strange segmentation fault

alltime · 11-09-2005, 04:13 PM

I am creating a program function to display the Ethernet cards MAC address. For some reaon, I am getting a segmentation fault. Shouldn't displaying the info be as simple as what I have below?

Code:

        struct net_device *dev;
        struct sk_buff *skb;
        struct smap_chan *smap;
        
/* Gather information from the kernel */
int cpu_info( char *buf, char **start, off_t off, int count )
{              
        printk(KERN_INFO "%s - %02x:%02x:%02x:%02x:%02x:%02x\n", dev->name
                                                               , dev->dev_addr[0]
                                                               , dev->dev_addr[1]
                                                               , dev->dev_addr[2]
                                                               , dev->dev_addr[3]
                                                               , dev->dev_addr[4]
                                                               , dev->dev_addr[5]);
}

I am declaring the following header files

Quote:

#include <linux/pci.h> /* For gathering MAC information */
#include <linux/kernel.h> /* for struct sysinfo */
#include <linux/unistd.h> /* for _syscallX macros/related stuff */
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/ioport.h>
#include <linux/skbuff.h>
#include <asm/processor.h>
#include <asm/bitops.h>
#include <asm/io.h>

Matir · 11-09-2005, 05:26 PM

Where do you get "dev" from?

alltime · 11-09-2005, 05:59 PM

Quote:

Originally posted by Matir
Where do you get "dev" from?

dev is from struct net_device *dev;

dev_addr should display the hardware address (MAC).

deiussum · 11-09-2005, 06:49 PM

I think he means, where do you set dev to something. Just declaring it as a global as you have done there makes it a dangling pointer. Dangling pointers are bad and cause segfaults, which is what you are seeing. Somewhere in your code, you must have something on the order of:

dev = somefunctiontosetdev();

or maybe:
dev = malloc(sizeof(net_device));
somefunctiontosetdevproperties(dev);

Or maybe even:
somefunctiontofillindev(&dev);

Just having:

net_device *dev;

You can't expect it to magically have memory allocated for it and get all it's data filled in for you...

Not knowing much about the net_device struct, I can't say what exactly you need to call to initialize it with what you want, but maybe someone else here does.

enthusiast_abdul · 11-10-2005, 02:49 AM

first of all you have to initialise dev.
This can be done by :
struct net_device *dev=__dev_get_by_name("eth0");
This fn. is defined in /usr/src/linux-2.4.20-8/net/core/dev.c.
eth0 is the name of the ethernet interface.
This should prevent the seg. fault.