Re: Segmentation Fault on printf()

Sabu,

I think that in this case the packet length is zero which will result in
nothing from mtod.

Daniel


On Sun, Mar 16, 2014 at 3:03 PM, sabu kurian [off-list ref] wrote:

Hello chris,

Thanks for your reply. I tried dumping the contents of mbuf as you have
suggested.
This is what I get:

dump mbuf at 0x0x6c7620, phys=7f34d91eba28, buf_len=59488
pkt_len=0, ol_flags=0, nb_segs=0, in_port=0

and soon after that I get a segmentation fault. Do you have any idea on
what could be causing it ?

Regards



On Fri, Mar 14, 2014 at 9:40 PM, Chris Wright [off-list ref] wrote:

quoted

* sabu kurian (sabu2kurian-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org) wrote:

quoted

Hello friends,

I'm trying to print the ether_type for a packet that I captured from a

port

quoted

on my machine. Suppose 'm' holds the packet. 'm' is of type 'struct
rte_mbuf'. Intels API reference for DPDK says 'ether_type' is of

uint16_t.

quoted

I used the following code to retrieve ether_type.

void * eth_type;
struct ether_hdr *eth;

eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = &eth->ether_type;

printf("\n Type is %" PRIu16 , *((uint16_t *)eth_type));

Looks ok, albeit slightly overly complicated.

struct ether_hdr *eth;
uint16_t eth_type;

eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = eth->ether_type;
printf("\n Type is %" PRIu16 , eth_type));

This would remove all the extra casting.  Perhaps the mbuf is the problem
here.  And if so, the above snippet would segfault on eth->ether_type
showing you that mbuf is invalid.

You could try to (above mtod):

rte_pktmbuf_dump(m, sizeof(struct ether_hdr));

as that will show key contents of mbuf and packet data (and do some
basic validation along the way).

thanks,
-chris