Thread: help printing sockaddr_in info

I've got the following populated struct and I'm trying to print it into the normal human readable IP and port address (e.g. 192.168.0.1, and port ... 25).

struct sockaddr_in *saddr;

saddr->sin_port;
saddr->sin_addr.s_addr;

I've been trying these, but none work.

printf("Numeric: %u\n", inet_ntohl(saddr->sin_addr.s_addr));
printf("sin_port: %d\n", saddr->sin_port);
printf("saddr = %d, %s: %d\n", saddr->sin_family, inet_ntoa(saddr->sin_addr), saddr->sin_port);
printf("s_addr: %d\n", ntohl(saddr->sin_addr.s_addr));

What doesn't work about them? I.e., are they printing complete garbage? Are they printing 0.0.0.0 or some other invalid address? Are they printing nothing at all?

None of them print in the format I'm looking for. A string format for ip xx.xx.xx.xx and an int for port.

So saddr->sin_port will be stored in network byte order (big endian). I'm guessing you're using an x86 based (little endian) architecture. The port is a 2 byte number, thus, if your port number is 25, the network byte order will be 0x0019. Printing that in %d format would give you 6400 on a little endian machine. You may need to call inet_ntohs on it before printing. As for the IP address, I don't see anything wrong with your call and print statement. Try printing the values in hex with all leading zeros (%04x for port, %08x for IP). It will be easier to see if you have byte order issues in hex instead of in decimal. Also, it might help us if we had a little more code to actually see how your structures get populated and if anything may be corrupting them (e.g. buffer overflow).

The only one of those that's even supposed to do that (for the IP address anyway) is inet_ntoa. Fortunately, that's exactly what it does. If it's not giving you the numbers you want, that's something else. But what you've got prints just fine.

Code:

int main(void) {
    struct sockaddr_in *saddr = malloc(sizeof(*saddr));
    inet_aton("192.168.10.45", &saddr->sin_addr);
    printf("%s\n", inet_ntoa(saddr->sin_addr));
    return 0;
}

prints exactly what you want.

Originally Posted by rrlangly

I'm having some issues troubleshooting a bug. That very code segfaults on my linux system.

Code:

int main(void) {
    struct sockaddr_in *saddr = malloc(sizeof(*saddr));
    inet_aton("192.168.10.45", &saddr->sin_addr);
    printf("%s\n", inet_ntoa(saddr->sin_addr));
    return 0;
}

Lose the &

Originally Posted by CommonTater

Lose the &

So I dropped the & and same results. Even before it segfaulted on the printf line.

Code:

int main(void) {
    struct sockaddr_in *saddr = malloc(sizeof(*saddr));

    inet_aton("192.168.10.31", saddr->sin_addr);

    printf(": %s\n", inet_ntoa(&saddr->sin_addr)); // <-- segfauts on this line

    return 0;
}

Originally Posted by rrlangly

That gives me the following compile error.

Dang... now I've got a headache. (sockets can be a real pain!)

I actually tested this one in winsock...

Code:

int main(void)
  {

    struct sockaddr_in *saddr = malloc(sizeof(*saddr));

    saddr->sin_addr.S_un.S_addr = inet_addr("192.168.10.31");

    printf("Stored address is: %s\n", inet_ntoa(saddr->sin_addr));


    return 0; }

Well, I do know that I don't have S_un on my linux box. So I can't get that working either.

Originally Posted by rrlangly

Well, I do know that I don't have S_un on my linux box. So I can't get that working either.

Did you even try it?

And yes, I tried it. It's unrecognized.

Originally Posted by rrlangly

And yes, I tried it. It's unrecognized.

Then I guess you can't do that on Linux