this both on Red Hat 5.1 with kernel 2.0.34 on a generic
i586 tower and on Slackware 3.4 with kernel 2.0.30 running on a Toshiba
T4700CT laptop.
Please include email to me (bob@cavu.com) in any replies as I do not
normally follow the kernel mailing list.
I enclose a demonstration program and its output for your consideration.
------------------------------- addr.c -----------------------------
#include <string.h>
#include <sys/socket.h>
#include <asm/ioctls.h>
#ifndef SIOCGIFADDR
#include <ioctls.h>
#endif
#include <net/if.h>
#include <stdio.h>
main()
{
zzz();
zzz();
exit(0);
}
zzz()
{
int inet_sock;
int i;
static
struct ifreq ifr;
static
int n;
inet_sock = socket(AF_INET, SOCK_DGRAM, 0);
strcpy(ifr.ifr_name, "eth0");
if (ioctl(inet_sock, SIOCGIFADDR, &ifr) < 0)
perror("zzz: ioctl");
printf("zzz%d: &ifr.ifr_addr=0x%X SIOCGIFADDR=0x%X dev=%s\n",
n, &ifr.ifr_addr, SIOCGIFADDR, "eth0");
printf("zzz%d: dump:\n", n);
for (i=0; i<sizeof ifr; i++)
printf("%3d ", ((unsigned char *) &ifr)[i]);
printf("\n");
printf("==============================\n");
n++;
}
------------------------------- output of ./addr -----------------------------
zzz0: &ifr.ifr_addr=0x8049810 SIOCGIFADDR=0x8915 dev=eth0
zzz0: dump:
101 116 104 48 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 96
8 137 123 90 0 0 0 0 0 0 0 0
^^^^^^^^^^^^^^^^ bogus inet addr
==============================
zzz1: &ifr.ifr_addr=0x8049810 SIOCGIFADDR=0x8915 dev=eth0
zzz1: dump:
101 116 104 48 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0 0
139 162 250 186 0 0 0 0 0 0 0 0
^^^^^^^^^^^^^^^^ correct inet addr
==============================
------------------------------ output of "ifconfig eth0" --------------------------
eth0 Link encap:Ethernet HWaddr 00:60:08:89:7B:5A
inet addr:139.162.250.186 Bcast:139.162.250.255 Mask:255.255.255.0
^^^^^^^^^^^^^^^ correct inet addr from ifconfig
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:490829 errors:165 dropped:165 overruns:1584997 frame:165
TX packets:1970214 errors:0 dropped:0 overruns:0 carrier:4755428 coll:
1584858
Interrupt:3 Base address:0x300
----------------------------------------------------------------------
Thanks,
Bob
bob@cavu.com
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