Structure Pointing to a Character Array

nikon2k · 08-09-2010, 03:35 PM

Hi,

I am currently writing a program using raw sockets. This program is used to send out ARP reply frames.

The problem that I have is that I do not seem to be able to fill out a structure that is pointing to part of a character array.

Code:

struct eth_header *eth;
struct arp_header *arp;
char buf[8];

eth = (struct eth_header *)buf;
arp = (struct arp_header *)(buf+sizeof(struct arp_header));

Whilst I can fill out the variables of eth, I cannot fill out the variables of arp. If I fill them out and then send the contents of the array using sendto, only the first structure (the ethernet header) is sent out. The rest of the data appears to be the original junk data in the array.

I have checked the address of the structure pointer and it is pointing to the right place in the array.

If I write a raw sockets program that uses an IP header struct and TCP header struct I can point these into the array and write to both of them without any issues.

The difference here is that the call to socket is different:

Code:

/* For ethernet */
fd = socket(AF_INET, SOCK_PACKET, htons(ETH_P_ARP));

/* For IP */
fd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);

If I just create one structure containing all of the varaibles needed this gets around the issue, however, I would like to know what I am doing wrong in the first instance.

Can anyone help?

Cheers

johnsfine · 08-09-2010, 03:47 PM

Quote:

Originally Posted by nikon2k

char buf[8];

You intend that array to hold both an eth_header and an arp_header?

What source did you use for the definitions of those two headers? What are their sizes? Why do you think they will fit in 8 bytes?

Quote:

eth = (struct eth_header *)buf;
arp = (struct arp_header *)(buf+sizeof(struct arp_header));

Why do you add sizeof(struct arp_header) to the buffer address??

You seem to intend that to advance past the eth_header that you put at the start of the buffer.

PTrenholme · 08-09-2010, 04:03 PM

From the code you posted, you start out with a array of pointers ("buf") and calculate another pointer to the sizeof(struct arp_header) element in that array. So, eth=buf[0] and arp=buf[sizeof(struct arp_header)].

I presume that your code initializes the values of buf[0] and buf[sizeof(struct arp_header)] to some valid memory locations? But I don't understand your question about the contents of buf[1]...buf[7]. Why would you expect that they would have anything in them except the uninitialized values? And, unless sizeof(struct arp_header) is < 8, buf[sizeof(struct arp_header)] is someplace in unallocated memory, so writing to it is somewhat risky.

nikon2k · 08-09-2010, 05:53 PM

Hi,

Thanks for the responses.

The code I posted was merely for the purpose of illustrating my issue.

The actual array is 42 bytes, 14 bytes for the ethernet header and 28 bytes for the ARP header.

The intention was to point the ethernet structure to the start of the array and the ARP structure immediately after it in the array.

The only reason that I was doing it this way is because this is how I had seen some TCP raw sockets programs coded and had worked in a couple programs that I subsequently wrote (pretty new to C/programming in general)

Quote:

I presume that your code initializes the values of buf[0] and buf[sizeof(struct arp_header)] to some valid memory locations? But I don't understand your question about the contents of buf[1]...buf[7]. Why would you expect that they would have anything in them except the uninitialized values?

What i was doing was filling in the structures after they are pointed to the array. I also use memset to set the array to all zeroes first.

The reason I thought that the contents of the buffer would not be the uninitialized values is because the memory that the first structure gets in the array does get the values I fill the structure in with.

It is only the memory where the second structure is pointing to that shows the uninitialized values.

johnsfine · 08-09-2010, 06:02 PM

Quote:

Originally Posted by nikon2k

It is only the memory where the second structure is pointing to that shows the uninitialized values.

I explained that already.

You used the wrong offset for the second structure, leaving uninitialized memory between the end of the first structure and the beginning of the second.

Quote:

Originally Posted by nikon2k

The actual array is 42 bytes

So you wrote incorrect code that you need help with, but you decided to post different incorrect code.

nikon2k · 08-10-2010, 07:54 AM

Hi,

Sorry, typo in the sample code that I posted.

It will probably make things a lot clearer if I post the relevant sections of the actual code:

Code:

struct eth_header{

char dst_hw_addr[6];
char src_hw_addr[6];
short int frame_type;

};

struct arp_header{

short int hw_type;
short int proto;
char hw_addr_len;
char pr_addr_len;
short int opcode;
char src_mac[6];
char src_ip[4];
char dst_mac[6];
char dst_ip[4];

};

char buf[42]
struct arp_header *arp;
struct ethernet_header *eth;

eth = (struct ethernet_header *)buf;
arp = (struct arp_header *)(buf+sizeof(struct ethernet header));

Am I correct in saying that in the above code, the ethernet structure is the first 14 bytes of buf and the remaining 28 bytes of buf are the arp structure ?

johnsfine · 08-10-2010, 08:04 AM

Quote:

Originally Posted by nikon2k

typo in the sample code that I posted.

And another typo this time. You're missing the _ in ethernet header in the last line of the code you just posted.

Quote:

It will probably make things a lot clearer if I post the relevant sections of the actual code:

Are you still having a problem with that code? If so, post a larger amount including the part with the problem.

Quote:

Am I correct in saying that in the above code, the ethernet structure is the first 14 bytes of buf and the remaining 28 bytes of buf are the arp structure ?

On any architecture in which a short is two bytes, all that should be correct.

On both x86 and x86_64 a short is two bytes.

nikon2k · 08-10-2010, 03:03 PM

Hi,

Damn typos.

Turns out I was being an idiot and my actual code had the following:

Code:

char *buf[42];

Instead of:

Code:

char buf[42];

Thanks for the replies and help.

Cheers