inet_ntoa problem

Question

Hi

I am declaring following variables

unsigned long   dstAddr;
unsigned long   gateWay;
unsigned long   mask;

These variables contains ipaddresses in network byte order. So when I am trying to print the dot notation using inet_ntoa function for mask variable sometimes it is printing strange values. The below code is in a while loop .. which loops for n times.

printf("%s\t%s\t%s\t",inet_ntoa(dstAddr),inet_ntoa(gateWay),inet_ntoa(mask));

  192.168.122.0         0.0.0.0    0.255.255.255    

but it should be

  192.168.122.0         0.0.0.0    255.255.255.0

I printed the HEX values of the variables and it shows ..

007aa8c0    00000000      ffffff00  

So is this because of inet_ntoa ??

Actually I am trying to get the values of the declared variables from 254 routing table in kernel via NETLINKS. I guess I should still use inet_ntoa function to convert the value into dot notation .. ??

Answer 1

The only thing that makes sense is that your assumption regarding all the addresses being in network byte order is incorrect.

Answer 2

Well, given that it works for your non-mask values (including the first which also has the high bit set), I'd be looking at what mask actually contains.

What is it when you print it out as a normal unsigned long? My bet is that mask is actually not the correct value:

printf ("%08x\t%08x\t%08x\n", dstAddr, gateWay, mask);

(assuming you have four-byte longs).

For example, this little program (compiled under Cygwin):

#include <stdio.h>

int main (void) {
    unsigned long dstAddr, gateWay, mask;
    dstAddr = 0x007aa8c0;
    gateWay = 0x00000000;
    mask    = 0x00ffffff;

    printf("%-15s %-15s %-15s\n",
        inet_ntoa (dstAddr),
        inet_ntoa (gateWay),
        inet_ntoa (mask));

    printf("%-15s ",  inet_ntoa (dstAddr));
    printf("%-15s ",  inet_ntoa (gateWay));
    printf("%-15s\n", inet_ntoa (mask));

    printf ("%08x%8s%08x%8s%08x\n",
        dstAddr, "",
        gateWay, "",
        mask);

    return 0;
}

outputs:

192.168.122.0   192.168.122.0   192.168.122.0
192.168.122.0   0.0.0.0         255.255.255.0
007aa8c0        00000000        00ffffff

Note that I had to separate my calls to inet_ntoa as it appears to use a static buffer. When I was doing it all within a single printf, it overwrote the contents of that buffer before any of them were printed, hence I only got the last one processed. I don't think that's happening in your case since you're getting different values.