Is there a quick way, with these or similar methods to obtain the subnet mask of these interfaces ?

Thanks.
Lieven

HTH,
Adam Risi

I have Arch Linux and the compiler i use is gcc 4.2.2. And this is the output:

[yiannis@main Programs]$ gcc -o interface interface.c
[yiannis@main Programs]$ ./interface
Segmentation fault
============================================

I came across the same problem too.. In my case, it was the query for the MAC Address field for a loopback device, that was the culprit. In the code, the lines below:

printf(“%s: IP %s\n”,
item->ifr_name,
inet_ntoa(((struct sockaddr_in *)&item->ifr_addr)->sin_addr));

printf(“, MAC %s\\n”, ether_ntoa((struct ether_addr *)item->ifr_hwaddr.sa_data));

were the culprits in causing the segfaults. Since loopback device does not have a MAC value associated, it might have led to ifr_hwaddr being NULL. So the access to the sa_data might have led to the Segfault. I still need to go through the 2 structures though.

Commenting the above 2 printf’s and just printing item->ifr_name works fine.

Regards,
Prameeth

also, that first printf should have a \n

i tried your code but unfortunately only the loopback interface is found, though the other two are up.
If you got an idea what could be the problem, please let me know.

thank you

It will compile.

Unfortunately, I don’t think you can find every interface via the ioctl method so /proc/net/dev is probably your best bet for what you want.

/proc/net/dev is not so hard to parse. Here’s some code which can do it – at least on Debian. I don’t know if the format of the file is different on other distros:

/* listif.c - List all known interfaces on a Linux System */

#include <stdio.h>
#include <string.h>

main()
{
	char line[512];
	char *colon;
	char *name;
	FILE *fp;

	if(0 == (fp = fopen("/proc/net/dev", "r")))
		return 1;

	while(0 != (name = fgets(line, 512, fp)))
	{
		while(isspace(name[0])) /* Trim leading whitespace */
			name++;

		colon = strchr (ame, ':');
		if(colon)
		{
			*colon = 0;
			printf("%s:\n", name);
		}
	}

	fclose(fp);
	return 0;
}

To get infos about *all* interfaces, I know of no other way than to read the file /proc/net/dev which, unfortunately, is not quite parse-friendly.

1. Segfaults are usually caused by a bad pointer or a buffer overflow. There is only one buffer in this code, so try changing buf[1024] to a much larger number (eg. buf[16384]) and see if that helps.

2. Compile the code with the debug option (gcc -g -o ifenum ifenum.c) and then step through it with a debugger such as kdbg. Then you can see what line is causing the problem.

[yiannis@main Programs]$ gcc -o interface interface.c
[yiannis@main Programs]$ ./interface
Segmentation fault

A straight segmentation fault! (btw, a very ignoring error don’t u think? :P)
Thank u for the quick answer! You have done an excellent work here! ;)