UNIX for Dummies Questions & Answers

I'm getting the following Error:

prepare_pcap.c: In function `prepare_pkts':
prepare_pcap.c:127: error: dereferencing pointer to incomplete type
prepare_pcap.c:138: error: dereferencing pointer to incomplete type

====================================

This is the part of the relevant Code:

-----PREPARE_PCAP.H-----------

#if defined(__HPUX) || defined(__DARWIN) || defined(__CYGWIN) || defined(__FreeBSD__)
struct iphdr
{
#ifdef _HPUX_LI
unsigned int ihl:4;
unsigned int version:4;
#else
unsigned int version:4;
unsigned int ihl:4;
#endif
u_int8_t tos;
u_int16_t tot_len;
u_int16_t id;
u_int16_t frag_off;
u_int8_t ttl;
u_int8_t protocol;
u_int16_t check;
u_int32_t saddr;
u_int32_t daddr;
/*The options start here. */
};

------PREPARE_PCAP.C---------

int prepare_pkts(char *file, pcap_pkts *pkts) {
pcap_t *pcap;
struct pcap_pkthdr *pkthdr = NULL;
.....
struct iphdr *iphdr;

.....
iphdr = (struct iphdr *)((char *)ethhdr + sizeof(*ethhdr)); <<<LINE 127
if (iphdr && iphdr->version == 6) {
//ipv6
pktlen = (u_long) pkthdr->len - sizeof(*ethhdr) - sizeof(*ip6hdr);


----------------------------
This is where the error occurs. Can anyone please give me suggestion?

Thank you in Advance.

Code:

int prepare_pkts(char *file, pcap_pkts *pkts) {
pcap_t *pcap;
struct pcap_pkthdr *pkthdr = NULL;
.....
struct iphdr *iphdr;

.....
iphdr = (struct iphdr *)((char *)ethhdr + sizeof(*ethhdr)); <<<LINE 127
if (iphdr && iphdr->version == 6) {
//ipv6
pktlen = (u_long) pkthdr->len - sizeof(*ethhdr) - sizeof(*ip6hdr);

let's see here....

This: *ethdr is totally illegal. You cannot use a pointer syntax on a plain old struct.
I think you'd want this:

sizeof( struct ethdr )

in order to get any useful information.

substitute "struct whatever" wherever you have these sizeof(*thing) things.

----

BTW -- what in the world are you trying to do here?
Whatever it is .... it's the wrong way.

If you describe what it really is you're trying to accomplish,
I can show you a better way.

Best regards,
We're all learning,
quirk

Thank you for your reply. First a background on what I'm trying to do - I'm trying to compile SIPp (VoIP load generator) on Solaris Platform. This is an open Source code - however, a majority of the usage is on Linux platform - where it compiles fine.

I tried your suggestion of replacing sizeof((ethhdr) with sizeof( struct ethhdr ) - however, it fails with the following error:

prepare_pcap.c:126: error: invalid application of `sizeof' to an incomplete type

Source Line 126:
iphdr = (struct iphdr *)((char *)ethhdr + sizeof(struct ethhdr));

Please let me know if you have other suggestions.

thank you,
--------------------------------------------------

omg -- this is actual open source code? that's amazing.

Well -- one error at a time. Fix one -- on to the next.
That first suggestion did what it was supposed to do.

This statement:

Code:

iphdr = (struct iphdr *)((char *)ethhdr + sizeof(struct ethhdr));

Is complete insane. It's relying on typecasting a pointer of a structure,
jumping over the size of itself and assuming that the next spot in memory
is a pointer to iphdr. This simply is dreaming. Although it ~could~ happen,
given that the compiler writer places his variables on after the other in
memory but if it did... that means that the memory must've been explicitly
declared somewhere and if so... why not explicitly cast that memory here?

But whatever. The same problem exists, more or less. But to get it to compile,
try this:

Code:

iphdr = (struct iphdr *)((long)&ethhdr + (long)sizeof(struct ethhdr));

or:

Code:

iphdr = (void *)((long)&ethhdr + (long)sizeof(struct ethhdr));

good luck with this!

Modified your suggestion to:

iphdr = (void *)((long)&ethhdr + (long)sizeof(&ethhdr));

And I don't see the error :

prepare_pcap.c:127: error: invalid application of `sizeof' to an incomplete type


--------

Any suggestion on the original error : dereferencing pointer to incomplete type.

thx.

why modify my suggestion and then complain when it doesn't work?

Here is how this stuff is supposed to work:

Code:

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

struct hdr {
  int x;
  long y;
  char z[16];
  };

struct hdr *iphdr;
struct hdr ethhdr;



int main()
{

ethhdr.x = 7;
ethhdr.y = 81;
strcpy( ethhdr.z, "frogs" );

iphdr = (void *)((long)&ethhdr + (long)sizeof(&ethhdr));

(void)printf( "address becomes: |%x|\n", &ethhdr );
(void)printf( "address becomes: |%x|\n", iphdr );

return 0;
}

OUTPUT:

Code:

address becomes: |20e54|
address becomes: |20e58|

But this line:

Code:

iphdr = (void *)((long)&ethhdr + (long)sizeof(&ethhdr));

can serve no useful purpose.
it is a bug. it's wrong. etc...

I forgot one thing: I had tried w/ ur suggestion:
iphdr = (struct iphdr *)((long)&ethhdr + (long)sizeof(struct ethhdr));
and also with
iphdr = (void *)((long)&ethhdr + (long)sizeof(struct ethhdr));

However, I got this error again:

prepare_pcap.c:127: error: invalid application of `sizeof' to an incomplete type