Compare Const char* and char *

Question

I'm trying to compare the values in these two char pointers but i'm getting strange outputs:

The first one is (libpcap IP address):

const char* ip_source = inet_ntop(AF_INET, &ip->ip_src, buffer1, sizeof(buffer1)); //192.168.56.1

The second one is:

char *host_ip = inet_ntoa(((struct sockaddr_in*)a->addr)->sin_addr); //192.168.56.1

I've tried using if (*ip_source == *host_ip), if (strcmp(ip_source, host_ip) == 0) and if (strncmp(ip_source, host_ip, strlen(host_ip)).

How do I compare the IP addresses stored in these two variables to see if both IP addresses are the same?

This is the code:

if (strncmp(ip_source, host_ip, strlen(host_ip)) == 0) // if sent local > remote
{
    printf("   RST/ACK Detected [Local > Remote]\n");
}
else // if sent remote > local
{
    printf("   RST/ACK Detected [Remote > Local]\n");
}

This is the result:

Packet number 2386:
current time: 2015-04-11 15:07:59.412 
  From(src): 192.168.56.1 <---- Local (IP stored in *host_ip)
    To(dst): 192.168.56.2 <---- Remote
   Protocol: TCP
   Src port: 1864
   Dst port: 49750
    Seq Num: 0
    Ack Num: 3556812524
   RST/ACK Detected [Remote > Local] <--- Wrong

In this case it's returning -2

It looks like its failing the test but it depends on the network data coming in, i'm trying to find out if its a problem with this `if` test, the database or the data. — Crizly, Apr 11 '15 at 13:56
I found the problem, the host_ip variable was being over written with every packet captured. — Crizly, Apr 11 '15 at 15:00
@IgnacioVazquez-Abrams - strncmp() takes in a number of characters you want to compare doesn't it? - I want to compare them all so use strlen() of the variable? — Crizly, Apr 11 '15 at 15:02
The point of using `strncmp()` instead of `strcmp()` is that you know how many characters there are and you want to prevent accidental overruns. Using `strlen()` as the length completely undermines that. — Ignacio Vazquez-Abrams, Apr 11 '15 at 15:03
@IgnacioVazquez-Abrams - Ah right, so if i do strcmp() it'll compare the whole string in an if statement, not just like 1 character or something? — Crizly, Apr 11 '15 at 15:08
@Crizly Since you have found the answer, post that answer and accept it to close this issue. — chux - Reinstate Monica, Apr 11 '15 at 15:50

score 0 · Answer 1 · answered Apr 11 '15 at 13:58

0

Ths first would just compare the first character of each, while strcmp looks right to me. Should not matter if one is const and the other not.

answered Apr 11 '15 at 13:58

Bjorn Munch

496
2
6

I've tried using strncmp(ip_source, host_ip, strlen(host_ip) to compare all characters but it's still not working – Crizly Apr 11 '15 at 14:20

Crizly · Answer 2 · 2015-04-11T17:26:49.390

0

As it turns out the issue was with inet_ntoa rather than the string test.

The variable host_ip was being overwritten with every packet captured.

When I got the IP address I used this:

inet_ntoa(((struct sockaddr_in*)a->addr)->sin_addr);

But that dynamically overwrites every time a packet is captured, so changing it to the static:

const char* host_ip = inet_ntop(AF_INET, &(((struct sockaddr_in*)a->addr)->sin_addr), buffer1, sizeof(buffer1));

Method worked.

edited Apr 11 '15 at 17:26

answered Apr 11 '15 at 16:00

Crizly

971
1
12
33

That's an issue with `inet_ntoa()`, not libpcap. The man page on OS X says "The string returned by inet_ntoa() resides in a static memory area.", which is true on other OSes as well, and that means that if *any* other code calls `inet_ntoa()`, it overwrites the previous value. – Apr 11 '15 at 17:03

score 0 · Answer 3 · answered Apr 11 '15 at 17:07

Why not just compare the addresses directly, as they're both in network byte order?

if (memcmp(&ip->ip_src, &((struct sockaddr_in*)a->addr)->sin_addr.s_addr, 4) == 0) {
    printf("   RST/ACK Detected [Local > Remote]\n");
} else {
    printf("   RST/ACK Detected [Remote > Local]\n");
}

Compare Const char* and char *

3 Answers3